Life on the lattice

Lattice 2013 in Mainz

2011-12-20T15:16:00.000Z

If you are at all tuned in to the gossip of the lattice community, you will probably have heard that Mainz will be organising the annual lattice conference in 2013. I can now confirm that LATTICE 2013 (The XXXI International Symposium on Lattice Field Theory) will take place at the Johannes-Gutenberg-University in Mainz in the week July 29 to August 3, 2013. We look forward to welcoming you here, and I expect to keep you updated on the progress of our preparations as the date approaches.

Virus-related things

2011-09-21T10:10:00.000+01:00

Viruses are dreadful things. The digital ones destroy your data, steal your login, send spam in your name and empty your bank account. The biological ones cripple and kill you. Bad news are that a particularly dreadful virus, viz. polio, has been reported to have spread to China from Pakistan. This is a reminder that wide-spread vaccination against polio (and other diseases) is crucial to avoid the devastating impact they have on the lives of those affected.

Unfortunately, not all preventative measures always work as intended. In particular, another recent piece of bad news (of an entirely unrelated kind) is that the encryption protocols SSL and TLS (1.0) used to secure https connections are vulnerable to attack. An attacker who gets to intercept the encrypted data and who has some control over the user's browser (e.g. via a virus) can use a Cross-Site-Scripting (XSS) attack to hijack an encrypted connection and, e.g., steal from the user's online banking or PayPal account.

This attack is called BEAST. Another "beast", namely DRACO (Double-stranded RNA Activated Caspase Oligomerizer) may become for viruses (speaking of the biological sort again) what penicillin is for bacteria: researchers at MIT have developed this substance that selectively kills cells infected by viruses, curing mice infected with lethal viruses with apparently no serious side-effects seen so far. That sounds like one of those rare pieces of good news.

Lattice 2011, Day Six

2011-07-17T03:15:00.001+01:00

The last day of the conference had two last plenary sessions in the morning. The first began with a talk on lattice QCD with classical and quantum electrodynamics by Brian Tiburzi. In order to measure the electric polarisabilities of hadrons, their energy shift in a constant external electrical field is measured. Classical magnetic fields are also of interest, since they may affect the phase diagram of QCD by catalysing chiral symmetry breaking, possibly creating exotic superconducting phases of QCD matter. Quantum corrections to charged particle properties are also being studied using QED coupled to quarks, but this is still rather hard to do.

Next was John Bulava with a talk on excited hadrons. In order to study excited states, an approach like the GEVP is mandatory, which requires the measuring of multiple correlators with a suitable basis of operators. Since this basis eventually also needs to include multi-hadron states, some form of all-to-all propagators is neeeded, and John presented the distillation and the stochastic LapH approaches, which are based on an expansion in the low modes of the covariant Laplacian on a time slice.

After that, Dru Renner spoke about ETMC's recent work on QCD corrections to electroweak observables, in particular the (g-2) work for which they had been awared the Ken Wilson Award, but also new work on hadronic contributions to the running of α_e.m. and new NLO results for (g-2), which however exclude the light-by-light contribution.

In the second plenary, Hartmut Wittig gave the review talk about low-energy particle physics and chiral extrapolations. The most recent results from the BMW collaboration on the light and strange quark masses are consistent with the FLAG averages, and this remains the case if BMW's lightest (physical and lighter) pion masses are omitted in the chiral extrapolation (or interpolation), indicating that pion masses below 250 MeV are light enough for few-percent accuracy in this area. There are, however, uncertainties in the overall scale of the pion and kaon decay constants which may be due to combined pion mass and discretisation effects. Hartmut also presented recent progress in the determination of g_A of the nucleon.

A review of kaon physics was given by Robert Mawhinney. I'm afraid I can't adequately summarise his talk (there was just too much material).

The final talk was given by Anna Hasenfratz, who spoke about reweighting in the quark mass. Reweighting is an old idea, but recently it has picked up steam in lattice QCD and is now widely used to achieve lighter quark masses, to stabilise simulations, or to incorporate electromagnetic effects. Since the overlap between the simulated and the target distribution must not be too small, the Hasenbusch trick has to be used when reweighting to small quark masses. A new, quadrature-based, approach avoiding the need for inversions has been introduced at this conference by Abdel-Rehim et al.

After this, the conference closed with a round of well-deserved applause for the Local Organising Committee.

Lattice 2011, Day Five

2011-07-17T03:14:00.000+01:00

Sorry for the delayed update; I was too tired to blog last night.

The first plenary of the fifth day started with a talk by David Kaplan with the intriguing title "Listening to Noise". The topic of the talk was in fact noise, which of course affects baryonic correlators particularly badly. Studying unitary fermions as a toy model, David Kaplan showed that the distribution of the measured correlator values approaches a log-normal distribution, i.e. their logarithms approach a normal distribution. Exploiting this, one can attempt to use the cumulants of the measured distribution to extract an effective mass with reduced noise, and this does indeed work in the case of unitary fermions. For QCD, additional tricks may be needed.

The next talk was given by Kostas Orginos, who gave a review of hadron interactions on the lattice. This is still a very difficult problem, and new and better methods will be needed to make progress.

The last talk before the break was on a non-scientific topic, namely the situation in Japan after the great earthquake, presented by Shojo Hashimoto. Besides the terrible loss of life and the large number of people made homeless by the tsunami, the subsequent nuclear meltdown at Fukushima has further worsened the impact of the disaster. Not only have numerous towns been contaminated by Cs-137 (it takes a real physicist to show a curve of the measured radiation and remark upon the perfect exponential curve described by the decay of I-131), but also the power supply has been adversely affected by the shutdown of the nuclear power plants; a shortfall of 10-15% is expected in the summer, and hence power-intensive scientific facilities such as PACS-CS can only run at night. The US and the UK have stepped into the gap and have donated computer time on their machines to Japanes colleagues.

The second plenary was devoted to flavour physics. Enrico Lunghi spoke about the tensions observed in the unitarity triangle fits between sin(2β) and the branching ratio B --> τν, in (g-2)_μ, φ_{B_s}, and the branching ratio B_s --> μ⁺μ^-. The LHCb experiment should be able to clarify the situation soon.

This was followed by a review of heavy-flavour physics on the lattice by Christine Davies, who summarised the different approaches (NRQCD, HQET, Fermilab, relativistic heavy quarks on fine lattices with highly improved actions) and results for the charm and bottom masses and the decay constants and form factors of charm and bottom mesons, as well as for the B meson mixing parameters.

The plenary session closed with the invitation to LATTICE 2012 to be held in Cairns, Australia, from 24th to 29th June 2012.

In the afternoon there were parallel sessions one last time (this included my own talk in the last possible slot).

Lattice 2011, Days Three and Four

2011-07-15T06:29:00.001+01:00

Wednesday was the customary short day, without any plenaries and with morning parallel sessions. The afternoon was free for excursions. I joined some colleagues on a self-organised hiking trip on the Five Lakes Route, which was a short drive from the Village. The view from the upper parts of the trail was very nice, and the hike not too strenuous. At the end, the path got kind of lost in snow, so we only saw one of the five lakes before descending again.

Today's plenaries were almost entirely devoted to finite-temperature QCD. The first speaker of the first session was Ludmilla Levkova, who gave the review talk on finite temperature and density. Since it is always hard to summarise a summary, I'll refrain from tyring, and instead just highlight some of the things in her talk that I found particularly interesting. One is that there are efforts to understand the effects of magnetic fields on the nature of the QCD phase transition; this never occurred to me as a question, but once you realise that the magnetic fields in off-axis heavy-ion collisions are of the order of 10¹⁴ T, it seems quite a natural problem. The other was that the equation of state obtained from different lattice actions comes out significantly different. Some hope to resolve those differences may come from a new method to determine the equation of state that has recently been introduced by Giusti and Meyer.

The next talk was another experimental talk, given by Barbara Jacak of the PHENIX experiment. It is now known that the quark-gluon plasma is a nearly perfect liquid, and there is evidence that all strongly coupled plasmas are alike in some sense. Important remaining questions on which input from the lattice is needed are whether there are quasiparticles in the QGP and if so, what they are, as well as whether there are any relevant screening lengths.

The second plenary was opened with Swagato Mukherjee speaking about fluctuations and correlations at finite chemical potential. Since the fermionic determinant is in general no longer real in the presence of a chemical potential, no direct Monte Carlo evaluation of the path integral is possible in this case. A way around this is to consider the Taylor-expansion around zero chemical potential, and in this case generalised susceptibilities arise as Taylor coefficients. These can be related to moments of fluctuations of the baryon number, which are accessible experimentally. In order to connect the experiments, which controlled by the center of mass energy \sqrt{s}, to theoretical determinations which are controlled by the temperature T and the chemical potential μ, the hadron gas model is used, apparently with good success.

Next was a talk about U(1)_A in hot QCD by Prasad Hegde. At zero temperature, the axial U(1) symmetry of QCD is broken by the axial anomaly, which among other things gives rise to the η/η' mass splitting. Since the spontaneously broken chiral SU(N_f)_LxSU(N_f)_R symmetry is restored at finite temperature, it may be natural to ask if the same happens for the axial U(1) symmetry. Indeed, since the axial anomaly is related to the topological charge of the fields, it is known that the axial U(1) symmetry is restored in the infinite-temperature limit by the screening of the chromoelectric fields (as the topological charge density is proportional to E.B). However, studies using both staggered and domain wall quarks indicate clearly that U(1)_A remains broken above the critical temperature.

The last talk of the morning was by Balint Jóo, who gave a review of the role of GPUs in lattice simulations. By now, many lattice groups have discovered GPUs as a cost-effective means of accelerating computations, which however have their own issues (in particular related to the programming model and to the PCIe bus as a bottleneck in transferring data between GPUs and the CPU). A number of QCD codes have been or are being ported to GPUs (QUDA, QDP++ for GPUs).

In the afternoon there were parallel sessions again. In the evening, we took the cable car to High Camp, which is located at an altitude of about 8100 ft (ca. 2500 m) for the conference banquet. The buffet was good, the desserts very rich, the wine rather effective due to the reduced oxygen pressure at high altitude (for which reason I ask to be forgiven for any mistakes in this summary), and the view from the cable car truly spectacular.

Lattice 2011, Day Two

2011-07-13T16:15:00.000+01:00

Hello again from Squaw Valley.

Today's first plenary was devoted entirely to beyond-the-Standard-Model physics. The first speaker was Aleksi Kurkela, who spoke about large extra dimensions and the lattice. Extra dimensions are phenomenologically appealing, but since gauge theories in d>4 are non-renormalisable, they are defined only up to a regularisation. Results from the ε-expansion suggest the existence of a non-Gaussian UV fixed point in higher dimensions, but since d=5 is well outside of the expected convergence radius of the expansion, lattice studies are needed to check this; for the isotropic case it does not appear to be true, but for the anisotropic case there is evidence that it is indeed true. When the fifth dimension is compactified, new effects arise; in some cases, knowledge of the correlation length of the dimensionally reduced theory can give bounds on the compactification radius.

The second plenary talk was the traditional experimental talk, delivered by Adam Martin from Fermilab. With 1 fb^-1 of data both ATLAS and CMS can exclude the Higgs mass range from 130 GeV to 460 GeV at the 95% confidence level; with 5-10 fb^-1, they should be able to either exclude the full mass range up to 600 GeV or else claim a 5σ discovery. In the low mass range, the Tevatron is currently still more sensitive; CDF has seen a bump in the W/Z+jj cross section, which appears to be ruled ou by D0, so this seems to be a case where backgrounds need to be understood better before reaching any conclusions. Other interesting discrepancies include the tt forward-backward asymmetry and the like-sign dimuon charge asymmetry. We should "stay tuned this summer for exciting results".

The BSM theme was continued in the second plenary. Ethan Neil gave a talk about new physics models on the lattice, giving an account of the (N_c, N_f, representation) space of models studied in the search for the conformal window, and of the methods used to study them, including spectral studies, studies of finite-T phase transitions and the Monte Carlo Renormalisation Group.

In the next talk, Daniel Nogradi spoke about a specific model that has particular phenomenological appeal, namely the SU(3) theory with N_f=2 fermions in the sextet representation. This theory has exactly three Goldstone bosons, allowing for Higgs-less electroweak symmetry breaking, and may allow for a small S-parameter (unacceptably large values for the S-parameter being a problem plaguing many technicolor-like models).

At the end of the plenary sessions, the first Ken Wilson lattice award was awarded to Xu Feng, Karl Janssen, Marcus Petschlies and Dru Renner for their recent paper on the anomalous magnetic moment of the muon.

In the afternoon, there were parallel sessions, and in the evening, the poster session took place.

Lattice 2011, Day One

2011-07-12T06:25:00.000+01:00

Hello from The Village at Squaw Valley, where I am at the Lattice 2011 conference.
Having arrived late yesterday (actually early today), I still feel rather tired and would like to ask my readers to ascribe any glaring errors or omissions in todays post to that fact.

The welcome was in a different style from the usual speeches -- we were shown a short movie by Massimo Di Pierro that combined elements of "Star Wars" and the "Powers of 10" educational film with images of topological charge densities measured on the lattice. Also unusual was the announcement of a Tesla card raffle sponsored by nVidia.

After that, the first plenary session started with a talk by Eigo Shintani on the determination of α_s from lattice QCD. In fact, currently lattice determinations are dominating the world average for α_s(M_Z²), although there are some discrepancies with other methods. Shintani focussed mainly on the efforts of the JLQCD collaboration, which is based on measuring the light quark vacuum polarisation using dynamical overlap fermions, which then can be compared directly to an operator product expansion performed in the continuum, and α_s can be determined by matching to continuum perturbation theory. Other determinations that have been performed have used the Schrödinger functional (ALPHA, PAC-CS), Wilson loops and lattice perturbation theory (HPQCD), and moments of heavy quark current-current correlators (also HPQCD).

The next speaker was Shou-Cheng Zhang from Stanford, who spoke about a topic condensed matter theory that has some interesting connections to lattice QCD, namely topological insulators and superconductors. These are "materials that realise theoretical ideas" in that they cause concepts that are otherwise the realm of theory to appear in an experimentally accessible context. Examples included the appearance of the 3-dimensional Wilson-Dirac operator in the description of a two-dimensional topological insulator, the possibility to have a QED θ-term with θ=π in a topological superconductor, or the appearance of a Dirac monopole as the image charge of a point charge in front of a topological superconductor. These materials also have the possibility to have an enormous technological impact by creating the possibility of having dissipation-free electron flows at room temperature, which could revolutionised electronics and lead to much faster computers.

The last speaker of the session was Mithat Ünsal talking on large-N volume independence and related ideas. Provided that translation invariance and centre symmetry are not spontaneously broken, there is the possibility of reducing QCD in the limit of infinitely many colours to a large-N matrix model. While the Eguchi-Kawai model and its various extensions have failed due to centre symmetry breaking, there appears to be some hope that some other kinds of matrix models could give new insights into gauge theories.

After the coffee break, the second plenary of the day began with Laurence Yaffe speaking about an approach to heavy-ion collisions that begins with simplifying the complicated situation to the much simpler of colliding shockwaves in N=4 super-Yang-Mills theory, which has a dual description as a collision of gravitational waves via the AdS/CFT correspondence. After thus reducing a non-equilibrium problem in a strongly coupled QFT with an initial-value problem in a classical field theory, it turns out that after applying a number of tricks, Einstein's equations for this situation can be converted into a set of nested ODEs that can be solved numerically.

Next was a talk by Jack Laiho on Asymptotic Safety and Quantum Gravity. The concept of asymptotic safety as introduced by Weinberg states that a perturbatively non-renormalisable theory may still be well-defined and possess predictive power if its renormalisation group flow has an ultraviolet fixed point with a finite number of relevant directions. There is some numerical evidence that gravity might be asymptotically safe with only three parameters. In a Euclidean framework, asymptotic safety corresponds to the existence of a critical point. This scenario has been studied in a number of different formulations, including the Euclidean dynamical triangulations of Ambjorn et al. (which have a crumpled phase with infinite Hausdorff dimension and a branched polymer phase with Hausdorff dimension 2, separated by a first-order phase transition, and hence no hope to describe continuum physics) and the Causal Dynamical Triangulations of Ambjorn and Loll (which have a large-scale solution in the form of de Sitter space, and where the spectral dimension runs from 2 at short scales to 4 at large scales). Jack and his student have studied what happens if one adds a measure term to the Regge action, and have found that there are three phases (collapsed, extended, and branched polymer phase) with the possibility of a critical end point in the phase diagram, which could realise the scenario of asymptotic safety. There is also evidence that the spectral dimensions runs from 4 at large scales to 3/2 at short scales, where the dimension 3/2 would reconcile the requirements of holography and the Bekenstein-Hawking entropy.

The last plenary speaker of the day was Paul Rakow, who spoke about flavour-blindness and the pattern of flavour breaking in N_f=3. Since the masses of the light and strange quarks are not identical, the SU(3) flavour symmetry is explicitly broken. Expanding in this breaking around the symmetric theory and exploiting the representation theory of SU(3) allows one to understand the way the physical point is approached in lattice simulations.

In the afternoon there were parallel sessions.

What's new at the fermion zoo?

2011-02-11T16:07:00.000Z

If there is anything more typical of the landscape of lattice QCD than collaboration acronyms that mean something very different (like a car manufacturer, a color model, or an old DOS command), to people from outside the lattice community, it has to be the fact that each of the aforementioned collaborations uses a fermion action that is in some way different from those of all other collaborations. For gauge actions, there isn't all that much variety (Wilson, tree-level Symanzik, Lüscher-Weisz with or without O(N_fα_sa²) corrections, and Iwasaki), but for fermions there is a veritable zoo.

Of course, for every zoo, there is a Linnean system establishing a taxonomy, so the fermion zoo can be ordered by grouping the fermion actions into different classes:

Wilson fermions get rid of the doublers by adding a term (the Wilson term) to the action that explicitly breaks chiral symmetry and thus lifts the degeneracy of the doublers, giving them masses of the order of the cut-off. Wilson fermions can be subdivided further firstly into straight Wilson fermions (which have O(a) discretisation effects and hence are rarely used) and O(a)-improved Wilson fermions, which add another term, the Sheikholeslami-Wohlert term, to reduce the lattice actifacts to be O(a²). The numerous individual actions being used then differ mainly by the kind of links that go into the discretised derivatives (and possibly into the SW term), whether they are thin links for rigorous locality and positivity properties, or different kinds of smeared links for empirically better statistical behaviour of various observables.

twisted-mass fermions are close relatives of Wilson fermions, consisting of a doublet of unimproved Wilson fermions with a twisted mass term of the form τ₃γ⁵; the doublet is interpreted as the up/down isospin doublet. One of the attractive features of twisted fermions is that spectral observables are automatically O(a)-improved. On the other hand, isospin and parity are violated by cut-off effects, which leads to potentially undesirable features such as a neutral pion with the quantum numbers of the vacuum.

staggered fermions reduce the number of doublers to four by redistributing the degrees of freedom between sites. Also here, improvement by adding an additional three-link term (the Naik term) is commonly employed. Significant use is made of smearing to reduce the impact of high-momentum gluons whose exchange results in interactions mixing the different "tastes" of remaining doublers. An advantage of the staggered formalism is the preservation of a residual chiral symmetry; a disadvantage is the need to take the root of the determinant of the Dirac operator (unless one wants to simulate with N_f=4 degenerate flavours), and issue that has been surrounded by some controversy. The actions in current use are the asqtad and HISQ actions.

overlap fermions are constructed as an exact solution to the Ginsparg-Wilson relation by means of the overlap operator, which is essentially the matrix sign function of the Wilson Dirac operator. While having the obvious theoretical advantage of exact chiral symmetry at finite lattice spacing, overlap fermions are very expensive to simulate, and thus are not in widespread use yet.

domain-wall fermions use a fictitious fifth dimension to realise chiral symmetry by localising the opposite chiralities on different "branes" or domain walls in the fifth direction. They are likewise rather expensive to simulate.

Of course, life being incredibly diverse, every taxonomist will sooner or later run into a creature which defies the existing taxonomic scheme. The past year has, I think, been such an occasion for the fermion zoo, which was increased by the addition of what may become two new families of fermions that straddle the boundaries between the classes outlined above.

One is the family of minimally doubled fermions, which are being championed by Mike Creutz and by people here at Mainz. The idea is to find an action which has the minimal number of doublers permitted for a chirally symmetric Dirac operator by the Nielsen-Ninomiya theorem, i.e. a doublet of fermions that can then be interpreted as the up/down doublet. There are two realisations of this idea, now known as Karsten-Wilczek and Creutz-Borici fermions, respectively, both of which rely on the addition of a Wilson-like term to the action. In a way, this puts them somewhere between Wilson and staggered fermions, the latter because of the existence of taste-changing interactions; of course, no rooting is required to simulate an N_f=2 theory with minimally doubled fermions. The price paid is that, because the line connecting the two poles in momentum space defines a preferred direction, at least one of the discrete spatiotemporal symmetries must be broken; this leads to the possibility of generating additional (relevant in the RG sense) dimension-3 operators in the action, which have to be fine-tuned away. Simulations with minimally doubled fermions are in preparation and will have to deal with these questions; it remains to be seen if this formulation will have practical relevance beyond its obvious theoretical impact.

The other new fermion family are the staggered overlap fermions introduced at this year's lattice conference by David Adams, and which as suggested by the name close the gap between staggered and overlap fermions. The idea here is to perform a similar construction to that used to obtain the overlap operator from the Wilson Dirac operator, but taking the staggered Dirac operator as the starting point. As it turns out, this results naturally in a theory with two fermion flavours, so again no rooting is required to simulate an up/down doublet in this fashion.

Like all taxonomy-defying creatures, these new fermion actions hold the potential to reveal hitherto unknown connections between previously unconnected classes of entities, in this case perhaps by establishing new connections between the number of flavours, chiral symmetry, doubling and the staggered formalism.

Lattice 2011 website online

2010-12-21T08:41:00.000Z

This Christmas season being rather snowy, at least here in Germany, many people will be thinking of winter sports. Thinking of winter sports, they might (possibly) be thinking of the Squaw valley ski resort, thinking of which they might (if they happen to be lattice theorists) think of Lattice 2011. All of which is just a roundabout way of saying that the Lattice 2011 website is now online, and while still under construction will soon contain a wealth of relevant information for participants.

Just some links

2010-10-14T11:39:00.001+01:00

Another ultra-lazy links-only post; I promise there'll be some new content soon, though.

Tim Gowers has observed a little physics problem in the ever interesting subfield of bathtub hydrodynamics. Perhaps some fluids expert is reading this and can offer an explanation to his questions.

Nobel prize for a (kind of) lattice

2010-10-05T12:34:00.000+01:00

The Nobel Prize in physics 2010 has been awarded to Andre Geim and Konstantin Novoselov of the University of Manchester "for groundbreaking experiments regarding the two-dimensional material graphene".

Graphene is a novel form of carbon, in which the carbon atoms are bound into a hexagonal lattice covering a single flat two-dimensional layer. Graphite consists of lots of pieces of graphene jumbled together into a three-dimensional whole, so graphene is actually quite common, but Geim and Novoselov were the first to systematically isolate it and elucidate its unusual properties.

Graphene has a number of unique properties, not the least of which is that it has gapless excitations which are described by a Dirac equation -- massless electrons, so to speak. It is this particular feature of the graphene lattice which has inspired the study of graphene-like structures in higher dimensions as a means of obtaining minimally doubled fermions, i.e. lattice fermions that have the minimal number (=2) of doublers prescribed by the Nielsen-Ninomiya theorem. So even if the technological promise of graphene (described e.g. at the Nobel site) were not to be realised, it has at least given theoretical particle physicists something to think about.

Bloggalia varia mixtaque

2010-09-01T10:14:00.001+01:00

Conference blogging from the best of the best: Fields medalist Tim Gowers has been covering the International Congress of Mathematicians (ICM 2010) in Hyderabad on his blog

John Baez has turned from higher category theory to saving the planet and blogs about it at his new blog

Rob Knop has taken up blogging again at the new Scientopia website, which hosts the bloggers that left ScienceBlogs as a consequence of "PepsiGate" or for other reasons

Blogging ICHEP 2010

2010-07-24T10:50:00.001+01:00

I'm currently at the ICHEP 2010 conference in Paris, from where I'm blogging at the official ICHEP 2010 blog. I'll post a summary here later, but for now come over and follow me and the wonderful other bloggers at Blogging ICHEP 2010!

Lattice 2010, Day Five

2010-06-19T09:06:00.000+01:00

The day started with plenary sessions again. The first plenary speaker was Chris Sachrajda on the topic of phenomenology from the lattice. Referring to the talks on heavy and light quarks, spectroscopy and hadron structure for those topics, he covered a mix of various phenomenologically interesting quantities, starting from those that have been measured to good accuracy on the lattice and progressing to those that still pose serious or perhaps even unsurmountable problems. The accurate determination of V_us/V_ud from f_K/f_π and of V_us from the K_l3 form factor f⁺(0), where both the precision and the agreement with the Standard Model are very good, clearly fell into the first category. The determination of B_K is less precise and there is a 2σ tension in the resulting value of |ε_K|. Even more challenging is the decay K --> ππ, for which however progress is being made, whereas the yet greater challenge of nonleptonic B-decays cannot be tackled with presently known methods. Chris closed his talk by reminding the audience that at another lattice conference held in Italy, namely that of 1989 (i.e. when I was just a teenager), Ken Wilson had predicted that it would take 30 years until precise results could be attained from lattice QCD, and that given that we still have nine years we are well on our way.

The next plenary talk was given by Jochen Heitger, who spoke about heavy flavours on the lattice. Flavour physics is an important ingredient in the search for new physics, because essentially all extensions to the Standard Model have some kind of flavour structure that could be used to find them from their contributions to flavour processes. On the lattice, "gold-plated" processes with no or one hadron in the final state and a well-controlled chiral behaviour play a crucial role because they can be treated accurately. Still, treating heavy quarks on the lattice is difficult, because on needs to maintain a multiscale hierarchy of 1/L << m_π << m_Q << 1/a. A variety of methods are currently in use, and Jochen nicely summarised results from most of them, including, but not limited to, the current-current correlators used by HPQCD, ETMC's interpolation of ratios between the static limit and dynamical masses, and the Fermilab approach, paying special attention to the programme of non-perturbative HQET pursued by the ALPHA collaboration.

The second plenary session started with a talk by Mike Peardon about improved design of hadron creation operators. The method in question is the "distillation" method that has been talked about a lot for about a year now. The basic insight at its root is that we generally use smeared operators to improve the signal-to-noise ratio, and that smearing tends to wipe out contributions from high-frequency modes of the Laplacian. If one then defines a novel smearing operator by projecting on the lowest few modes of the (spatial) Laplacian, this operator can be used to re-express the large traces appearing in correlation functions with smaller traces over the space spanned by the low-modes. If the smearing or "distillation" operator is D(t)=V(t)V(t)⁺, one defines the "perambulator" τ(t,t')=V(t)⁺M^-1(t,t')V(t') that takes the place of the propagator, and reduced operators Φ(t)=V(t)⁺ΓV(t), in terms of which to write the small traces. Insertions needed for three-point functions can be treated similarly by defining a generalised perambulator. Unfortunately, this method as it stands has a serious problem in that it scales very badly with the spatial volume -- the number of low-modes needed for a given accuracy scales with the volume, and so the method scales at least like the volume squared. However, this problem can be solved by using a stochastic estimator that is defined in the low-mode space, and the resulting stochastic method appears to perform much better than the usual "dilution" method.

The last speaker of the morning was Michele Pepe with a talk on string effects in Yang-Mills theory. The subject of the talk was the measurement of the width of the effective string and the observation of the decay of unstable k-strings in SU(2) gauge theory. By using a multilevel simulation technique proposed by Lüscher and Weisz, Pepe and collaborators have been able to perform these very challenging measurements. The results for the string width agree with theoretical expectations from the Nambu-Goto action, and the expected pattern of k-string decays (1 --> 0, 3/2 --> 1/2, and 2 --> 1 --> 0) could be nicely seen in the plots.

The plenary session was closed by the announcement that LATTICE 2011 will be held from 10-16th July 2011 at the Squaw Valley Resort in Lake Tahoe, California, USA.

In the afternoon there were again parallel sessions.

Lattice 2010, Day Four

2010-06-18T11:06:00.000+01:00

Today's first plenary session was started by Kazuyuki Kanaya with a talk on finite-temperature QCD. Many groups are looking for the transition temperature between the confined and deconfined phases, but since in the neighbourhood of the physical point, the transition is most likely a crossover, the value of the "critical" temperature found may be dependent on the observable studied. There was further some disagreement even between different studies using the same observables, but those discrepancies seem to have gone mostly away.

Next was Luigi Del Debbio speaking about the conformal window on the lattice. The motivation for those kinds of studies is the hope that the physics of electroweak symmetry breaking by originate not from a fundamental scalar Higgs, but from a fermionic condensate similar to the chiral condensate in QCD arising from a gauge theory ("technicolor") living at higher energy scales, perhaps around 1 TeV. To make these kinds of models viable, the coupling needs to run very slowly. One is then motivated to look for gauge theories having an infrared fixed point. Lattice simulations can help studying the question which combinations of N_c, the number of colours, and N_f, the number of fermion flavours, actually exhibit such behaviour. The Schrödinger functional can be used to study such questions, but while there are a number of results, no very clear picture appears to have emerged yet.

The second plenary session of the morning was opened with a talk on finite-density QCD by Sourendu Gupta. QCD at finite density, i.e. finite chemical potential, is plagued by a sign problem because the fermionic determinant can no longer be real in general. A number of ways around this problem have been proposed. The most straightforward is reweighting, the most ambitious a reformulation of the theory that manages to eliminate the sign problem entirely. On the latter front, there has been progress in that the 3D XY model, which also has a sign problem, has been successfully reformulated in different variables in which it does no longer suffer from its sign problem; whether something similar might be possible for QCD remains to be seen. Other approaches try to exploit analyticity to evade the sign problem, either by Taylor-expanding around zero chemical potential and measuring the Taylor coefficients as susceptibilities at zero chemical potential, or by simulating at purely imaginary chemical potential (where there is no sign problem) and extrapolating to real chemical potential. In this way, various determinations of the critical point of QCD have been performed, which agree more or less with each other. All of them lie in a region through which the freeze-out curve of heavy-ion experiments is expected to pass, so the question of the location of the critical point may become accessible experimentally.
The last plenary talk of the morning was Takeshi Yamazaki talking on a determination of the binding energy of helium nuclei in quenched QCD. The effort involved is considerable (there are more than 1000 different contractions for ⁴He, and the lattices considered have to be very large to be able to accommodate a helium nucleus and to distinguish between true bound states and attractive scattering states), even though the simulations were quenched and the valence quarks used corresponded to a pion mass of about 800 MeV. The study found that helium nuclei are indeed bound.

In the afternoon there were parallel sessions.

Lattice 2010, Days Two and Three

2010-06-17T09:01:00.000+01:00

Yesterday was an all-parallels day, so there are no plenary talks to summarise. In the evening there was the poster session.

The internet connection at the resort does not really have the capacity to deal with 360 computational physicist all reading their email, checking on their running computer jobs, browsing the hep-lat arXiv or writing their blog at the same time; this may lead to late updates from me, so please be patient.

Today's first plenary session was the traditional non-lattice plenary. The first talk was by Eytan Domany, who spoke about the challenges posed to computational science by the task of understanding the human genome. A large part of his talk was an introduction to the biological concepts involved, such as DNA, chromosomes, genes, RNA, transcription, transcription factors, ribosomes, gene expression, exons, introns, "junk" DNA, regulation networks and epigenetics. These days, it is possible to analyse the expression of thousands of genes in a sample by means of a single chip, and the data obtained by performing this kind of analysis on large numbers of samples (e.g. from different kinds of cells or from different patients) can be seen as an expression matrix with rows for genes and columns for samples. The difficult task is then to use this kind of large data matrix to infer regulation networks or connections between gene expression and phenotypes. Apparently, there are physicists working in this area together with the biologists, bringing in their computational expertise.

The second plenary talk was an LHC status summary given by Slawek Tkaczyk. The history of the LHC is of course well known to readers of this blog; so far, the first data are being analysed to "rediscover" the Standard Model with the aim of discovering new physics in the not too distant future, but there was no evidence of e.g. the Higgs or SUSY shown (yet?).

The second plenary session was devoted to non-QCD lattice simulations. The first talk was Renate Loll speaking on Lattice Quantum Gravity, specifically on causal dynamical triangulations. This approach to Quantum Gravity starts from the path integral for the Einstein-Hilbert action of General Relativity and regularises it by replacing continuous spacetime with a discrete triangulation. The discrete spacetime is then a simplicial complex satisfying certain additional requirements, and the Wick-rotated path integral can be treated using Monte Carlo techniques. In one phase of the (three-parameter) theory, the macroscopic structure of the resulting spacetime has been found to agree with de Sitter-space. Another surprising and interesting result of this approach has been that the spectral dimension associated with the diffusion of particles on the discrete spacetime is continuously going from around 2 at short (Plackian) to 4 at large distances.

Next was a talk on exact lattice SUSY by Simon Catterall. Normally, a lattice regularisation completely ruins supersymmetry, but theorists have found a way to formulate certain classes of supersymmetric theories (including N=4 Super-Yang-Mills) on a special kind of lattice, giving a local, gauge-invariant action with a doubler-free fermion formulation. This may offer a chance to study quantum gravity by simulations of lattice SUSY via the AdS/CFT correspondence.

In the afternoon there were excursions. I had signed up to the only excursion for which places were still available, which was a tour of a Sardinian winery with a wine tasting. The tour was not too interesting, as everything was very technologically modern, and as somebody said, we can go and look at the LHC if we want to see modern technology. The wines tasted were very nice, though.

Lattice 2010, Day One

2010-06-14T20:39:00.001+01:00

Hello from the Atahotel Tanka Village Resort in Villasimius, Sardinia, Italy, where I am at the Lattice 2010 conference.

The conference started this morning with a talk by Martin Lüscher about "Topology, the Wilson flow and the HMC algorithm". It is by now well known in the lattice community that Monte Carlo simulations of lattice QCD suffer from a severy problem with long autocorrelations of the topological charge of the gauge field. This problem affects the HMC algorithm and its variants that are used in lattice simulations with dynamical fermions just as well as the simple link updating schemes (Metropolis, heat bath) that can be used for pure gauge or quenched calculations. The autocorrelation time of the topological charge grows roughly like the fifth power of the inverse lattice spacing a as a is taken to zero. This is a real problem because it indicates the presence in the system being simulated of modes that are updates only very slowly, and as a consequence the statistical errors of observables measured from Monte Carlo simulations may be seriously underestimated, because the contribution to the error coming from the long tails of the autocorrelation function that stem from those modes are not properly taken into account. Martin Lüscher then introduced the Wilson flow, which is an evolution in field space generated by the Wilson plaquette action, and which can in some sense be seen as consisting of a sequence of infinitesimal stout link smearings. For the case of an abelian gauge theory, the flow equation can be solved exactly via the heat kernel, and it can be shown that it gives renormalised smooth solutions. For QCD, the same can be seen to be true numerically. Defining a transformed field V(U) by running with the Wilson flow for a specified time t₀, it can then be shown that the path integral over U is the same as the path integral over V(U) with an additional term in the action that comes from the Jacobian of the transformation and is proportional to g₀/a times the integral of the Wilson plaquette action along the flow trajectory. As a goes to zero, the latter term will act to suppress large value of the plaquette. An old theorem of Lüscher shows that the submanifold of field space with a plaquette values less than 0.067 divides into topological sectors, and hence the probability to be "between" topological sectors decays in line with the suppression of large plaquettes by the g₀/a term. This explains the problem seen, but also offers hope for a solution, since one might now try to develop algorithms that make progress by making large changes to the smooth fields V.
This was followed by two review talks. The first was a review of the state of the art in hadron spectroscopy and light pseudoscalar decay constants by Christian Hölbling emphasizing the reduction of systematic errors achieved by decreasing lattice spacings and pion masses and increasing simulation volumes.

The second review talk of the morning was given by Constantia Alexandrou, who reviewed hadron structure and form factor calculations from the lattice, drawing attention to the many remaining uncertainties in this important area, where in particular the axial charge g_A of the nucleon is consistently measured to be significantly lower on the lattice than in nature.

The last plenary speaker of the day was Gregorio Herdoiza, who spoke about the progress being made towards 2+1+1 flavour simulations. The collaborations currently pursuing the ambitious goal of including a fully dynamic charm quark in their simulations are ETMC and MILC. MILC is using the Highly Improved Staggered Quark (HISQ) action to reduce discretisation errors, whereas ETMC is relying on a variant of twisted mass fermions with an explicit breaking of the mass degeneracy for the strange/charm doublet. In the former case, the effects of reduced lattice artifacts are clearly seen, while in the latter case the O(a²) mass splitting between the neutral and charged pion increases with the number of flavours. In either case, a significant effort is necessary to tune the strange and charm quark masses to their physical values, but the effort is definitely well-spent if it leads to N_f=2+1+1 predictions from lattice QCD that include all effects of an active charm quark.

In the afternoon there were parallel talks. Two that I'd like to highlight were the talk of Bastian Knipschild from Mainz, who presented an efficient method to strongly reduce the systematic error on nucleon form factors coming from excited state contributions, and David Adam's talk in which he presented a generalisation of the overlap operator to staggered fermions that gives a chiral two-flavour theory.

Another chink in the armor of the Standard Model?

2010-05-18T14:54:00.000+01:00

Via Resonaances: The D0 collaboration has a new paper on the arXiv in which they present their observations of a like-sign muon charge asymmetryin B meson decays.

Neutral B mesons can decay into an antimuon, a mu neutrino and other stuff (B⁰ --> μ⁺ν_μ X_c) via the weak interaction \bar{b} --> \bar{c} W⁺, and neutral anti-B mesons can accordingly decay into an muon, a mu antineutrino and other stuff. However, neutral B mesons can oscillate into their antiparticles and back, so that if a B-Bbar pair is created in a collision, and one particle of the pair decays into a muon-neutrino pair while in its original state whereas the other decays into a muon-neutrino pair while turned into the antiparticle of its original state, both of them will decay into muons, or both into antimuons -- a like-sign muon decay.

If CP was an exact symmetry of nature, the rates for the oscillation and decays would be equal between B and anti-B mesons, but since it is not, CP violation leads to a difference in the rate at which the initial B-Bbar pair decays into positive and negative like-sign muon pairs -- a charge asymmetry. The Standard Model predicts a very small such charge asymmetry stemming from the complex phase in theCKM matrix.

What the D0 collaboration have done is to measure the charge asymmetry, carefully subtracting all (hopefully) sources of background, and obtained a result thatis about two orders of magnitude larger than the Standard Model prediction! Of course the experimental result has statistical and systematic errors, and thus the relevant measure of deviation from the Standard Model is only about 3σ ... still, this is another chink in the armor of the Standard Model.

What I find interesting is that all of the evidence of flavour physics beyond the Standard Model comes from particles containing a strange (rather than an up or down) quark besides a heavy flavour. The contribution to the charge asymmetry from B⁰_d decays is well constrained by other experiments, so most of the D0 result would appear to be coming from the B⁰_s system. I'm not a BSM phenomenologist, but I could imagine this to be relevant input for an understanding of possible BSM physics.

The Standard Model predictions rely on hadronic quantities such as decay constants, form factors and mixing parameters of the B meson, which must be determined nonperturbatively in lattice QCD. Better accuracy here could have real impact on the most stringent tests of the Standard Model that we have so far, and this is an area where significant progress is being made.

Bloggy stuff

2010-05-10T17:53:00.000+01:00

This will be an unusaully bloggy post for this blog, consisting as it does of two unrelated remarks one of which is not terribly relevant to anything.

Firstly (and irrelevantly), I noticed that Google are now classifying the blogs on blogspot by some sort of content-matching algorithm -- if you click on "next blog" in the title bar, you are most likely going to see another physics blog, or at least a blog by some physicist; if that physicist happens to blog about cooking, the next blog after that might be another culinary blog, or if he happens to be based in Texas, it might be a blog about a trip to Texas, and so forth. That's a neat trick that makes the "next blog" feature actually at least somewhat useful.

Secondly, when people talk about how the Euro is loosing against the Dollar, which has to mean the end of the EU or even of civilisation as we know it, I wonder how short their memory spans are. Here's a little memory aid -- click on "10y" on the chart and behold, the Euro is 42.4% higher against the Dollar than it was in early 2000, when it was actually worth less than $1 ...

ICHEP 2010 has a blog

2010-05-06T16:10:00.001+01:00

As my readers will know, this blog is most active during the conference season, when I blog from the annual lattice conference and possibly also from other meetings. I believe that conference blogging is both a service to those members of the physics community who for whatever reasons cannot personally attend the conference, and also to the wider public, who can get an insight into what scientists do and talk about at their meetings. It is thus a great pleasure for me to be able to announce that ICHEP 2010 will have an official conference blog, where bloggers from the high energy particle physics community will post on the conference and on current topics in high energy physics in general.

Excited states from the lattice, 1 of n

2010-01-29T18:12:00.000Z

This post is intended as the first in a series about techniques for the extraction of information on excited states of hadrons from lattice QCD calculations.

As a reminder, what we measure in lattice QCD are correlation functions C(t)=<O(t)O(0)> of composite fields O(t). From Feynman's functional integral formula, these are equal to the vacuum expectation value of the corresponding products of operators. Changing from the Heisenberg to the Schrödinger picture, it is straightforward to show that (for infinite temporal extent of the lattice) these have a spectral representation C(t)=Σ_n |ψ_n|² e^-E_nt, which in principle contains all information about the energies E_n and matrix elements ψ_n=<0|O|n> of all states in the theory.

The problem with getting that information from the theory is twofold: Firstly, we only measure the correlator on a finite number of timeslices; the task of inferring an infinite number of E_n and ψ_n from a finite number of C(t_k) is therefore infinitely ill-conditioned. Secondly, and more importantly, the measured correlation functions have associated statistical errors, and the number of timeslices on which the excited states' (n>1) contributions are larger than the error is often rather small. We are therefore faced with a difficult data analysis task.

The simplest idea of how to extract information beyond the ground state would be to just perform a multi-exponential fit with a given number of exponentials on the measured correlator. This approach fails spectacularly, because multi-exponential fits are rather ill-conditioned. One finds that changing the number of fitted exponentials will affect the best fit values found rather strongly, leading to a large and unknown systematic error; moreover, the fits will often tend to wander off into unphysical regions (negative energies, unreasonablely large matrix elements for excited states). This instability therefore needs addressing if one wishes to use a χ²-based method for the analysis of excited state masses.

The first such stabilisation that has been proposed and is widely used is known as Bayesian or constrained fitting. The idea here is to augment the χ² functional by prior information that one has about the spectrum of the theory (such as that energies are positive and less than the cutoff, but if one wishes also perhaps more stringent constraints coming e.g. from effective field theories or models). The reason one may do this is Bayes' theorem, which can be read as stating that the probability distribution of the parameters M given the data D is the product of the probability distribution of the data given the parameters times the probability distribution of the parameters absent any data: P(M|D)=P(D|M)/P(D) P(M); taking the logarithm of both sides and maximising of M, we then want to maximise log(P(D|M)) + log(P(M)). Now log(P(D|M)) is known to be proportional to -χ², so if P(M) was completely flat, we would end up minimizing χ². If we take P(M) to be Gaussian instead, we end up with an augmented χ² that contains an additional term Σ_n (M_n-I_n)²/σ_n² that forces the parameters M_n towards their initial guesses ("priors") I_n, and hence stabilises the fit -- in principle even with an infinite number of fit parameters. The widths σ_n are arbitrary in principle; fitted values M_n that noticeably depend on σ_n are determined by the priors and not the data and must be discarded. In practice the lowest few energies and matrix elements do not show a significant dependence on σ_n or on the number of higher states included in the fit, and may therefore be taken to have been determined by the data.

Bayesian fitting is a very powerful tool, but not everyone is happy with it. One objection is that adding any external information, even as a constraint, compromises the status of lattice QCD as a first-principles determination of physical quantities. Another common worry is the GIGO (garbage in-garbage out) principle with regards to the priors.

A way to address the former concern that has been proposed is the Sequential Empirical Bayes Method (SEBM). Here, one first performs an unstabilised single-exponential fit at large times t, where the ground state is known to dominate. Then one performs a constrained two-exponential fit over a larger range of t using the first fit result as a prior (with its error as the width). The result of this fit is then used as the prior in another three-exponential fit over an even larger time range, and so forth. (There is some variation as to the exact procedure followed, but this is the basic idea). In this way, all priors have been determined by the data themselves.

In the next post of this series we will look at a completely different approach to extracting excited state masses and matrix elements that does not rely on χ² at all.

New book on the lattice

2010-01-08T16:06:00.000Z

There was a time when the only textbooks on lattice QCD were Montvay&Münster and Creutz. Not so any more. Now the new textbook "Quantum Chromodynamicson the Lattice: An Introductory Presentation" by Christof Gattringer and Christian Lang (Lecture Notes in Physics 788, Springer) offers a thorough and accessible introduction for beginners.

Gattringer and Lang start from a derivation of the path integral in the context of Quantum Mechanics, and after deriving the naive discretisation of lattice fermions and the Wilson gauge action present first the lattice formulation of pure gauge theory, including the Haar measure and gauge fixing, with Wilson and Polyakov loops and the static quark potential as the observables of interest. Numerical simulation techniques for pure gauge theory are discussed along with the most important data analysis methods. Then fermions are introduced properly, starting from the properties of Grassmann variables and a discussion of the doubling problem and the Wilson fermion action, followed by chapters on hadron spectroscopy (including some discussion of methods for extracting excited states), chiral symmetry on the lattice (leading through the Nielsen-Ninomiya theorem and the Ginsparg-Wilson relation to the overlap operator) and methods for dynamical fermions. Chapters on Symanzik improvement and the renormalisation group, on lattice fermion formulations other than Wilson and overlap, on matrix elements and renormalisation, and on finite temperature and density round off the volume.

The book is intended as an introduction, and as such it is expected that more advanced topics are treated briefly or only hinted at. Whether the total omission of lattice perturbation theory (apart from a reference to the review by Capitani) is justified probably depends on your personal point of view -- the book clearly intends to treat lattice QCD as a fully non-perturbative theory in all respects. There are some other choices leading to the omission or near-omission of various topics of interest: The Wilson action is used both for gluons and quarks, although staggered, domain wall and twisted mass fermions, as well as NRQCD/HQET, are discussed in a separate chapter. The calculation of the spectrum takes the front seat, whereas the extraction of Standard Model parameters and other issues related to renormalisation are relegated to a more marginal position.

All of these choices are, however, very suitable for a book aimed at beginning lattice theorists who will benefit from the very detailed derivations of many important relations that are given with many intermediate steps shown explicitly. Very little prior knowledge of field theory is assumed, although some knowledge of continuum QFT is very helpful, and a good understanding of general particle physics is essential. The bibliographies at the end of each chapter are up to date on recent developments and should give readers an easy way into more advanced topics and into the research literature.

In short, this book is a gentle, but thorough introduction to the field for beginners which may also serve as a useful reference for more advanced students. It definitely represents a nice addition to your QCD bookshelf.

Old News

2009-12-09T16:50:00.000Z

Since I'm still too busy to write something real, I'll just dump a few links that I stumbled over during the last few days into a list:

The world's most energy-efficient computer stands in Germany -- QPACE (QCD Parallel Computing on the Cell) at Jülich leads the Green500 list

The Royal Society has brought the back issues of its Philosophical Transactions online -- now you can link to papers from 1665 via their DOI ...

The LHC is just being cranked up again, and the ALICE collaboration already have the first paper using LHC collision data accepted for publication

A propos unusual papers, this paper has probably most unusual one-line abstract ever seen on the arXiv

Even the best may need some rest

2009-11-26T10:12:00.000Z

Being named one of the 50 best physics blogs should probably serve as inspiration for writing a bit more frequently again, I suppose.

Still, you'll have to bear with the current blogging pause a little longer: I have spent each of the past five weekends in a different place, and have two papers to finish, a couple thousand lines of code to rewrite and a bunch of new data to analyse, all before the Christmas break. But you may look forward to a new series of posts on a group of related topics in the spirit of the "lattice quarks" series.

Silly science policies threaten progress

2009-10-29T10:44:00.000Z

Suppose you were a politician in charge of shaping your country's science policy. Let's also suppose you are actually interested in promoting the welfare of the nation and humanity at large (hopefully not all politicians are driven by sociopathic greed, and after all, we are talking about you here). Let's also suppose that you are not entirely stupid. What kind of science policy would you make?

Presumably, you would not come up with the kind of ultra-shortsighted policy that the UK has now come up with in determining to weight research proposals' (short-term) 'economic and social impact' by 25% in assessing their merits.

The point with fundamental research, however, is that one just simply cannot make any reliable statement about its likely impact on society. When Dirac postulated the existence of the positron on the basis of his equation, he didn't think of positron emission tomography revolutionising cancer diagnostics. When Einstein described stimulated emission of radiation, he certainly didn't have DVDs in mind. And while Peter Grünberg might have had some applications in mind when he made his discovery of giant magnetoresistance, he probably didn't imagine the iPod (otherwise he'd be very, very rich).

The only research that will fare well under such a short-sighted policy is industrial and quasi-industrial research that has a clear product (i.e. a product that can be readily imagined with current knowledge) in mind. Such applied research is important, sure. But fundamental research is far more important for the overall progress of the human race, because it creates the foundations upon which the applied research of the future is going to rest. Moreover, applied research generates revenue for industry, and therefore it behooves industry to fund it. The government's job in science is the support of fundamental research that will not easily get industry support -- corporations are notoriously short-sighted, rarely looking beyond next year's balance sheet. The government should have more foresight.

Nobel Laureates are leading the fight against this silly policy; you can hear from Chemistry Nobel Laureate Venki Ramakrishnan at nature.com. UK-based readers can sign a petition against the silliness at ucu.org.uk.

On Deadlines

2009-10-21T10:46:00.000+01:00

Human behaviour being what it is, conference proceedings tend to be written only when the deadline is practically elapsed. This can be seen on hep-lat every year. I remember reading somewhere (in PhysicsWorld, maybe?) that someone had studied the number of proceedings submitted per time period as a function of time until the deadline and had found a power-law behaviour. If that is correct (Google fails me for this, and the paper appears not to be on the arXiv), it would show again that even aggregates of (presumably free-willed) humans can be described well by statistical physics (thankfully, because if the free-will theorem is to be believed, spin-1 particles have free will if humans have). Does anyone know what the reference for the power-law behaviour of conference submissions was?

LATTICE 2009 reports open thread

2009-07-29T08:02:00.000+01:00

It appears that this blog is not accessible from within the Great Firewall of China (I suppose blogs are inherently subversive, even if they are about as innocuous a topic as Lattice QCD). There is thus little point in having lots of empty comment threads for participants to report their impressions from the Lattice 2009 conference in Beijing, and those threads have accordingly disappeared. This post exists for any comments that people may have about Lattice 2009; comments may also be sent to me by email, and will then be published here (anonymously, if desired).

LATTICE 2009 programme online

2009-07-02T13:16:00.000+01:00

The programme for the LATTICE 2009 conference in Beijing is up on the Web (here). This gives me the opportunity to remind my readers of the need for guest bloggers to cover the conference, since I will not attend this year.

我不去 LATTICE 2009 北京

2009-06-14T14:20:00.000+01:00

I hope I didn't maltreat the 汉字 above too badly in my ignorance -- in any case, what I'm trying to say is that I am not going to the Lattice conference this year. Yes, that means no conference blog from me.

"But how are we going to survive without the annual conference blog?" I hear a reader exclaim. To which I reply, without so much as batting an eyelid: "You will have to write it yourself." Okay, I admit that imaginary exchange is just silly, but the reality is that I'd still like to cover the Lattice meeting in Beijing, but obviously can't do the reporting myself. So I would like to encourage my readers to volunteer as guest bloggers and cover a session or two from their own point of view. Any conference reports (except for those of a libelous, ~~pornographic~~ or defamatory nature) submitted as comments or by email will be published, with or without attribution as desired by their respective authors.

To make this a more tempting offer I will add a prize for the most productive (by number of sessions covered, by words in case of equal numbers of sessions) guest blogger, who will win my Lattice 2008 Williamsburg baseball cap (autographed or unautographed at the winner's discretion).

Austria will not leave CERN

2009-05-19T11:43:00.000+01:00

Public pressure has been successful: Austrian chancellor Faymann has decided to veto the pull-out from CERN that his science minister had announced. Over 32,000 people have signed the online petition against the proposed withdrawal, and together with open letters by Nobel laureates and other public figures, this seems to have been enough to convince the Austrian government that leaving CERN would have been bad PR. I suppose that's another example of "yes, we can!"

Openness >> fraud

2009-05-17T19:52:00.000+01:00

In the most recent edition of PhysicsWorld, there are two articles that on the face of it have little to do with each other: one is about Jan Hendrik Schön, the physicist formerly famous for creating the first organic superconductor and the first single-molecule transistor, and now most famous for having simply made up all of those results out of thin air, the greatest kind of scientific fraud in physics. The other article, by Michael Nielsen, is about how the internet is transforming scientific communications, looking at which new means of scientific communication failed (such as Physics Comments and scientists contributing to Wikipedia -- although Scholarpedia is taking off quickly at the moment, probably because its signed and peer-reviewed authorship model is more in line with academic customs than Wikipedia's semi-anarchistic one) and which succeeded (the arXiv, of course) in making the dissemination of scientific results quicker and more transparent.

At first glance these two topics appear to have little to do with each other. At second glance, however, they are closely intertwined.

Schön's deception was only possible because the researchers who tried and failed to replicate his results didn't have access to his primary data. Once doubts had been raised over the appearance of two completely identical graphs supposedly representing two completely different sets of experimental data, Schön's primary data were subjected to close scrutiny and were found to be non-existent -- his labbooks had been destroyed, and his samples were damaged beyond recovery. This raises the question whether it would have been possible to even contemplate such a fraud in an environment where scientists are genuinely expected to hide nothing, and in particular to make their primary data publicly available after publication.

The more radically open schemes, where raw data are being made public before publication, are unlikely to take off largely because of concerns over the enormous plagiarism potential. But once results have been published and priority has thus been established by the original authors, there is no immediately obvious reason not to allow other researchers to perform their own analyses of the primary data, either to confirm (or possibly to refute) the original analysis, or to use their own methods to obtain results from the data that the original authors didn't (either because they weren't interested or because they didn't have the relevant analysis methods at their disposal). Some access controls are needed, of course, in order to ensure that the later researchers will duly acknowledge the use of the original group's datasets.

It is hard to see how a fraud like the Schön case could have occurred under a scheme like this; the groups who wasted years on trying to replicate his results to no avail would likely have realised the fraud if they had had access to Schön's lab books.

Just like with the arXiv (which after all started out as a specialised High Energy Physics preprint server and now has revolutionised publishing in most of physics and mathematics, plus assorted other areas), particle physicists are pushing ahead with schemes to open access to raw data, and lattice QCD is right at the forefront of the movement: since the most expensive step in unquenched simulations is the actual generation of the gauge configurations, using those just once for whatever analysis or analyses interests one specific group would be an irresponsible waste of computer resources, postdocs' lifetime and taxpayers' money.

It has therefore been common for a long time now for lattice theorists to form larger collaborations that pool their resources to generate their configurations and then perform different analyses on them (policies differ: some collaborations publish all of their papers as a collaboration, some break up into smaller groups for most analyses). But with the huge effort needed for unquenched simulations on large ultrafine lattices with very light quarks, even that becomes inefficient; in particular, groups that don't belong to any of the major collaborations would be left out in the quenched darkness. Therefore, it is becoming an increasingly common policy to make gauge configurations available to the larger lattice community after performing some initial analyses that the collaboration generating the ensemble is particularly keen on doing (generally, that includes the hadron spectrum, plus some other stuff).

Configurations have been available for a while at NERSC's Gauge Connection, and are now quickly beginning to be available on the International Lattice Data Grid (ILDG). This way the many CPU cycles that have been invested in generating these ensembles are put to even better use by enabling other groups to run their analyses on them.

Just like in the case of the arXiv, it may take a while for other disciplines to follow suit, but it appears likely that if and when more and more scientists choose to make their raw data public after publication (and those that don't therefore become increasingly subject to suspicion by their peers), a fraud case like that of Jan Hendrik Schön will become quite impossible at some point in the future.

Austria to leave CERN?

2009-05-12T17:00:00.000+01:00

Austria's Science Minister Johannes Hahn has announced that Austria is to pull out of CERN. The reason given is that the 20 M$/year that Austria contributes to CERN is too much in these economically difficult times. Given the amounts handed out to bankrupt banks these days, that seems like an odd argument against participating in the greatest international endeavour of our time, an endeavour whose spinoffs include among others the WWW, no less. Maybe the crackpots convinced Mr Hahn that the LHC was going to end the world? Or maybe the Austrian government just wants to reap the global benefits of international research without contributing anything to it? You can sign a petition against this piece of political silliness here.

arXiv goes Facebook

2009-04-09T09:13:00.000+01:00

Remember my slightly tongue-in-cheek question when we'd see a "My arXiv papers" application for Facebook? Well, it has happened, and apparently not on April Fool's Day. In fact, it is a part of the new author identifiers system introduced by the arXiv, which will help identify papers by the same author (since not everyone has the advantage of having a name that is unique within his field).

MAMI and beyond, Day Five

2009-04-04T18:45:00.001+01:00

Today's first talk was by Savely G. Karshenboim (Garching and St Petersburg) who spoke about hadron physics' impact on precision in atomic physics. Atomic physics is famously precise in its measurements, with relative precisions of order 10^-12 now being achieved for some quantities. The largest uncertainty in theoretical predictions there now comes from uncertainty about the effects of nuclear and proton structure.

The second speaker was Wolfgang Gradl (Mainz) with a talk on hadronic uncertainties in flavour physics. Flavour physics is about quark-level quantities (CKM matrix elements), but only hadronic decay and oscillation processes are experimentally accessible; thus one needs good control of QCD effects contained in form factors, decay constants and the B meson bag parameter. Lattice QCD is an important ingredient here, in particular when coupled with effective field theories such as HQET.

This was followed by discussion sessions about the prospects for MAMI, about the prospects for an electron-nucleon collider, and about the impact of hadronic physics on high-energy physics. The good news for lattice theorists is that there is a high demand for precise lattice predictions by experimentalists. The not so good news is that most of that demand is in areas where the lattice is not in a position to make accurate predictions in the near future, such as resonances, hadronic scattering lengths or hadronic light-by-light scattering amplitudes.

MAMI and beyond, Day Four

2009-04-02T18:58:00.001+01:00

Today's first talk was Fabio Ambrosino (INFN Napoli) speaking about flavour physics at the 1 GeV scale. Of course, flavour physics here does not mean charm or B-physics -- the topic was instead the accurate determination of |V_ud| and |V_us| from nuclear transitions and Kaon decays. The very accurate results obtained there confirm the unitarity of the first row of the CKM matrix to great accuracy, as well as confirming universality (via a comparison of G_μ and G_F).

The next talk was by Christoph Hanhart (Forschungszentrum Jülich), who spoke about QCD exotics such as hybrids, glueballs, tetraquarks and hadronic molecules. Here I learned what the physical difference between a tetraquark and a mesonic molecule (who after all both consist of two quarks and two antiquarks) is: since hadrons (as opposed to quarks) can go on-shell, the S-matrix elements for a hadronic molecule (but not a tetraquark state) would contain non-analyticities.

The remainder of the day had talks mostly about hypernuclear physics (hypernuclei are nuclei with a nucleon replaced by a strange baryon), which I feel unable to summarise (I only remember that hypernuclei are smaller and more tightly bound than normal nuclei), and accelerator physics, which I skipped in order to look after my email and a couple of papers that are in the final pre-arXiv stages of the pipeline.

MAMI and beyond, Day Three

2009-04-02T18:57:00.003+01:00

Today's morning session was filled with experimental talks making the case for an electron-nucleon collider to study the structure of the nucleon.

The short afternoon session had a talk by Akaki Rusetsky (Bonn) about the determination of resonance properties from finite-volume spectroscopy using a combination of Lüscher's formula and heavy-baryon chiral perturbation theory applied to lattice simulations near (or ideally at) the physical quark masses.

After that there was an excursion to Kloster Eberbach, a nearby former Cistercian monastery, where a guided tour was combined with a wine tasting. After that, the conference dinner took place in a castle hotel on the Rhine.

MAMI and beyond, Day Two

2009-04-02T18:57:00.001+01:00

Hello again from Mainz, where I am at the conference "MAMI and beyond".

Today's first talk was by Barry Holstein (UMass Amherst), who spoke on "Hadronic physics and MAMI: past an future". The hadronic physics was cast mainly in the language of Chiral Perturbation Theory and its extensions. An interesting detail was the magnetic polarisability of the nucleon, which suggest that the nucleon in 10,000 times "stiffer" electromagnetically than a typical atom; this is in spite of the fact that the ability of the nucleon to transition to a Δ resonance ought to give it strongly paramagnetic properties from the quark spins; heutistically this is countered by the diamagnetism of the nucleon's pion cloud. Another feature that I found interesting was that the experimental determination of hadronic scattering lengths seems to be rather involved (possibilities mentioned involved the decay of pionium, or an analysis of the cusp structure in the energy dependence of K->3π or η->3π decays), and that the best way to determine them from theory is apparently from the lattice via Lüscher's formula for the volume-dependence of two-particle state energies.

The next speaker was Rory Miskimen (also UMass Amherst) talking about the measurement of nucleon polarisabilities in real and virtual Compton scattering. Real Compton scattering is, well, Compton scattering, virtual Compton scattering is the production of a photon in the scattering of an charged particle by a proton: γ^*p -> pγ. Apparently the results from MAMI lie on a different curve from those from other experiments at other energies, which might suggest that there is something interesting happening around energies of Q²=0.3 GeV².

The next two talks were by Bernard Pire (CPHT/Polytechnique) and Diego Bettoni (INFN Ferrara), who both talked about timelike processes. Due to my limited understanding of the relevant physics, I feel unable to give a summary of those talks, except that apparently it is quite difficult to disentangle the different form factors experimentally.

After that Fred Jegerlehner (Katowice and DESY Zeuthen) spoke about the running of the fine structure constant α. The running of α, which at zero energy is known to astounding precision, is of particular interest around the muon mass (where it enters the determination of the muon anomalous magnetic moment) and around the Z boson mass. The difficult part is to determine the contributions to the running of α coming from hadronic loops, the uncertainty about which causes a loss of five significant figures when evolving α from 0 to M_Z. Using a method based on the Adler function (essentially a derivative of the self-energy with respect to the momentum squared), it should be possible to get a much more precise running of α by improving the understanding of low-energy hadronic contributions. Since most of the information needed in this approach would come from the Euclidean momentum region, the lattice might be able to help here.

After the lunch break, I skipped a couple of experimental talks to go over to the IWHSS workshop held next door and listen to a talk by Chris Michael about hadronic physics on the lattice. Chris presented approaches that can enable the determination of the nature of resonances and even the description of ρ -> ππ decays on the lattice.

After the coffee break, the lattice session of the MAMI conference took place: Meinulf Göckeler gave a summary of recent work towards the determination of generalised parton distributions on the lattice; Dru Renner at DESY Zeuthen works on this kind of thing, so I have heard about it a few times; it seems very hard each time I hear it, but I suppose saying "let's wait a few more years before starting on something like this" is not really an option.

Mike Peardon spoke about hadron spectroscopy on the lattice, giving a great introduction to lattice spectroscopy for the non-latticists in the audience. The highlight for lattice theorists was his mention of a new method that might replace noisy estimators for all-to-all propagators: a redefinition of quark smearing as a projection on the subspace spanned by the low modes of the Laplacian on a timeslice, enabling one to then exactly calculate all elements of the quark propagator out of this (relevant) subspace. The results shown looked rather promising, and the cost for diagonalising the Laplacian on a timeslice is of course much lower than that for diagonalising the Dirac operator as needed for the Dublin method of all-to-all correlators with low-modes.

Andreas Jüttner gave a talk about ongoing work to study mesonic form factors and (g-2). Using twisted boundary conditions to induce a momentum, he obtained very nice pion and K->π form factors. The (g-2) work is still in progress, but looks promising.

Silvia Necco gave an introduction to the links between Lattice QCD and Chiral Perturbation Theory, covering the extraction of SU(2) and SU(3) low-energy constants from N_f=2 and N_f=2+1 lattice simulations, and of the leading-order couplings Σ and F from simulations in the ε-regime.

Finally, Johann Kühn (Karlsruhe) spoke about precision physics in e⁺e^- interactions, where the perturbative determination of the hadron-to-muon ratio R(s) has made it possible to precisely determine α_s, m_c and m_b from experimental data (and the former two also from lattice simulations via the moments of current-current correlators).

In the evening, there was a social event: A string quartet played for us at the university's faculty of music in Mainz. The program was Mozart (Divertimento No. 1, KV 135), Schubert (String quartet No. 13 "Rosamunde) and Shostakovich (String quartet No 8 op. 110), the first two pieces quite pleasant, the last rather harrowing.

MAMI and beyond, Day One

2009-04-02T18:55:00.000+01:00

Hello from Schloss Waldthausen near Mainz, where I am attending the conference "MAMI and beyond".

The meeting started this morning with welcome speeches by the VP for research of Mainz University, the VP of physics of the German Research Foundation, and the acting director of the Mainz nuclear physics institute. This was followed by the first talk, given by Ulf G. Meissner (Bonn University), who spoke about "Hadron physics at the 1 GeV scale and its impact". He paid particular attention to isospin violating effects, which can come from both QED and QCD sources, since the up and down quarks differ in both mass and charge. MAMI experiments could measure isospin violating effects in πN scattering, η -> 3π and η' -> ηππ decays, and in Kaon photoproduction on the proton, for all of which there are higher-order predictions from some versions of chiral perturbation theory. Beyond MAMI, interesting isospin violating effects are the mass splittings of heavy baryon mutliplets, where the mass of the cdd Σ_c⁰ is greater that that of the cud Σ_c⁺, even though m_d>m_u, but the ordering of the Σ_s baryons is the normal one, an effect that may be explained by the presence of a new operator appearing in the O(p²) χPT effective Lagrangian for heavy quarks, which has a different sign for c and b quarks because of their different electrical charge.

After the coffee break, Jens Erler (UNAM, Mexico) talked about "Low-energy tests of the Standard Model and beyond". Low-energy probes, such as leptonic decays, flavour-changing neutral current contributions to Kaon decays, first row CKM matrix unitarity tests, tests of CP violation, search of nucleon and lepton electric dipole moments, atomic parity violation, (g-2) measurements and many more from particle, nuclear and atomic physics, can surprisingly probe very high energies by placing extremely stringent limits on various kinds of beyond-the-Standard-Model physics, excluding in many cases BSM contributions from scales below a few 100 TeV or so. This makes them a very useful complement to high-energy collider experiments that search for BSM particles and processes in a more direct manner.

The next talk was an overview of form factors given by Carl Carlson (College of William and Mary). The point that stuck to my mind most prominently was that measurements of hydrogen hyperfine splitting when combined with proton structure measurements and calculations are accurately predicted to more than 1 p.p.m. and show now evidence of new physics to such accuracy.

After the lunch break, Constantia Alexandrou (Cyprus University) gave an overview of nucleon structure on the lattice, concentrating on N_f=2 studies using dynamical twisted mass, Wilson clover or overlap fermions. Special attention was drawn to the fact that it is now becoming possible to simulate at the physical pion mass, and that the first such simulations have recently been done by the Wuppertal group.

This was followed by another experimental talk by Volker Burkert (Jefferson Lab). What I took home from this talk was that there is experimental support for the notion that the Roper resonance is a radial excitation of the nucleon, and that there is such a thing as femtotomography, where an image of the charge distribution inside a hadron is created from the Fourier transform of its structure functions.

After this, Mauro Anselmino (INFN Torino) spoke about the spin structure of the nucleon from a mostly theoretical point of view, followed after the coffee break by Klaus Rith (Erlangen-Nürnberg University) speaking about the same from a mostly experimental point of view. The "spin crisis" caused by the discovery that the quark spins only contribute about 33% of the nucleon's spin still appears somewhat unresolved. The gluons appear to contribute very little, and the contributions of the angular momenta of up and down quarks, which must make up the remainder, interestingly have opposite sign. A lot of research still seems to be ongoing in this very complex area, and I honestly don't understand enough of it to be able to give a decent summary of the enormous amount of information contained in these talks.

The same is true (and to an even larger extent) of the experimental talks that followed, and to which I didn't pay the necessary attention in any case, since I had to deal with several pressing matters by email.

The World Wide Polymath

2009-03-13T13:17:00.000Z

20 years ago today, the World Wide Web was first invented by (now Sir) Tim Berners-Lee. The most successful "spin-off" to come from particle physics in recent times, the WWW has transformed one after another the way we inform ourselves, do our shopping, and (with social networking sites becoming the great wave) keep in contact with (or even find) friends.

The reason the web was invented was of course the large-scale collaborative nature of experimental particle physics research, which created a need to connect the various bits and pieces of information that were created by different collaborators into a coherent whole. Other areas of science have become more and more collaborative as well, and researchers in those areas have also profited from the existence of the web, as have countless groups of people inside and outside academia who are connected by some common interest, but separated by geography.

One discipline which so far has withstood the tendency towards large-scale collaboration has been pure mathematics, which is still dominated by single-author papers and sometimes two-person teams. Fields Medalist Tim Gowers has now set out to use the web in order to transform the way mathematical research is (or at least may be) done: on his blog, he has started a series of discussion threads aiming for a new proof of the density Hales-Jewett theorem for k=3 (which apparently is of genuine interest to combinatorialists) by a large-scale collaboration of mathematicians connecting their ideas through the web. This project appears to have succeeded. Whether this is the beginning of a new era of massively-collaborative mathematics remains to be seen, but it is fair to say that the World Wide Web's power to transform many and diverse areas of human endeavor by bringing people and information together in new ways still hasn't found its limits.

Hey, that guest post should be here!

2009-03-06T17:19:00.000Z

There's a guest post at Shores of the Dirac Sea about exact lattice supersymmetry. It's good to see that physicists in other areas are interested in what lattice field theory has to say.

Brief sign of life

2009-03-06T09:09:00.000Z

There hasn't been much going on on this blog recently since I've been busy with other stuff and am still busy with still more other stuff, which I'm afraid has to take precedence over blogging. Guest posts, however, would be more than welcome!

Anti-Crackpot Blog

2008-09-17T09:10:00.001+01:00

There is a new physics blog devoted exclusively to debunking the claims of crackpots (such as that the LHC will create black ~~dragons~~holes that will devour the earth). I'm glad somebody is doing this.

LHC online

2008-09-10T09:39:00.000+01:00

As you probably already know, the LHC went online today with the first beam circulating. CERN has video (which unfortunately didn't work for me due to lack of the right plugin), and there are several liveblogs at Resonaances, Superweak, Cosmic Variance and the US/LHC blog.

Lattice 2008, Day six

2008-07-24T12:57:00.001+01:00

Saturday was the last day of the conference. The first plenary, chaired by Andreas Kronfeld, was devoted to the quest for new physics. Luca Silvestrini spoke about the observed discrepancies between lattice and experiment: A 4.4σ difference in the CP asymmetries in B to K π decays, a 3.8σ difference in f_{D_s}, and a 3σ difference in the phase of the B_s to J/ψ φ decay. Although the LHC is expected to give access to the low-lying part of the particle spectrum of the expected new physics, a Super-B factory will be needed to map the new physics out in detail (the MSSM has 160 parameters). Lattice QCD determinations of quantities of interest at the <1% accuracy level will be needed for these purposes.

Then George Fleming spoke about strong interactions beyond the Standard Model, where technicolor is making a comeback, since only some QCD-like versions of it have been ruled out. The interest in this area centers on "walking" theories with a very slowly running coupling. For SU(3), it is believed that there is a "conformal window" of N_f, where the coupling runs to an IR fixed point in the infrared. Simulations using unrooted staggered fermions to simulate N_f=4,8,12,16 indicate that this window lies somewhere around N_f=12.

The last plenary had Michael Teper speaking about Large-N QCD using old-fashioned OHP slides. N=&infinity; QCD is a theoretical laboratory for ideas about QCD, both because it turns out that as far as the N-dependence of observables is concerned, N=3 is close to N=&infinity;, and because at N=&infinity;, quenched QCD is full QCD, because fermion loops are infinitely suppressed by their colour factors; also, resonances become infinitely narrow as N goes towards infinity, allowing accurate measurements of e.g. the ρ mass, which turn out to be quite close to the real world at N=&infinity;.

This was followed by Hermann Krebs's talk about nuclear effective theories on the lattice. The lattice as a regulator is of course not unique to gauge theories, and nuclear theorists are now performing simulations of effective theories of pions and nucleons to determine the properties of light nuclei and nuclear matter from first principles. The low-energy constants can be either fitted to experiment by giving up an a number of predictions, or can be taken from lattice QCD (once they are determined accurately enough) for a truly first-principles treatment of nuclear physics.

After the end of the session, there was an announcement of the Les Houches Summer School on Lattice QCD in 2009. Then Kostas Orginos thanked the support staff and volunteers, before handing over to the representative of the Lattice 2009 organising committee, who thanked Kostas and his team. Everybody got their well-deserved applause, and then the lattice community was invited to come to Beijing for the Lattice 2009 conference, which is to be held July 26-31, 2009. It was also announced that Lattice 2010 will be held at a yet-to-be-determined location in Europe. And then the conference was over, and everybody said their goodbyes before leaving.

Since my flight only left the next day, I took the opportunity to visit the "Colonial Williamsburg" open-air museum, which I liked a lot better than the Jamestown one, largely because the colonials/locals just went quietly about their business without too much show or spectacle, which I thought gave one a much better impression of what life in the American colonies might have been like.

My flight back went fine, but I didn't get to post the last two summaries earlier.

Lattice 2008, Day five

2008-07-24T12:56:00.001+01:00

Friday's first plenary started with a summary talk on heavy-flavour physics on the lattice given by Elvira Gamiz. The most striking point there is what is now being called the "f_{D_s} puzzle", i.e. the difference between lattice predictions (241(3) MeV [HPQCD N_f=2+1], 249(11) MeV [FNAL/MILC N_f=2+1], 244(4)(11) MeV [ETMC N_f=2 preliminary], 251(6)(?) [Alpha N_f=2 preliminary]) and experimental measurements (268(8)(4) MeV [CLEO-c, most recent]) of the D_s meson decay constant, for which new physics is being invoked as an explanation by many. Other topics were semileptonic decays of heavy mesons, which are quite hard to study on the lattice, and B-Bbar mixing parameters, where some also raise the possibility of new physics to explain discrepancies between theory and experiment that have recently arisen.

The next talk was Laurent Lellouch speaking about Kaon physics, and comparing the usefulness SU(2) and SU(3) chiral perturbation theory.

The second plenary session was started by Rob Pisarski speaking about heavy-ion collisions ar RHIC, where the study of the strongly interaction quark-gluon plasma (if it may be called that, since it does not really appear to be the state of matter formerly imagined under the name of quark-gluon plasma) requires methods from non-equilibrium thermodynamics and non-ideal hydrodynamics. One of the long-standing puzzles that appear to be experimental signatures of the QCD phase transition is the suppression of J/ψ final states and jets. Some explanation for these phenomena in terms of the "elliptic flow" of the quark-gluon plasma seems to have been found, but it appears to me that a fundamental undestanding of what is going on in these highly out-of-equilibrium situations involving strongly interacting matter is still a fair way off.

The next talk was related to this topic, as Harvey Meyer spoke about the extraction of hydrodynamical transport coefficients from the spectral functions of correlators of energy-momentum tensors, which requires some clever tricks to get the continuous spectral functions from the correlators measured only at a few discrete points.

After this, Yoshinobu Kuramashi gave a talk about PACS-CS's simulations of N_f=2+1 QCD at and near the physical pion mass. They were thus able to test the applicability of SU(2) and SU(3) chiral perturbation theory, and my interpretation of their results was that both might not be sufficiently well-behaved to be truly valid even at the physical point.

The last talk of the session was given by Tomoteru Yoshie, who gave an introduction and status update on the International Lattice Data Grid (ILDG), which now contains 183 ensembles with a total of 193,000 configurations using 41 Terabyte of storage space.

In the afternoon, there were again parallel sessions, including one in which Rainer Sommer spoke about our group's recent work on the Generalised Eigenvalue Problem for correlator matrices and how to use it in the most efficient manner to get ground and excited state masses and matrix elements, both in QCD and in effective theories such as HQET, and another on in which I talked about a preliminary analysis of D_s physics on the large and fine CLS lattices. Rainer's talk was certainly very well received, and since the potential criticisms of the work that I presented were easy to anticipate, I would say that my talk also went quite well.

Lattice 2008, Day four

2008-07-18T02:55:00.000+01:00

Today was the customary short day. There were no plenaries, only parallel sessions. I went to the sessions on Standard Model parameters and renormalisation, where Peter Lepage presented the HPQCD collaboration's new method and results for the heavy quark masses from moments of current-current correlators, and after the cofee break the session on weak decays and matrix elements where Paul Mackenzie presented the Fermilab group's new result for f_{D_s}, which is larger than, but in agreement with, the HPQCD result, but at the moment has larger error bars than the latter. Notable talks in these sessions were also given by Ian Allison on results from high-β simulations and by Ruth van de Water on extracting an accurate number for |V_ub| from QCD simulations by making use of variable transformations and complex fitting procedures.

After the end of the last session, we picked up our boxed lunches at the reception desk and climbed into the busses that took us to Jamestown settlement, where we got a tour of the museum, where the most remarkable exhibit were watercolours by John White, an artist who accompanied an early expedition to Virginia and depicted the flora, fauna and native population as they appeared to an English artist encountering the New World for the first time, while everything else was certainly informational and presented very nicely, but nothing unusual compared to the other historical museums. The reconstructed settlement was a bit too Disneylandish for my taste -- while the ships and buildings certainly gave a good idea of life in an early English colony in North America, the costumed show was more funny than informational, although I am sure the kids who were there were having a lot of fun, which is probably the main purpose of these kinds of reenactments. After that, the busses took us to the historical site of Jamestown, where we could see the ruins of the buildings and the rebuilt church and walk around in the heat until the busses took us back.

The banquet was a buffet dinner in one of the big rooms that can be divided to serve as three meeting rooms each. The menu is likely of no interest to readers, so I'll end here for today.

Lattice 2008, Day three

2008-07-17T03:35:00.000+01:00

Today's first plenary, chaired by Rainer Sommer, started off with the much-anticipated plenary talk about the state of dynamical fermion simulations given by Karl Jansen. Simulations with dynamical fermions are of course a hugely controversial subject in the community at the moment, with each group fiercely defending its particular fermion discretisation as the best, or even the only way to solve QCD (whatever that may mean). Karl managed to navigate this minefield (or to stay closer to the analogies he had chosen for his talk, this swamp of alligators) with great impartiality and fairness, pointing out the strengths and potential weaknesses of each formulation in an accurate and diplomatic manner; even regarding the great staggered controversy, he was very fair to both sides, pointing out that the concerns raised by Mike Creutz almost certainly don't affect the simulations being done now, given that they are being performed at fermion masses which are safe with regard to maintaining the order of the continuum (first) and chiral (second) limits, but that these concerns are also valid and worthy of further investigation in the context of theoretical studies in which the order of the limits is intentionally violated. A very nice picture shown was the scaling of the nucleon mass in different lattice fermion formulations, which looked like great evidence of universality; unfortunately, the same picture for the pion decay constant looked a lot worse, possibly because some formulations with Z_A not equal to one may have used incorrect value of Z_A. The progress made on reducing the cost of dynamical fermion simulations through algorithmic improvements (especially deflation) is also very impressive.

The next talk was Shoji Hashimoto speaking about simulations with dynamical overlap fermions. The overlap operator being the most expensive to simulate formulation of lattice fermions, the lattices being simulated are still pretty small, but it is nevertheless quite impressive that this has become possible at all.

After the coffee break, Ami Katz gave the non-QCD theory plenary talk, about AdS/QCD models. The AdS/CFT correspondence is maybe the most important (some might say the only) result coming out of string theory; as far as I understand it, it states that string theory in anti-de Sitter (AdS) space is dual in an apparently well-defined way to Large-N super-Yang-Mills theory on the boundary of AdS. A consequence of this appears to be that by introducing extended objects such as branes or black holes into AdS one can break the conformal symmetry or supersymmetry of the theory on the boundary, thus potentially being able to construct a supergravity theory that is dual to QCD. Since the AdS/CFT duality relates weak and strong coupling, this would allow to describe the low-energy spectrum of QCD by perturbative calculations in the dual theory. Some models that attempt to do this have achieved quite reasonable agreement with QCD spectra, but none of them are truly dual to QCD, so all of this amounts to model-building. I may well have misunderstood some things here, though, and corrections or better explanations in the comments are greatly appreciated.

The last plenary talk of the morning was Ken-Ichi Ishikawa talking about recent devlopments in the algorithms and machines field, such as the Cell processor, lattice QCD running on graphics cards, deflation and adaptive multi-grid methods for lattice QCD. His projections for the future seemed a little off, though: I am currently responsible for running a simulation which according to his final slides would require Petaflop computing.

After lunch, I attended the parallel session on hadron spectroscopy in which the different collaborations presented results on heavy-quark states. Christine Davies presented the HPQCD results on pseudoscalar decay constants, among which the result for f_{D_s} is remarkable for showing a 3σ deviation from the experimental value, which some believe is a possible sign of new physics.

After the afternoon coffee break, there was a public lecture by Rajan Gupta on the global energy problems facing the human species in the 21st century. The numbers are always very depressing, as is the political cloud of war, imperialism, famine and general oppression surrounding the fossile and nuclear energy resources of our planet. As a little contribution towards improving the level of discourse on energy issues he presented a project called OpenModel Global Obervatory that aims to create a Wikipedia-like database of the global energy infrastructure as a tool for policymakers and an instrument of educating the public. The fact remains that if 8 billion people all are to live the American lifestyle including its (ab)use of non-renewable fossile ressources, the next generation is probably going to be the last to be able to enjoy any kind of lifestyle above the level of "poor, nasty, brutish and short", if at all, so huge advances in energy efficiency and the use of renewable energies (photovoltaic, solar-thermal, wind, hydro, tidal) will be absolutely crucial -- economically viable fusion power would also be nice, but appears unlikely to appear soon enough to help us very much.

In order to cheer ourselves up after considering this, some colleagues and I had a beer with our dinner and discussed the most cheerful topics we could think of (which included cannibalism, assisted suicide and homelessness).

Lattice 2008, Day two

2008-07-16T03:20:00.000+01:00

Today was a long day, so this will be a short summary. Any typos and inaccuracies in content are to be blamed exclusively on the wine served during the poster session.

After another continental breakfast, the first plenary session of the daym chaired by John Negele, started with a talk by Marc Vanderhaegen about nucleon structure studies. This was this year's experimental talk, for which reason I find myself too ignorant to give a good summary of it; there were lots of plots of experimental results of observables such as the ratio of the electric to magnetic form factors of the proton (which appears to be quite different if measured by unpolarised or by polarised probes for reasons to do with two-photon exchanges), the generalised parton distributions of the nucleon, and the magnetic dipole moment of the Δ(1232) resonance (which is apparently very hard to measure, because the Δ decays strongly and hence is far too short-lived to measure its magnetic moment by the precession method that can be used for stable or quasi-stable particles).

Next was James Zanotti who gave an overview of the work that has been done on hadronic structure from the side of lattice QCD. Again there were lots of plots of the same quantities, this side from lattice simulations, but I have to freely admit that I am way too ignorant of hadronic structure studies to appreciate this work very well. A better summary of the progress in this area that might be given in the comments would be appreciated.

After the coffee break, the second plenary session of the morning continued with Martin Savage in the chair. The nuclear thread of the previous session was continued by Silas Beane speaking about Hadronic interactions and nuclear physics. This was probably the funniest talk I have ever heard at a lattice conference (it included inter alia a picture of a crying baby held by G.W. Bush, illustrating the exponential growth with time of noise in baryonic channels, and of a live rabbit being pulled from a top hat). Multi-hadron states are now being targetted by lattice simulations, but dealing with the noise will require petascale computing.

The final plenary talk of the day was by Colin Morningstar, who talked about studies of excited hadronic states. His talk concentrated almost exclusively on the very extensive work done in this area by his collaborators, who have indeed made some remarkable progress on this very difficult problem; however, there are also some other approaches to extracting information on excited states, which may well turn out to not be competitive with the variational method, but might still deserve a mention at least in this blog.

After lunch with some colleagues, taken at an Asian buffet place that was both better and much more reasonably priced than the university canteen, I attended parallel sessions. The more remarkable talks included Constantia Alexandrou presenting a new method to extract excited states, which seemed to work remarkably well given that it appeared to be largely a rather glorified form of uniform random search. Progress towards using the HISQ action for simulations with staggered quarks including dynamical charm was presented by Alexei Bazavov. Michael Clark spoke about adaptive multi-grid methods for QCD as potential competitors to deflation methods. A presentation an a new high-performance computing architecture was given by John Mucci, the CEO of SiCortex, the company producing it -- it sounded a bit like marketing, but if their computers really only use 200 W per 100 GFlops and can run with ordinary air-cooling that would be quite amazing.

Finally, the poster session closed the day. My poster on the determination of the O(N_fα_sa²) improvement coefficients for the Lüscher-Weisz action with dynamical HISQ fermions appeared to be received quite well by its intended audience. The food was gone quickly, and the wine not much more slowly. By an amusing coincidence there was a poster from another group about pretty much exactly the same work as I am doing with people at Zeuthen at the moment.

Lattice 2008, Day one

2008-07-15T00:14:00.001+01:00

Hello from the College of William and Mary in Williamsburg, Virginia, where I am at the Lattice 2008 conference.

After a continental breakfast that was provided by the conference in the central meeting room, and registration (where we got a very nice conference bag, probably better even than the excellent one from Tucson), the meeting started with the usual welcomes from the Vice-Provost of Research and the head of the physics department.

Then the first plenary session started with Frithjof Karsch in the chair. The first talk was by Carleton DeTar, who talked about developments in finite-temperature QCD on the lattice. A he pointed out, the N_τ=4 simulations that were still fairly standard in that area rather recently correspond to lattice spacings of about a=0.27 fm at T=180 MeV, so by today's standards they are unacceptably coarse. A point of contention in finite-temperature QCD is the nature of the phase transition; while it is generally agreed to be a crossover and not a real transition at the physical point, for massless N_f=2 QCD there are some who believe it to be first order, while most groups find it to be second order. The fact that the transition is a crossover at the physical point poses the problem of how to determine the critical temperature, since for a crossover there is no uniquely defined transition point. The observables used to study the critical temperature can be divided into confinement-type (such as Polyakov loops) and chiral-type (such as the chiral condensate). A chiral-type observable that has led to some uncertainty about the critical temperature is the chiral susceptibility, which can be understood as the integrated correlator of the chiral condensate. Since this needs to be renormalised, it picks up a mass dependence which makes it difficult to pin down its precise temperature-dependence, thus leading to systematic errors in the determination of the critical temperature from its peak. Other interesting points raised in this talk were the ongoing effort to try to extract information on the transport coefficients of the quark-gluon plasma from lattice simulations, and the observation that dimensional reduction seems to work surprisingly weel down to about T=1.5 T_c, which is really completely unexpected, since dimensional reduction is strictly a high-temperature effective theory.

After that, Shinji Ejiri spoke about lattice QCD at finite density, an area that is known to be very difficult since the fermionic determinant becomes complex if the chemical potential is non-zero, thus runining the probability interpretation of the path integral measure and making Monte Carlo simulations impossible without some groundbreaking new idea that has apparently not arrived yet.

The coffee break was followed by another plenary session, chaired by Richard Brower. The first speaker was Shailesh Chandrasekharan who spoke about the worldline approach to simulating lattice field theories as an alternative to cluster algorithms for scalar and fermionic models, with the possibility of extending it to gauge theories as a worldsheet approach.

The next talk was by Uwe-Jens Wiese who talked about lessons for QCD to be drawn from solid state physics. Various solid-state physics models, such as the Hubbard model on a hexagonal lattice, can be described in terms of effective theories strongly resembling chiral perturbation theory, which in term can be reduced to qunatum mechanical rotors, whose spectra are known analytically. A similar reduction can be performed for χPT, and the nucleon can be incorporated in that approach as a Dirac monopole contained inside the sphere on which the rotor degrees of freedom live.

The final talk of the morning was Andre Walker-Loud speaking about Heavy Baryon Chiral Perturbation Theory. The main message I took from this talk was that a totally unphysical straight line fit appears to describe the pion-mass dependence of the nucleon mass just as well as involved HBχPT calculations, which is somewhat disconcerting.

After lunch with some colleagues at the somewhat expensive university canteen, the afternoon saw me attending parallel sessions. The cookies in the coffee creak were very delicious and probably had way too many calories. I also finally finished my talk. That's it for today, stay tuned for more tomorrow.

Lattice 2008 programme online

2008-06-26T12:28:00.000+01:00

The plenary program and preliminary schedule for the Lattice 2008 conference are now online.

Update: Meanwhile, the parallel and poster programs are also available.

I'm still here (and now we can finally be sure the LHC won't change that)

2008-06-25T18:57:00.000+01:00

Yes, I have been ~~lazy~~busy doing research, which takes priority over blogging (after all the German taxpayer is giving me money to do research, not to blog, and I don't want to disappoint her too much). But I'm still here, and I fully intend to blog from the Lattice 2008 conference in Williamsburg.

Fortunately most of us will still be alive by then, and even by the time Lattice 2009 in Beijing comes around, since the LHC is unlikely to swallow us up into a black hole. Okay, that's not really news to anybody who knows any real physics at all, but still CERN felt the pressure from misinformed segments of the public was enough to warrant yet another safety report, which comes to the conclusion that

[t]here is no basis for any concerns about the consequences of new particles or forms of matter that could possibly be produced by the LHC.

This hardly surprising conclusion is based on the fact that the flux of cosmic rays at energies far exceeding the energy of the LHC is such that if stable black holes, strangelets, vacuum decay bubbles or other doomsdayons could be produced at the LHC, the solar system would have been completely converted into those doomsdayons a long time ago. Since the sun, the moon and, indeed, the earth still consist after billions of years of being bombarded by cosmic rays with energies many orders of magnitude above that of the LHC, the probability for creating doomsdayons must be very, very low. Or, as the LHC safety report nicely puts it,

the continued existence of the Earth and other astronomical bodies can be used to constrain or exclude speculations about possible new particles thatmight be produced by the LHC.

Not that that will stop the silly trial in Hawaii from going ahead, but since it appears to be scheduled to begin in 2009, the LHC will have been turned on by then, and the continued existence of the Earth will make the doomsdayonistas look pretty stupid on their day in court.

Trento

2008-05-18T19:48:00.000+01:00

So I've obviously been a really bad blogger recently, but I was quite busy. One of things I was doing was attending a workshop at the ECT* (not sure what's up with the star; I suppose ECTRNPARA was a little too long, so they used a shell wildcard) in Trento, Italy. The workshop was sort of a miniature version of the lattice conference, with representatives from all major collaborations talking about the state of the art in simulations with dynamical fermions. I briefly considered live-blogging it like I do with the lattice conferences, but in the end decided against it for various reasons. The ECT* is very nicely located in a historical villa a little outside of Trento itself; the meeting room is in the basement of a side building, though, so there is nothing to distract one from the talks. The workshop was very well organised, with hotels, meals and everything arranged in advance, so five stars to the organisers and ECT* staff for that.

Contentwise, the workshop brought few real surprises, but a lot of confirmation of the fact that dynamical fermion simulations are now pretty far advanced due to a combination of algorithmic advances and ever greater and faster parallel computers. To all but eliminate systematic errors, ultimately, one will need to simulate at small lattice spacings (0.04 fm, say), large volumes (5 fm, say) and at the physical light quark masses. At the moment, each major group is accomplishing at least one of these, with some approaching two out of the three. In three or four years at the latest, somebody will have an ensemble of configurations fulfilling all three. Given that lattice spacings this small, or quark masses anywhere in the vicinity of the physical point, were considered completely out of reach just three years ago, it is fair to say that the lattice has come a long way in a short time.

Some people will therefore sometime use phrases like "when we will have solved QCD", but great as that sounds one first needs to consider what solving QCD means. Even when we have predictions for the hadronic ground state mass spectrum with essentially zero systematic error, there will still be excited states, decay constants and widths, scattering lengths, form factors, multi-hadron states and potentials, and so forth coming from QCD, and many of these will likely require considerable effort in terms of new theoretical developments in order to make it viable to extract them from lattice simulations. So unless "solving QCD" means "computing the hadronic ground state mass spectrum", we won't solve it for a fair while to come. Which is good news, because otherwise I'd really have to start looking for a different job, and I actually like this one.

And of course then there is the often-mentioned possibility that the LHC might find evidence of technicolor or some other new strongly coupled physics at higher energies, putting lattice theorists at the cutting edge of the energy frontier. That sounds more like some kind of dream though.

I've also been doing other interesting things, but I'll save those for a different post. If everything goes as hoped for, there may also be an exciting guest post on this blog in the not too distant future.

The LHC is making waves

2008-03-28T12:18:00.000Z

The LHC is making waves on the web already before it is even being switched on. A rather unusual concept of what experimentalists really care about has been featured in this comic, but it is interesting to know that there are people who are genuinely worried that the LHC might "give helicopters cancer" or even swallow up the entire solar system. Peter Steinberg at the US LHC blog has more about them, including a link to this fine example of paranoia in action -- or is it intentional misinformation, or maybe satire? Of course, a well-known thriller writer who likes to claim his fictions as truth has written about CERN as an antimatter factory potentially useful to therrorists looking for an antimatter bomb, so what wonder is it if some members of the woefully uninformed public are willing believe this kind of stuff? More effective outreach is clearly needed.

Word on the arXiv

2008-02-21T12:00:00.000Z

The arXiv have announced that they now support submissions of "Microsoft Word DOCX or other OOXML (Office Open XML) document[s]". While I am perfectly aware that high-energy physicists (or indeed any kind of physicists) are not the only users of the arXiv, and that usage of TeX is not terribly common outside the physics/mathematics field (though I know a few philosophers and economists, and even one historian, who were won over by the superior look of texts typeset in LaTeX), I find this a little worrying, especially given that the arXiv acknowledges support from the Microsoft Technical Computing Initiative. What worries me is the possibility that this might be the first step towards a less open information architecture at the arXiv, and by implication in the high-energy physics communications sector. Will Microsoft try to gain a foothold, leading to the eventual establishment of their "open" (not) formats as the only accepted submission and download format? One sincerely hopes not.

arXiv catchup

2008-02-14T10:05:00.001Z

I have been too ~~lazy~~busy recently to blog anything. However, in the spirit of the day, I'd like to share a romantic little poem extolling the nonabelian nature of strong attraction:

Roses are red, violets are blue
quarks come in colours, and so does glue.

No, I won't give up physics and become a card designer for H$llm$rk, don't worry. But after softening your hearts with this touching verse, I'd like to blog about some rather old stuff, which I hope hasn't gone stale in the meantime.

One paper on the arXiv that struck me as interesting in the last couple of months was this paper by Jeffrey Mandula (of Coleman-Mandula No-Go fame), who discusses the consequences of Lüscher's nonlinear realisation of chiral symmetry for Ginsparg-Wilson fermions. We recall that this symmetry can be written in two inequivalent ways by putting the phase factor e^iαγ₅ either on the quark field ψ or its conjugate $$\bar{\psi}$$. The crucial fact that Mandula points out is that both of these are independent symmetries of the lattice theory, and they don't commute! Hence, we have to look for the symmetry algebra generated by them, which turns out to be infinite-dimensional. Hence the lattice symmetry has an infinite number of conserved currents, a structure quite different from the continuum theory. However, it would really appear that the differences between any two of these lattice currents are just lattice artifacts of order a or higher that should disappear in the continuum limit, if the latter is properly defined. So some of the objections that the paper raises are likely a lot less serious than stated (especially the non-locality exhibited for free overlap fermions [eq. (38)] goes away once one realises that the continuum limit must be taken with the negative mass s constant in lattice units), but it appears that Ginsparg-Wilson fermions may have their own set of problems beyond just being expensive to simulate. Any comments on this from Ginsparg-Wilson specialists would be of great interest.

Another interesting paper was this one by Mike Creutz who proposed a new fermion discretisation based on features of the electronic structure of graphene. Apparently the low electronic excitations of a grpahene layer are described by the massless Dirac equation, and a lattice model based on this (by reducing the links in one of the three graphene hexagonal directions to points, and rescaling eveything to make the lattice rectangular again) exploits this to achieve the minimum number (two) of doublers permitted in an conventional chiral lattice theory by the Nielsen-Ninomiya theorem, and this construction can be extended to four dimensions and gauged to get a lattice discretisation of QCD with two light quark flavours. This was quickly followed up by a similar proposal for a minimally-doubling quark action, and by this paper which shows that any minimally-doubling chiral lattice theory necessarily has to break either of the discrete symmetries P or T such that their product PT is broken; this allows the generation of additional (relevant) dimension 3 operators that have to be removed by fine-tuning, precluding the use of minimally-doubling chiral actions in practice (unless some additional non-standard symmetry should conspire to do that fine-tuning itself, a possibility hinted at in the conclusion).

Spammers strike

2008-01-12T11:57:00.000Z

My apologies to our readers for the very nasty spam post that headed this blog for the last few hours. Somehow a spammer managed to break into Matthew's unused Blogger account and used it to post his garbage (in giant bold italics). I have suspended Matthew's account from posting at this blog for the time being, and I hope that this will put an end to the spammers' incursion into this forum. Apologies to all offended parties again.

Algorithms for dynamical fermions -- Hybrid Monte Carlo

2007-12-09T16:49:00.000Z

In the previous post in this series parallelling our local discussion seminar on this review, we reminded ourselves of some basic ideas of Markov Chain Monte Carlo simulations. In this post, we are going to look at the Hybrid Monte Carlo algorithm.

To simulate lattice theories with dynamical fermions, one wants an exact algorithm that performs global updates, because local updates are not cheap if the action is not local (as is the case with the fermionic determinant), and which can take large steps through configuration space to avoid critical slowing down. An algorithm satisfying these demands is Hybrid Monte Carlo (HMC). HMC is based on the idea of simulating a dynamical system with Hamiltonian H = 1/2 p² + S(q), where one introduces fictitious conjugate momenta p for the original configuration variables q, and treats the action as the potential of the fictitious dynamical system. If one now generates a Markov chain with fixed point distribution e^-H(p,q), then the distribution of q ignoring p (the "marginal distribution") is the desired e^-S(q).

To build such a Markov chain, one alternates two steps: Molecular Dynamics Monte Carlo (MDMC) and momentum refreshment.

MDMC is based on the fact that besides conserving the Hamiltonian, the time evolution of a Hamiltonian system preserves the phase space measure (by Liouville's theorem). So if at the end of a Hamiltonian trajectory of length τ we reverse the momentum, we get a mapping from (p,q) to (-p',q') and vice versa, thus obeying detailed balance: e^-H(p,q) P((-p',q'),(p,q)) = e^-H(p',q') P((p,q),(-p',q')), ensuring the correct fixed-point distribution. Of course, we can't actually exactly integrate Hamilton's in general; instead, we are content with numerical integration with an integrator that preserves the phase space measure exactly (more about which presently), but only approximately conserves the Hamiltonian. We make the algorithm exact nevertheless by adding a Metropolis step that accepts the new configuration with probability e^-δH, where δH is the change in the Hamiltonian under the numerical integration.

The Markov step of MDMC is of course totally degenerate: the transition probability is essentially a δ-distribution, since one can only get to one other configuration from any one configuration, and this relation is reciprocal. So while it does indeed satisfy detailed balance, this Markov step is hopelessly non-egodic.

To make it ergodic without ruining detailed balance, we alternate between MDMC and momentum refreshment, where we redraw the fictitious momenta at random from a Gaussian distribution without regard to their present value or that of the configuration variables q: P((p',q),(p,q))=e^{-1/2 p'²}. Obviously, this step will preserve the desired fixed-point distribution (which is after all simply Gaussian in the momenta). It is also obviously non-ergodic since it never changes the configuration variables q. However, it does allow large changes in the Hamiltonian and breaks the degeneracy of the MDMC step.

While it is generally not possible to prove with any degree of rigour that the combination of MDMC and momentum is ergodic, intuitively and empirically this is indeed the case. What remains to see to make this a practical algorithm is to find numerical integrators that exactly preserve the phase space measure.

This order is fulfilled by symplectic integrators. The basic idea is to consider the time evolution operator exp(τ d/dt) = exp(τ(-∂_qH ∂_p+∂_pH ∂_q)) = exp(τh) as the exponential of a differential operator on phase space. We can then decompose the latter as h = -∂_qH ∂_p+∂_pH ∂_q = P+Q, where P = -∂_qH ∂_p and Q = ∂_pH ∂_q. Since ∂_qH = S'(q) and ∂_pH = p, we can immediately evaluate the action of e^τP and e^τQ on the state (p,q) by applying Taylor's theorem: e^τQ(p,q) = (p,q+τp), and e^τP = (p-τS'(q),q).

Since each of these maps is simply a shear along one direction in phase space, they are clearly area preserving; so are all their powers and mutual products. In order to combine them into a suitable integrator, we need the Baker-Campbell-Hausdorff (BCH) formula.

The BCH formula says that for two elements A,B of an associative algebra, the identity

log(e^Ae^B) = A + (∫₀¹ ((x log x)/(x-1))_{{x=e^{ad A}e^{t ad B}}} dt) (B)

holds, where (ad A )(B) = [A,B], and the exponential and logarithm are defined via their power series (around the identity in the case of the logarithm). Expanding the first few terms, one finds

log(e^Ae^B) = A + B + 1/2 [A,B] + 1/12 [A-B,[A,B]] - 1/24 [B,[A,[A,B]]] + ...

Applying this to a symmetric product, one finds

log(e^{1/2 A}e^Be^{1/2 A}) = A + B + 1/24 [A+2B,[A,B]] + ...

where in both cases the dots denote fifth-order terms.

We can then use this to build symmetric products (we want symmetric products to ensure reversibility) of e^P and e^Q that are equal to e^τh up to some controlled error. The simplest example is

(e^{δτ/2 P}e^{δτ Q}e^{δτ/2 P})^τ/δτ = e^τ(P+Q) + O((δτ)²)

and more complex examples can be found that either reduce the order of the error (although doing so requires one to use negative times steps -δτ as well as positive ones) or minimize the error by splitting the force term P into pieces P_i that each get their own time step δτ_i to account for their different sizes.

Next time we will hear more about how to apply all of this to simulations with dynamical fermions.

Algorithms for dynamical fermions -- preliminaries

2007-11-29T21:06:00.000Z

It has been a while since we had any posts with proper content on this blog. Lest my readers become convinced that this blog has become a links-only intellectual wasteland, I hereby want to commence a new series on algorithms for dynamical fermions (blogging alongside our discussion seminar at DESY Zeuthen/Humboldt University, where we are reading this review paper; I hope that is not too lazy to lift this blog above the waste level...).

I will assume that readers are familiar with the most basic ideas of Markov Chain Monte Carlo simulations; essentially, one samples the space of states of a system by generating a chain of states using a Markov process (a random process where the transition probability to any other state depends only on the current state, not on any of the prior history of the process). If we call the desired distribution of states Q(x) (which in field theory will be a Boltzmann factor Z^-1e^-S(x)), and the probability that the Markov process takes us to x starting from y P(x,y), we want to require that the Markov process keep Q(x) invariant, i.e. Q(x)=Σ_y P(x,y) Q(y). A sufficient, but not necessary condition for this is the the Markov process satisfy the condition of detailed balance: P(y,x)Q(x)=P(x,y)Q(y).

The simplest algorithm that satisfies detailed balance is the Metropolis algorithm: Chose a candidate x at random and accept it with probability P(x,y) = min(1,Q(x)/Q(y)), or else keep the previous state y as the next state.

Another property that we want our Markov chain to have is that it is ergodic, that is that the probability to go to any state from any other state is non-zero. While in the case of a system with a state space as huge as in the case of a lattice field theory, it may be hard to design an ergodic Markov step, we can achieve ergodicity by chaining several different non-ergodic Markov steps (such as first updating site 1, then site 2, etc.) so as to obtain an overall Markov step that is ergodic. As long as each substep has the right fixed-point distribution Q(x), e.g. by satisfying detailed balance, the overall Markov step will also have Q(x) as its fixed-point distribution, in addition to being ergodic. This justifies generating updates by 'sweeping' through a lattice point by point with local updates.

Unfortunately, successive states of a Markov chain are not really very independent, but in fact have correlations between them. This of course means that one does not get truly independent measurements from evaluating an operator on each of those states. To quantify how correlated successive states are, it is useful to introduce the idea of an autocorrelation time.

It is a theorem (which I won't prove here) that any ergodic Markov process has a fixed-point distribution to which it converges. If we consider P(x,y) as a matrix, this means that it has a unique eigenvalue λ₀=1, and all other eigenvalues λ_i (|λ_i+1|≤|λ_i|) lie in the interior of the unit circle. If we start our process on a state u=Σ_i c_iv_i (where v_i is the eigenvector belonging to λ_i), then P^Nu = Σ_i λ_i^N c_iv_i = c₀v₀ + λ₁^Nc₁v₁ + ..., and hence the leading deviation from the fixed-point distribution decays exponentially with a characteristic time N_exp=-1/log|λ₁| called the exponential autocorrelation time.

Unfortunately, we cannot readily determine the exponential autocorrelation time in any except the very simplest cases, so we have to look for a more accessible measure of autocorrelation. If we measure an observable O on each successive state x_t, we can define the autocorrelation function of O as the t-average of measurements that are d steps apart: C_O(d)=<O(x_t+d)O(x_t)>_t/<O(x_t)²>_t, and the integrated autocorrelation time A_O=Σ_d C_O(d) gives us a measure of how many additional measurements we will need to iron out the effect of autocorrelations.

With these preliminaries out of the way, in the next post we will look at the Hybrid Monte Carlo algorithm.

arXiv API

2007-11-01T09:12:00.000Z

Via Jacques Distler: The arXiv now has an API intended to allow web application developers access to all of the arXiv data, search and linking facilities. They have a Blog and a Google group about it, as well. Anybody wants to guess when we'll see a "My arXiv papers" application for Facebook?

Warp processors

2007-10-22T09:36:00.000+01:00

Via Cosmic Variance, this sounds pretty cool.

Brief post

2007-09-24T17:06:00.000+01:00

This is just a brief note saying that I am still alive and still blogging here, but that I have been too busy (moving across the Atlantic, settling into my new job here in Zeuthen, finding a flat in Berlin, and so forth) to write anything since my last post.

To catch up with the aftermath of the lattice meeting: the slides for the plenary and parallel talks are now online (just click on the "pdf" link next to each talk), as are some photos (including one showing me looking into the empty air above, thinking deeply about what to write here). The proceedings are also in progress (the deadline for submissions is 1st October, so expect a flurry of preprints on the lattice arXiv this week).

I'll be back with post that have actual content at some point, but don't expect to much for the next month or so.

Lattice 2007 -- Day Six

2007-08-18T03:36:00.000+01:00

My apologies for the delay in posting this. A cold and various personal matters kept me from posting it earlier.

The first plenary talk today was Walter Wilcox speaking about deflation methods for fermion inverters. Deflation methods like GMRES-DR are based on Krylov subspace ideas, where the Krylov space is augmented by some (approximate) eigenvectors to remove the corresponding eigenvalues from the system, thus improving convergence.

Next was Falk Bruckmann, who spoke about exploring the QCD vacuum with lattice QCD. The nonperturbative degrees off freedom relevant for the QCD vacuum are topological objects (vortices, monopoles and instantons). Studying these on the lattice is hard, but progress is being made.

The third talk of the session, about renormalization-group flows in multi-parameter in φ⁴ theories, was given by Ettore Vicari. Critical phenomena can be described in terms of a few critical exponent; one way to determine these is by looking at fixed points of renormalisation group flows. Since there are only a certain number of universality classes into which those critical points can fall, one can study these by looking at φ⁴ models falling into different classes (Landau-Ginzburg-Wilson models); this may even have some applications to determining the nature of the QCD phase transition.

After the coffee break, Michele Della Morte got a plenary session of his own for his talk about determining heavy quark masses. A number of determinations of heavy-quakr observables were summarised, and a more detailed overview of recent progress in determining the b-quark mass using HQET was given.

After that, the organisers thanked the staff who had made the conference possible, and they received a round of well-deserved applause. The organisers got some equally well-deserved applause of their own, and all partcipants were invited to attend Lattice 2008 in Williamsburg, VA, which will be held July 14-19, 2008. Looking forward beyond next year, Lattice 2009 was announced to take place in Beijing, and so the meeting adjourned.

Finally I had some time to look around the city properly, and so I visited the Johannes Kepler-Gedächtnishaus (Kepler's dying place, and today a museum about his life) with some colleagues. After that, highlights on our tour round the city were the romanesque Schottenkirche (the church of a monastery build in the 11th century by Iro-Scottish monks) and St. Emmeram (the church of a former monastery that now serves as the palace of the Princess of Thurn and Taxis). I will do some more sightseeing tomorrow morning, but since I don't think it will interest my readers too much, this closes my coverage of Lattice 2007.

Lattice 2007 -- Day Five

2007-08-03T11:33:00.001+01:00

The opulent banquet, late hours and probable overconsumption of Bavarian beer afterwards led to a notable decrease in the occupation number of the seats at the first plenary session today. The first plenary talk was Jo Dudek speaking about radiative charmonium physics. Experimentally theses are part of the research program at CLEO, but until now have been studied mostly in potential models. Radiative decays have now been studied on the lattice by analysing three-point function, but two-photon decays require some new theoretical developments based on combining QED perturbation theory and the LSZ reduction formula with lattice simulations.

The second speaker was Johan Bijnens talking about quark mass dependence from continuum Chiral Perturbation Theory at NNLO. After a quick overview of Chiral Perturbation Theory ideas and methods, he presented the results that have been obtained in NNLO light meson χPT during the past few years.

Next was Silvia Necco who spoke about the determination of low-energy constants from lattice simulations in both the p- and ε-regimes. The ε-regime is particularly useful because the influence of higher-order LECs is small there, so that the leading-order LECs Σ and F can be determined accurately.

After the coffee break, Philip Hägler talked about hadron structure from lattice QCD, giving a review of recent determinations of hadron electric polarisabilities and form factors, the nucleon spin fractions and other hadron structure observables.

The next talk was by Sinya Aoki, who spoke about the determination of hadronic interactions from QCD. ππ scattering can be studied on the lattice using Lüscher's finite-volume method, and this has been used to obtain results for the ρ meson decay width as well. Baryon-baryon potentials can be computed by computing the energy of a Qqq-qqQ system as a function of QQ separation, where Q denotes static quarks, and similarly for mesons. A different approach defines a potential from a measured wavefunction and its energy via an auxiliary Schrödinger equation.

The last plenary speaker for today was Gert Aarts with a talk about transport and spectral functions in high-temperature QCD. A prominent topic in this field is the fate of charmonium states in the quark-gluon plasma state. Another is the hyhdrodynamics of the QGP, which has been observed to be a nearly ideal fluid experimentally. Key to solving these problems is the analysis of spectral functions, which can be obtained from lattice correlators by means of a maximum extropy method.

In the afternoon there were parallel session again. The most remarkable talk was a summary of a proposed proof that SU(N) gauge theory is confining at all values of the coupling using a renormalisation group blocking technique by Terry Tomboulis. I am sure this proof will be closely scrutinised by the experts, and if it holds up, that would be a major breakthrough.

Lattice 2007 -- Day Four

2007-08-02T11:30:00.000+01:00

The first plenary session today started with a talk about Kaon physics on the lattice by Andreas Juettner. The leptonic decays of kaons are important in order ot determine the CKM matrix element V_us. A large number of determinations of |V_us| from Kl2 and Kl3 decays have been performed in the last couple of years, which are mutually compatible for the most part. An important feature of kaon physics is CP violation in neutral kaon decays. Determinations of B_K have been done in a number of different formulations, which show a number of minor discrepancies due to different error estimates, although they all seem to be compatible with the best global fit.

Next was a survey of large-N continuum phase transitions by Rajamani Narayanan. Large-N QCD in the 't Hooft limit (g²N fixed, g to 0, N to &infinity;) has been studied analytically in two dimensions where it can be reduced to an Eguchi-Kawai model, and numerically in three and four dimensions. It exhibits a variety of phase transitions in coupling, box size and temperature, too many in fact for me to properly follow the talk.

After the coffee break, a presentation on the BlueGene/P architecture and future developments was given by Alan Gara of IBM. The limits of the growth of supercomputer performance still seem to be far away, and Exaflop performance allowing dynamc simulations of 128³x256 lattices was predicted for 2023.

A talk on QCD thermodynamics by Frithjof Karsch followed. The question he addressed was whether there was evidence for different temperatures for chiral symmetry restoration and deconfinement, or whether these two transitions coincided. On the realtively coarse lattices that are available, improved actions are needed to approach the continuum limit. In spite of progress in the analysis of the various sources of systematic error, there appears to be a discrepancy in the answer to this question obtained by different groups.

A second QCD thermodynamics talk was given by Zoltan Fodor, who also addressed the nature of the QCD phase transition, outlining the evidence that the transition is in fact a crossover at zero chemical potential. Since a crossover does not have a unique transition temperature, the different transition temperatures found using chiral and deconfinement observables could be physical.

In the lunch break I was picked up by the police again in order to look at the suspect they had arrested in the meantime. It was the guy who had robbed me, and he apparently confessed even before I arrived to identify him. He "apologized" on seeing me, but at the same time tried to excuse the robbery with my refusal to hand over cash when asked "nicely" -- I suppose you can't afford to have too much of a conscience if your preferred lifestyle involves injecting yourself with illegal and poisonous substances on a regular basis. I must admit I feel a certain amount of pity for these guys, criminals though they are.

I also want to take this opportunity to sing the highest possible praises of the Regensburg police, who were incredibly polite and helpful and solved this case so quickly. Let me also add that apparently this kind of thing is very rare around here, so as not to give people a wrong impression of what is really a very lovely place.

There were two parallel sessions in the afternoon. Of note was the talk by Rob Petry, a graduate student at Regina, about work we had done on using evolutionary fitting methods to extract mass spectra from lattice correlators, which met with a lot of interest from the audience.

In the evening the conference banquet took place at "Leerer Beutel", apparently a former medieval storehouse that has been converted to an art gallery-and-restaurant. The banquet was a huge buffet dinner, with great German and Italian dishes, the surroundings were very nice, as was talking to people in a more relaxed environment.

Lattice 2007 -- Day Three

2007-08-02T07:20:00.000+01:00

Today was the traditional excursion day, so there were no plenaries in the morning. Instead there were parallel sessions, including the one with my talk (which went fine). A number of other lattice perturbation theory talks took place in the same session, and it was nice to see the methods from our paper get picked up by other groups.

At lunchtime, the police came to see me in order to have me pick the likely suspect in my robbery out of a photo array.

In the afternoon there were excursions. The one I was on went to Weltenburg Abbey, one of the oldest Benedictine abbeys north of the Alps, famous both for the beer from its 950 years old brewery, and for its beautiful baroque church, the latter a work of painter-architect Cosmas Damian Asam, his brother, sculptor Egid Quirin Asam, and his son, painter Franz Asam, members of the famous Asam clan of baroque churchbuilders in Germany. Particularly remarkable is the life-size statue of St. George on his horse, complete with dragon and saved princess. We went to the abbey by boat through the Danube gorge, a rock formation where the Danube broke through a layer of sedimentary rocks millions of years ago, drastically altering its course and leaving us both a testament to the earth-shaping power of water and a very scenic piece of valley. At the abbey, we had a guided tour of the church with a very nice and very well-informed guide who was apparently an art historian (a rather pleasant break from the common pattern of tour guides who could learn from some of the tourists they supposedly guide). After a pleasant snack and beer in the abbey's beergarden, we went back the same way we came.

Lattice 2007 -- Day Two

2007-07-31T08:41:00.001+01:00

The second day started with the annual experimental talk, which was given by Diego Bettoni, who spoke about FAIR (Facility for Antiproton and Ion Research). After an overview of the accelerator facilities involved, he spoke about charmonium spectroscopy. The advantage of studying charmonium systems in pbar-p annihilation reactions is that states of all quantum numbers can be produced directly, as opposed to e⁺-e^- annihilation which gives only 1^-- states directly and all others via radiative decays only. Studies of the χ_c and η_c states were presented. Planned studies are searches for exotic charmonium hybrids and for glueballs, measurements of the in-nuclear-medium mass shifts of the D meson mass, studies of double hypernuclei (nuclei with two nucleons replaced by hyperons), measurements of the proton form factor in the timelike region, and reversed deeply virtual Compton scattering, all at PANDA, and studies of nucleon structure with polarised antiprotons at PAX. As always, the experimental talk was somewhat sobering, as it pointed out the huge gaps in one's (or at least my) knowledge of experimental physics.

Next was Craig McNeile speaking about hadron spectroscopy. Topics were the η and η' mesons, the 0⁺⁺ spectrum, the controversial κ meson, distinguishing qqbar mesons from tetraquarks and molecules, the glueball spectrum and the search for glueballs within the meson spectrum, the changing and mixing in the 0⁺⁺ spectrum from unquenching, the f₀(600)/σ meson, and comparisons between different unquenched studies, including the different values obtained for r₀.

After the coffee break, we got to the "staggered wars" plenary. Mike Creutz opened with a talk on "why rooting fails". The crux of his argument as I understood it was that rooting averages over the four tastes, which have pairwise opposite chiralities, leading to a theory that is not a theory of a single chiral fermion. The postulated manifestation of this was an incorrect singular behaviour of the 't Hooft vertex in the rooted theory, which could lead to the wrong physics in singlet channels, particularly the mass of the η'.

The opposite point of view was presented by Andreas Kronfeld. He argued that the group structure of staggered symmetries is much more complex than usually considered, and that the "phantom" Goldstone bosons coming from the tastes removed by rooting cancel in physical correlation function. He then proceeded to counter the points raised in Creutz's criticism of rooted staggered quarks, arguing that rooting turns the quark mass m into its absolute value |m|, that the staggered taste-singlet chirality is not the same as naive chirality, and does in fact track the topological index correctly if the chiral and continuum limits are taken in the right order.

The final plenary talk was an ILDG status report delivered by Carleton DeTar. The ILDG (International Lattice Data Grid) is the union of national grid applications from Europe, the UK, Japan, Australia and the US, which is intended to allow sharing of lattice configurations, and eventually propagators, between collaborations. They have developped portable data formats (a markup language called QCDml and a binary format for lattice configurations), as well as the grid software. While the permissions policies of the various collaborations are still an issue in some cases, the general tendency seems to be that it is now easier to download unquenched configurations than to generate quenched configurations, which will put the last nail into the coffin of the (already quite dead) quenched approximation over the next couple of years.

After the lunch break, there were parallel sessions. Some remarkable talks were about non-QCD physics on the lattice: Julius Kuti talked about getting Higgs physics from the lattice by using a lattice theory as the UV completion of the Standard Model, Simon Catterall talked about exploring gauge-gravity duality through simulations of N=4 super-Yang-Mills quantum mechanics as the dual of a type IIa string theory with D0 branes, and Jun Nishimura talked about non-lattice Monte Carlo simulations of SYM quantum mechanics as the dimensional reduction of a theory that might be M-theory.

The poster session was interesting, if a tad chaotic, for which I blame the Bavarian beer. I didn't get to see all the posters, since I spent too much time talking to people I knew who had posters.

Lattice 2007 -- Day One

2007-07-30T08:47:00.001+01:00

Hello again from Regenburg. The conference opened at 9 with a brief address by a representative of the university, who said the usual things about how wonderful it is to have us here and so on.

A few brief announcements from the organisers followed, and then the first plenary session started with a talk by Peter Boule speaking for the RBC and UKQCD collaborations about simulations with dynamical domain wall fermions. There was a lot of comparison between domain wall and overlap with their respective topological and chiral properties. Preliminary results for the SU(3) and SU(2) chiral perturbation theory low-energy constants were presented, as were preliminary predictions for pseudoscalar decay constants, light quark masses, B_K and the K_l3 form factor. Nucleon form factors and structure were also mentioned, but I'm afraid a lot of it went too fast for me to follow, so you will have to wait for the proceedings.

Next was a talk about exploring the chiral regime with dynamical overlap fermions by Hideo Matsufuru speaking for the JLQCD collaboration. He started by discussing the properties of the overlap operator and the methods used to deal with the sign function discontinuity. The method they decided to use was including a topology fixing term. The results presented were for N_f=2 (an N_f=2+1 run is in progress), and included studies of the ε-regime, physics at fixed topology and its relation to θ=0 physics, the topological susceptibility and chiral extrapolations at NNLO.

After the coffee break, the theme of actions for light quarks continued with Carsten Urbach on behalf of the European Twisted Mass (ETMC) collaboration speaking about twisted mass QCD at maximal twist. After a brief overview of the general features of tmQCD at maximal twist, such as automatic O(a) improvement, he explained how to tune to maximal twist and presented some results on the behaviour and performance of simulation algorithms. Finally, there were some N_f=2 results for the pseudoscalar mass and decay constant including finite-size effects and comparisons with chiral perturbation theory. Other preliminary new results included a measurement of the pion mass splitting (which is difficult ot measure because of disconnected contributions for the neutral pion), a study of the ε-regime, and many others.

The plenary session concluded with a talk by Yoshinobu Kuramashi of the CP-PACS collaboration about using clover quarks and the Iwasaki gauge action to approach the physical point in N_f=2+1 simulations using Lüscher's domain-decomposed HMC algorithm.

I had to see the police again during the lunch break in order to go through photo arrays of potential suspects (without much success; I couldn't identify the robbers in the database, but there was a recent arrest which included a person I think was one of them; if he has extremely bad teeth, the police think it will be a sufficient ID to charge him, but that means I'll have to go to the police yet again to identify him in person as having the right kind of bad teeth; the economic damage from this robbery in terms of my time and the cops' time probably already greatly exceeds the 100 Euro taken in value...). This meant that I also missed the first parallel session.

From the second parallel session of the afternoon, I found Ulli Wolff's talk about cluster simulations of two-dimensional fermions very interesting. Basically, the partition function for theories of 2d fermions can be reformulated as the partition function for a theory of non-intersecting loops, which can be reformulated as a theory of Ising spins, which then can be simulated efficiently using cluster algorithms. Of course, 2d fermions are very special, so this is unlikely to carry over to 4d QCD.

Lattice 2007 -- Day Zero

2007-07-30T08:28:00.000+01:00

Hello from Regensburg, where the Lattice 2007 conference started with an evening reception in the old town. Things got off to a nice start, and Regensburg is a very beuatiful town. Unfortunately, a certain dampener was put on my enthusiasm for it when it became the scene for my being robbed of 100 Euros at knifepoint on one of the high streets by a couple of thugs. While physically unharmed, I was understandably rather shaken, and being questioned about the event by police until well past midnight didn't really enhance the experience.

Quick catching-up post

2007-07-20T19:42:00.000+01:00

I have been somewhat too busy to blog recently, because I had to work both on this paper, the ideas behind which I discussed in this post earlier on, and on my talk (about this work, also discussed here) for the Lattice meeting in Regensburg.

This is therefore mostly just a quick catching-up post with a few things I thought remarkable enough to note:

Using a massive brute-force computation, computer scientists at the University of Alberta have solved the game of checkers. More on the story here and here, or straight from the source.

The most recent issue of Physics World is all devoted to questions of energy, a topic which I have blogged about before both here and elsewhere. I am especially delighted at the "lateral thought" column pointing out the problem of electronic devices on stand-by, which make up for about 20% of the average household's energy consumption, a problem I am fond of pointing out to anyone listening. Go unplug your TV and stereo overnight!

I intend to cover the Lattice meeting in Regensburg in much the same manner as last year's meeting.

Lattice 2007 abstracts online

2007-06-29T20:40:00.000+01:00

The list of abstracts for the Lattice 2007 conference is now online. The JavaScript popup feature -- hover your mouse over a title and see the abstract as a tooltip -- is very nice.

Vexillology and the Moon Hare

2007-06-12T23:20:00.000+01:00

A friend of mine recently asked me a question regarding the moon, and I thought it might be good to share the answer with my readers.

The question was whether the fact that Europeans tend to see the face of a man in the moon (with the Mare Imbrium and Mare Serenitatis forming the eyes, and the Mare Nubium and/or Mare Humorum forming the Mouth), whereas Asians tend to see a hare or rabbit (with the Mare Foecunditatis and Mare Nectaris forming the ears), had any astronomical basis.

Now, longitude is of course a largely arbitrary quantity (for which reason it was historically so hard to determine that a large prize was offered for the development of a method to determine it reliably while at sea) and should not have any effect on the appearance of celestial bodies (except for the time at which they transit, some parallax and effects following from those, such as visibility of eclipses etc., but certainly not the size or orientation of a disc). Latitude, on the other hand, has a true astronomical meaning, and a moment's thought should show you that the angle that the crescent moon (which always points towards the sun, which moves accross the sky at different angles to the horizon at different latitudes) forms with the horizon varies with latitude -- in fact, it is something of a cliché that this is reflected in the flags of islamic countries at different latitudes: compare the flags of Turkey, Pakistan and Mauritania.

Since the part of the moon turned towards the sun is of course independent of the observer's latitude, it follows that from the point of view of an observer close to the equator the orientation of the moon disk is such that the rabbit in the moon is quite clearly visible, whereas an observer in the temperate zone sees the moon under an angle at which he would likely prefer the face, unless being told about the rabbit (which, at least for me, easily supersedes the face). I therefore hypothesize that the tradition of the moon rabbit spread into East Asia from South Asia, whereas the tradition of the face in the moon comes from Nothern Europe. Does anyone know whether that would appear to agree with the historical record? It seems rather plausible to me.

Unquenching meets improvement

2007-06-11T20:44:00.000+01:00

In a recent post, I explained how the fact that the vacuum in quantum field theory is anything but empty affects physical calculations by means of Feynman diagrams with loops, and specifically how one has to take account of these contributions in lattice field theory via perturbative improvement. In this post, I want to say some words about the relationship between perturbative improvement and unquenching.

To obtain accurate results from lattice QCD simulations, one must include the effects not just of virtual gluons, but also of virtual quarks. Technically, this happens by including the fermionic determinant that arises from integrating over the (Grassman-valued) quark fields. Since the historical name for omitting this determinant is "quenching", its inclusion is called "unquenching", and since quenching gives rise to an uncontrollable systematic error, unquenched simulations are absolutely crucial for the purpose of precise predictions and subsequent experimental tests of lattice QCD.

However, the perturbative improvement calculations that have been performed so far correct only for the effects of gluon loops. This leads to a mismatch in unquenched calculations using the perturbatively improved actions: while the simulation includes all the effects of both gluon and quark loops (including the discretisation artifacts they induce), only the discretisation artifacts caused by the gluon loops are removed. Therefore the discretisation artifacts caused by the quark loops remain uncorrected. Now, for many quantities of interest these artifacts are small higher-order effects; however, increased scaling violations in unquenched simulations (when compared with quenched simulations) have been seen by some groups. It is therefore important to account for the effects of the quark loops on the perturbative improvement of the lattice actions used.

This is what a group of collaborators including myself have done recently. For details of the calculations, I refer you to our paper. The calculation involved the numerical evaluation of a number of lattice Feynman diagrams (using automated methods that we have developed for the purpose) on a lattice with twisted periodic boundary conditions at a number of different fermion masses and lattice sizes, and the extrapolation of the results to the infinite lattice and massless quark limits. The computing resources needed were quite significant, as were the controls employed to insure the correctness of the results (which involved both repeated evaluations using independent implementations by different authors and comparison with known physical constraints, giving us great confidence in the correctness of our results). The results show that the changes in the coefficients in the actions needed for O(α_sa²) improvement caused by unquenching are rather large for N_f=3 quark flavours, which is the case relevant to most unquenched simulations.

QCD with cold atoms?

2007-05-22T18:06:00.000+01:00

Via Chad Orzel, a PhysicsWeb new story reports a recent proposal to use a rather different kind of lattice than the one usually discussed here for understanding QCD. The authors of this PRL paper propose that ultracold fermionic atoms with three possible hyperfine states trapped in an optical lattice (a periodic potential created by crossing laser beams) would behave like quarks in QCD, including forming "baryonic" states and showing the same phase transitions as QCD matter.

I don't know enough about atomic and optical physics to be able to tell whether this proposal is reasonable. If it is, it could be seen as one of the first examples of the use of an analogue quantum computer to simulate an otherwise experimentally inaccessible quantum system. However, I can see no real evidence that the atomic system would really be simulating QCD (which includes gluons and sea quarks) rather than some kind of quark model, so I remain a little sceptical regarding that claim. In any case, this proposal shows how far atomic and optical physics has come in its ability to finely control the states and interactions of atoms, so even if it isn't QCD, it's pretty cool.

Stakes

2007-05-14T18:02:00.000+01:00

Tomaso Dorigo has a great post on the difficult decisions experimentalists have to make when deciding on their triggers, and on the human behaviour that can be observed in the meetings where they discuss and make those decisions.

To someone looking into the academic world from outside, anecdotes like the one reported by Tomaso probably sound a lot like yet another example of "academic politics is so bitter because the stakes are so low," a quotation commonly misattributed to Henry Kissinger. But what needs to be kept in mind is that the stakes are in fact, extremely high: when a researcher devotes almost every waking minute to some research project, foregoing other (much better paying) career options and postponing, or even completely giving up on, such things as parenthood and home ownership, what is at stake in discussions about that project's future role in the greater structure of human knowledge and discovery is no less than that researcher's major purpose in life. And that is a huge stake for anyone, whether they are a lowly scientist (or historian or whatever) or a mighty CEO -- although the former are far more likely to face that kind of risk than the latter, who have their golden parachutes. So a certain amount of acrimony is really to be expected.

SciFi'ish Sunshine Scene

2007-05-03T17:48:00.000+01:00

Via Clifford Johnson, a link to a BBC report about the first commercially operating solar thermal power plant in Europe. Located in sunny Andalusia, it generates 11 MW of electrical power from sunshine alone by concentrating the light of the sun on the top of a 115 m tall tower by means of 600 heliostats, huge mirrors that track the Sun in the sky. The concentrated light is so intense that when scattered off the water vapour and dust in the air it creates the scifi-like special effect visible in the picture at the BBC link. If this kind of power plant is shown to work well commercially, the resulting increase in energy production could be a huge boost to the economy of developping countries in the subtropics, and the Gulf sheikhs will have nothing to worry about when the oil runs out -- they have plenty of sunshine in the Arabian desert after all.

Carl Friedrich von Weizsäcker 1912-2007

2007-04-28T20:53:00.000+01:00

The German physicist, philosopher and peace researcher Carl Friedrich von Weizsäcker has died on 28th April at the age of 94. The brother of former German president Richard von Weizsäcker was born on 28th June 1912.

Carl Friedrich von Weizsäcker studied physics under Werner Heisenberg and Niels Bohr. Working with Hans Bethe in nuclear physics, he discovered the Bethe-Weizsäcker formula for nuclear masses and the Bethe-Weizsäcker cycle of nuclear fusion that powers the heavy stars. He also developed a model for the evolution of the solar system.

During the Second World War, von Weizsäcker was a member of the elite team of German physicists working on the unsuccessful attempt to develop a nuclear weapon for Nazi Germany; later, von Weizsäcker always maintained that the failure of that project was due to the physicists' unwillingness to develop such a devastating weapon for the Nazis, rather than a lack of ability to do so.

After the war, von Weizsäcker was a prominent opponent of plans for the nuclear armament of West Germany, signing the declaration of the Göttingen Eighteen that publicly exposed and rejected defence minister Franz-Josef Strauß's plan to arm the newly refounded German Army (Bundeswehr) with tactical nuclear weapons, and that created enough public opposition to end those plans once and for all.

Carl Friedrich von Weizsäcker's opposition to nuclear weapons and his interest in the responsibility of scientists for the use of their research led him into peace research, and to founding and directing the Max-Planck-Institute for research into living conditions in a scientific-technical world. He also worked as a philosopher, trying to unify all of physics into a coherent system of Natural Philosophy based on the idea of the quantum dynamics of primal logical alternatives (Uralternativen) underlying physical reality.

Earth-2

2007-04-25T17:40:00.000+01:00

The science news story of today is that according to an ESO press release, the first extrasolar planet orbiting its host star in the "Goldilocks" zone, the zone of temperatures allowing for liquid water, and hence capable of supporting life, has been found.

The new planet, whose radius is estimated 1.5 times the Earth's radius, orbits a red dwarf called Gliese 581 on a close orbit with an orbital period of just 13 days; this close orbit is the reason why astronomers were even able to detect such a realtively low-mass planet. Because a red dwarf like Gliese 581 is much dimmer and cooler than a yellow dwarf like our Sun, however, this still lies in its "Goldilocks" zone with surface temperatures estimated to lie between 0 and 40 degrees Celsius, and hence Gliese 581 c (as the new planet is called) could have oceans. It should be noted, though, that the assumption of an earth-like rocky planet is based on models, not observations, so far. The next step will presumably be to attempt to detect telltale spectral lines that might reveal the existence of an atmosphere or of liquid water.

And at just 20.4 lightyears distance, the good news is that once alien civilisations have been found on Gliese 581 c, we will even be able to keep up a meaningful conversation with them. Yes, that was just a joke, but this is going to be big news in the popular press, and I am sure some tabloid will report this as "Alien life discovered!" or some such nonsense.

In other news, Life on the Lattice has now been moved to the new Blogger, and this time, things seem to work for the most part.

Some quick links

2007-04-19T18:47:00.000+01:00

Superweak has an interesting post on blind analysis, which is the first technique that has been carried over from medicine into nuclear and particle physics (rather than the other way, as were NMR, PET and a host of others). More on blind analysis techniques in experimental particle physics can be found in this review. Reading this, I was wondering wether any lattice groups used blinding in their data analyses; I am not aware of any that do, and the word "blind" does not appear to occur in hep-lat abstracts (except for phrases like "blindly relying on" and such). It may not be necessary, because we don't do the same kind of analyses that the experimenters do (like imposing cuts on the data), but the possibility of some degree "experimenter's (!?) bias" may still exist in the choice of operators used, priors imposed on fits etc.

There is a new paper on the arXiv which reports on tremendous gains in simulation efficiency that the authors have observed when using a loop representation for fermions instead of the conventional fermion determinant. Unfortunately their method does not work with gauge theories (except in the strong coupling limit) because it runs into a fermion sign problem, so it won't revolutionise QCD simulations, but it is very interesting, not least because it looks a lot like some kind of duality (between a theory of self-interacting fermions and a theory of self-avoiding loops) is at work.

Artificial "plants" invented?

2007-04-18T17:09:00.000+01:00

According to this press release, chemists at UCSD have realized the first step toward the creation of artificial "plants" that use solar radiation to convert CO₂ into fuel. Their prototype still needs additional energy input, but they believe they will be able to optimize it so it will run on solar power alone. The device creates carbon monoxide (CO), which is an extremely toxic gas which is commonly used in suicides, but which also serves an important basis material for the chemical industry and can even be converted into liquid fuel via the Fischer-Tropsch process. Artificial photosynthesis sounds like a great idea, but I am not an expert on this, so maybe there are hidden caveats that the inventors are not talking about. Any additional information from experts would be most welcome.

New identifiers at the arXiv

2007-04-03T17:57:00.000+01:00

The arXiv have changed their identifiers away from the familiar arch-ive/YYMMNNN (e.g. hep-lat/0605007) format to a new YYMM.NNNN (e.g. 0704.0274) format, which will be used across archives; the change was implemented on April Fool's Day. One consequence of the new identifiers is that the preprint numbers within an archive are no longer consecutive, making the "previous" and "next" functions on the abstract listings rather less useful. Existing papers will retain their old-style identifiers, though. It will remain to be seen how the community likes the change.

Another change, which at least I like quite a bit, is the new presentation format for abstracts. With the more commonly required pieces of information at the top, it looks a lot neater than the old one, which had a lot of less useful things (submission history etc.) in the first few lines.

The Quantum Vacuum, Loops and Lattice Artifacts

2007-03-31T23:10:00.000+01:00

This post was written for a general audience, and hence is written in a rather more popular language than our usual fare at Life on the Lattice. If you are familiar with the basic ideas behind perturbative improvement, you may want to skip this post.

When we think about the vacuum in classical physics, we think of empty space unoccupied by any matter, through which particles can move unhindered and in which fields are free from any of the non-linear interaction effects which make e.g. electrodynamics in media so much more difficult.

In Quantum Field Theory, the vacuum turns out to be quite different from this inert stage on which things happen; in fact the vacuum itself is a non-linear medium, a foamy bubble bath of virtual particles popping into and out of existence at every moment, a very active participant in the strange dance of elementary particles that we call the universe.

A metaphor which may make this idea a little clearer could be to think of the vacuum as a sheet of paper on which you write with your pen. Looked at on a large scale, the paper is merely a perfectly flat surface on which the pen moves unhindered. On a smaller scale, the paper is actually a tangle of individual fibers going in all directions and against which the pen keeps hitting all the time, thus finding the necessary friction to allow efficient writing.

In the case where the paper is the vacuum, the analogue of the paper fibres are the bubbles of virtual particle pairs that are constantly being created and annihilated in the quantum vacuum, the analogue of the pen is a particle moving through the vacuum, and the analogue of friction is the modification of the particle's behavior as compared with the classical theory which happens as a result of the particle interacting with virtual particle pairs.

At first sight, this description of the vacuum may appear like wild speculation, but it has in fact very observable consequences. In Quantum Electrodynamics (QED), the famous Lamb shift is a consequence of the interactions of the electron in a hydrogen atom with virtual photons, as are the anomalous magnetic moment of the electron and the scattering of light by light in the vacuum. In fact, none of the amazingly accurate predictions of QED (the most accurate theory we have) would work without taking into account the effects of the quantum vacuum.

In lattice QCD, we care about the vacuum because it affects how the discrete lattice theory relates to its continuum limit. By discretising a continuum theory, we introduce a discretisation error: When comparing an observable O_a measured on a lattice with lattice spacing a with the same observable in the continuum O₀, we find that they are related as

where μ is some energy scale that is typical of the reactions contributing to the observable O. In the classical theory (or at "tree level" as we say because the Feynman diagrams corresponding to classical physics have no loops in them), we can then tune the lattice theory so that as many of the c_i as we want to get rid of become zero, and the discrepancy between lattice and continuum becomes small.

At the quantum level, however, we get Feynman diagrams with loops in them that describe how particles traveling through the quantum vacuum interact with virtual particles; the problem with these is that the virtual particles exist at very short distances and hence can have very large momenta by virtue of Heisenberg's uncertainty relation. At very large momenta, the deviation of the lattice theory from the continuum becomes very evident, and hence the loops on the lattice contribute terms that differ a lot from what the same loops would contribute in the continuum. And then we find that this difference reintroduces the a-dependence that we got rid of classically by tuning our theory!

This is clearly no good. What we need to do is to get rid of the a-dependence (up to some order in a) in the quantum theory, too. There are a number of ways how to go about this, but the one most commonly used is called perturbative improvement. In perturbative improvement, we calculate the effect of the virtual particle loops by evaluating Feynman diagrams (a Feynman diagram isn't just a pretty picture: there is a well-defined mathematical expression corresponding to each Feynman diagram) on the lattice and extracting their contribution to the lattice artifacts c_i to some order in a. Once we have these contributions, we can then tune our theory again so that these contributions to the c_i are cancelled, and the discrepancy between lattice and continuum becomes small again.

Unfortunately, evaluating Feynman diagrams on the lattice is much harder than in the continuum in many ways, so that we need some rather advanced methods to do this, and there aren't very many people doing it. So this is an area where progress has been slow for a while. The next post will tell you how a group of collaborators including myself recently made some pretty significant progress in this field.

Fitness and Fitting

2007-03-12T19:57:00.000Z

I promised there were going to be some interesting posts, and I feel this is one of them. I want to talk about harnessing the power of evolution for the extraction of excited state masses from lattice QCD simulations.

OK, this sounds just outright crazy, right? Biology couldn't possibly have an impact on subnuclear physics (other than maybe by restricting the kinds of ideas our minds can conceive by the nature of our brains, which could of course well mean that the ultimate theory, if it exists, is unthinkable for a human being, but that is a rather pessimist view; I am also talking about QCD here). Well, biology doesn't have any impact on what is after all a much more fundamental discipline, obviously, but Darwin's great insight has applications far beyond the scope of mere biology. This insight, which I will roughly paraphrase as "starting from a set of entities which are subject to random mutations and from which those least adapted to some external constraints are likely to be removed and displaced by new entities derived from and similar to those not so removed, one will after a large enough time end up with a set of entities that are close to optimally adapted to the external constraints", is of course the basis of the very active field of computer science known as evolutionary algorithms. And optimisation is at the core of extracting results from lattice simulations.

What people measure in lattice simulations are correlators of various lattice operators at different (euclidean) times, and these can be expanded in an eigenbasis of the Hamiltonian as

(for periodic boundary conditions in the time direction the exponential becomes a cosh instead, but let's just ignore that for now), where the c_n measure the overlap between the eigenstates of the operator and those of the Hamiltonian, and the E_n are the energies of the Hamiltonian's eigenstates. Of course only states that have quantum numbers compatible with those of the operator O will contribute (since otherwise c_n=0).

In order to extract the energies E_n from a measurement of the correlator <O(t_i)O(0)>, one needs to fit the measured data with a sum of exponentials, i.e. one has to solve a non-linear least-squares fitting problem. Now, there are of course a number of algorithms (such as Levenberg-Marquardt) that are excellent at solving this kind of problem, so why look any further? Unfortunately, there are a number of things that an algorithm such as Levenberg-Marquardt requires as input that are unknown in a typical lattice QCD data analysis situation: How many exponentials should the fitting ansatz use (obviously we can't fit all the infinitely many states)? Which range of times should be fitted (and which should be disregarded as dominated by noise or disregarded higher states)? A number of Bayesian techniques designed to deal with this problem have sprung up over time (such as constrained fitting), and some of those deserve a post of their own at some point.

From the evolutionary point of view, one can simply allow evolution to find the optimal values for difficult-to-optimise parameters like the fitting range and number of states to fit. Basically, one sets up an ecosystem consisting of organisms that encode a fitting function complete with the range over which it attempts to fit the data. The fitness of each organism is taken to be proportional to minus its χ²/(d.o.f.); this will tend to drive the evolution both towards increased fitting ranges and lower numbers of exponentials (to increase the number of degrees of freedom), but this tendency is counteracted by the worsening of χ². The idea is that if one subjects these organisms to a regimen of mutation, cross-breeding and selection, evolution will ultimately lead to an equilibrium where the competing demands for small χ² and large number of degrees of freedom balance in an optimal fashion.

After Rob Petry here in Regina brought up this idea, I have been toying around with it for a while, and so far I am cautiously optimistic that this may lead somewhere: for the synthetic data sets that I let this method look at, it did pretty well in identifying the right number of exponentials to use when there was a clear-cut answer (such as when only finitely-many were present to start with). So the general method is sound; it remains to be seen how well it does on actual lattice data.

Around the blogs

2007-03-02T22:07:00.000Z

Bee at Backreaction has a post on snow, which is indeed an important topic here in Canada. Regina doesn't even get that much snow by Canadian standards, and it still is very snowy around here (although it is worse when it doesn't snow, because that is when it gets really cold).

Christine Dantas, well known for her background independence, has a new blog called Theorema Egregium, presumably in homage to Gauss.

Also new to our blogroll is Resonaances, by an anonymous particle physicist known as "Jester", who blogs from CERN and allows everybody who is interested to obtain a glimpse into CERN's seminars.

New Book on the Lattice

2007-02-16T18:42:00.000Z

There is a new book about lattice QCD by Tom DeGrand and Carleton DeTar (D&D). It is still quite new, and in fact I am still waiting for my copy to be delivered, but a senior colleague here in Regina was so nice to let me borrow his copy, so you can get my review.

D&D is a comprehensive overview of the current state of the art in lattice QCD. In the space of just 327 pages (excluding front and back matter) they manage to cover pretty much everything one needs to know about in order to be able to read the current research literature. To the best of my knowledge, this is the first lattice monograph to discuss such crucial topics as data analysis for lattice simulations, improved actions and operator matching, chiral extrapolations, and finite-volume effects.

Compared to Montvay and Münster (M&M) at 442 pages, and to Rothe at 481 pages, both of whom cover much less material, D&D are necessarily rather terse. There are no detailed derivations or proofs, and no discussion of the results of lattice simulations is given anywhere. The latter omission is very rightly justified by the authors, as to include them "would be to invite obsolescence". While the terseness of the presentation probably limits the usefulness of D&D as a graduate textbook, the authors' stated aim to bridge the gap between what a conventional (non-lattice) theorist already knows and the current research literature which often presupposes an enormous amount of specialised knowledge appear to have been met admirably well.

After a brief overview of continuum QCD and a quick introduction to path integrals for bosons and fermions, and to the renormalisation group, D&D turn to introducing the lattice discretisation of pure gauge theories, including topics such as gauge fixing and strong coupling expansions. A comprehensive overview of lattice fermion actions follows, covering naive, Wilson, twisted mass, staggered and exactly chiral fermions as well as heavy-quark actions (HQET, NRQCD and Fermilab action). This is succeeded by chapters discussing simulation algorithms for both gluonic and fermionic actions, including such state-of-the-art algorithms as RHMC, BiCGStab and Lüscher's implementation of Schwartz decomposition. Data analysis methods, including correlated fitting, bootstrap methods and Bayesian (constrained) fitting, are discussed in a chapter of their own. The design of improved lattice actions (covering both Symanzik and tadpole improvement, as well as "fat link" actions) also gets a chapter of its own, as does the design of measurement operators for spectroscopic quantities. This is followed by a chapter on Lattice Perturbation Theory (which even cites this paper) and one on matching operators between the lattice and the continuum. Chiral perturbation theory, including such difficult subjects as quenched and staggered χPT, also gets a chapter, as do finite-volume effects and their applications. An overview of Standard Model observables amenable to testing via lattice simulations and a brief introduction simulations of finite-temperature QCD round off this very comprehensive book (including even such specialised topics as dimensional reduction of thermal QCD or the maximum entropy method for extracting spectral functions). The bibliography and the index, while also rather terse, appear useful.

In short, D&D have written a comprehensive introduction to state-of-the-art lattice QCD, which should serve both as a useful introduction to those who know a little, but want to know much more, and as a quick reference for active researchers (although a more extensive bibliography will be missed by the latter). This book definitely belongs on the bookshelf of every lattice theorist as an important contemporary counterpoint to M&M's classic.

Alchemy

2007-01-29T20:33:00.000Z

Everybody knows that we could in principle make gold from lead using a particle accelerator such as the LHC -- at billions of dollars per ounce, it just would be extremely expensive gold. Everybody also knows that gold is a chemical element, and hence cannot be produced from other elements by chemical means; you really need a particle accelerator or nuclear reactor to transmute elements (except of course for naturally radioactive elements that sort of transmute themselves).

But there are still those brave souls who will valiantly ignore the insights into the nature of things that science has gathered over the past 300 years, and go on a crucible-sade for the philosophers' stone. I am speaking of the members of the International Alchemy Guild, who will be gathering in Las Vegas for their first conference this year (I wonder if the choice of location is symbolic -- just like the roulette tables are money sinks that only con the stupid, so is alchemy?)

I'd be inclined to think this is a joke, but apparently this is a real organisation, whose membership benefits for those who "[d]emonstrate successful creation of the Vegetable or Mineral Stone in private laboratory work" (or are otherwise co-opted into this exclusive society, such as on account of having bought a mail-order degree from a "hermetic college") include a "gilded Certificate of Membership (suitable for framing)" with an accompanying "license to practice alchemy" (I wonder if that license can serve as a legal defence against fraud charges).

Evil, bad, diseased, or just ugly?

2007-01-23T17:25:00.000Z

"Evil" is a word rarely heard in scientific discourse, at least among physicists, whose subject of study is after all morally neutral for pretty much any sensible definition of "morally". "Bad", "diseased" or "ugly" might be heard occasionally. But having all of them applied to a topic as relatively arcane as the fourth-root prescription for staggered fermions is, well, staggering. At last year's lattice meeting there was a lot of discussion as to whether this prescription was diseased or merely ugly. Now Mike Creutz has taken the discussion from the medicinally-aesthetic to the moral level by suggesting that rooting is actually evil. The arguments are much the same as before: The rooted staggered theory has a complicated non-locality structure at non-vanishing lattice spacing, and there is no complete proof (although there are strong arguments that many find very convincing) that this non-locality goes away in the continuum level. The debate will no doubt simmer on until a fully conclusive proof either way is found; the question is only, what kinds of unusual title words are we still going to see?

Still active

2007-01-22T21:09:00.000Z

It has been a little quiet around here, mostly because I was staying at home in Germany (admittedly not a good excuse in the days of almost universal WiFi internet access). I wasn't completely lazy, however. And there is a pile of half-finished blog-posts waiting to appear.

Meanwhile, please help me welcome Sujit Datta to the physics blogosphere. Sujit is a Master's student at the University of Pennsylvania who researches quantum transport in carbon nanotubes and is especially interested in condensed matter physics and the study of complex systems. He started his blog on January 1, 2007, and so far has been a very prolific poster of interesting articles.

PhysicsWeb 2.0

2007-01-03T14:46:00.000Z

The most recent issue of PhysicsWorld has a feature about how Web 2.0 technologies are transforming physics communications. Physicists' blogs feature quite prominently in this issue: besides a piece about blogs and wikis, which quotes physics bloggers Sabine Hossenfelder, Christine Dantas, Luboš Motl, Gordon Watts and Dave Bacon (apologies if I missed somebody) alongside the more critical voices of Nobel Laureates Philip Anderson and Jack Steinberger, they have an article by Sean Carroll of Cosmic Variance about his motivations for blogging.

And finally, they introduce a new column "Blog life", which starts with a profile of Chad Orzel's Uncertain Principles and is going to look at a different physics blog each month.

I suppose this makes physics blogging officially (at least almost) respectable. Now, if only PhysicsWorld would allow trackbacks ...

Update: Backreaction has some thoughts on this.

Tagged!

2006-12-22T03:35:00.000Z

I have been tagged by Robert with the instructions to

1. Grab the book closest to you.
2. Open to page 123, go down to the fifth sentence.
3. Post the text of next 3 sentences on your blog.
4. Name of the book and the author.
5. Tag three people.

The book closest to me at this moment is my address book, which has neither numbered pages nor sentences. So let's skip to the next-nearest one. That would be a notebook, which also doesn't have numbered pages. So let's skip to the next-next-nearest one. That one has numbered pages. Here it goes:

Sie fühlt, daß ich ganz sicher ein Genie,
Vielleicht wohl gar der Teufel bin.
Nun heute Nacht -- ?
FAUST. Was geht dich's an?
MEPHIST. Hab' ich doch meine Freude dran.

or, in translation for those who don't read German:

She feels that I am certainly a genius,
maybe even the Devil.
Now, tonight -- ?
FAUST. What do you care?
MEPHIST. I find pleasure in it.

The book is Goethe, Faust (Gesamtausgabe).

As to tagging three people, I must admit to not liking the idea of exponentially growing chain-letter type meme things. So I will go all Time Magazine on you, and nominate you, you and you as the next tag-bearers.

Off-Lattice Links

2006-12-14T18:33:00.000Z

JoAnne at Cosmic Variance has a nice introductory post about particle detectors. More about the subject, with a description of the CLEO-c detector, can be found at Superweak.

Peter Steinberg writes about nuclear toys, specifically an A.C. Gilbert U-238 Atomic Energy Lab. Nuclear toys for nuclear boys may sound like a bizarre relic from another age, but in fact, as recently as 2005, the Boy Scouts of America instituted a Nuclear Science merit badge with the advice of the APS Nuclear Physics Division.

Christmas presents

2006-12-11T18:48:00.000Z

I'm sure everyone working in theoretical physics knows the problem: Many, probably most, of the people who are most important in your life -- family, friends, loved ones -- don't have the background to understand what your research is all about. So what will you do? A strict secret-agent-style "don't talk about work when at home" policy is deeply unsatisfying; after all, you went into academic research rather than into a more lucrative field such as, say, finance, because you are driven by a consuming interest in finding out how the universe works. But any attempt at a conversation about our work tends to end in a muddle of confusion ("What do you mean by a symmetry? What on earth is a group? Why do you have to prove that, isn't it obvious (or clearly false)? What's a simulation? How can a computer solve the equations if you can't, I mean, don't you have to tell it how to solve them? ...?") or worse yet, misunderstandings ("--- studies particles that form groups and have links between them, or something like that!") when talking to people lacking the basic concepts necessary to understand contemporary research, i.e. to virtually all non-scientists. Of course there always is the pop-sci approach (leave out mathematics and every other bit of "technical" detail and just focus on the beauty and wonder of it all), but the people closest to you usually want more than that: they want to be able to ask "How was your day/week?" and get a meaningful answer that does not exclude your research, which is after all what you spend most of your time doing. But if their background is in the arts or humanities, they don't just lack the technical knowledge of, e.g. group theory -- that could be easily remedied; no, most of the time they are actually deeply unfamiliar with mathematical reasoning and the general modes and methods of scientific research.

At Christmastime, there's always the question "what can I give to ---", and what better than to give the gift of greater understanding? For introducing people with a humanities background to the kind of ideas and ways of thinking used in some of the more abstract fields of theoretical physics, I have tried titles from Oxford University Press's Very Short Introduction series of books, such as the one on Particle Physics by Frank Close, the one on Quantum Theory by John Polkinghorne, the one on Cosmology by Peter Coles, or the one on Mathematics by Fields medalist Tim Gowers. I think they do a very good job at giving a non-technical introduction to their respective subjects that goes a good way beyond the usual pop-sci stuff without trying to make experts out of their readers.

For the more down-to-earth kind of physics, the most recent issue of PhysicsWorld (the one with a lattice on the front page), contains a very positive review of Louis Bloomfield's "How Everything Works", which is said to be a general-market version of the author's textbook "How Things Work" for physics courses for non-scientists.

Lattice QCD makes title page

2006-12-07T19:28:00.001Z

The latest issue of PhysicsWorld has a feature article on Lattice QCD by Christine Davies describing the recent progress made in confronting theory with experiment through unquenched lattice simulations. Among the highlights she mentions are the correct prediction of the mass of the B_c and the fact that the determinations of the quark masses and the strong coupling constant α_s from unquenched lattice QCD are now more accurate than all other sources combined.

The article is very well written and should be easily understandable for anyone with a background in physics, and I would think that an informed layperson should also be able to learn something from it.

Axions discovered?

2006-12-07T17:53:00.000Z

There is a forthcoming paper which claims the discovery of two particles decaying into electron pairs, which are tentatively identified as axions, with masses of 7 and 19 MeV respectively.

I am a little sceptical of this claim, based as it is on two narrow peaks being 3 standard deviations above the best fit to the background. I'm no experimentalist, but the situation appears to me to be too similar to the "discovery" of the pentaquarks, which later dissolved into statistical fluctuations as better data became available.

If this discovery turns out to be real, though, this would be huge news: the discovery of the axion would solve the strong CP problem (why is the CP-violating θ-angle in QCD so close to, or even identically, zero, when it generically should be of order one?) and might also contribute to solving the riddle of dark matter. There are a number of experiments looking for the axion, the best known being PVLAS, who claimed to have found a candidate with a mass in the meV (milli-eV) range, and the new experiments at CERN aiming to test their results.

So we should wait and see if axions have indeed been observed. If so, it would be great news for theoretical particle physics: it would be, as far as I am aware, the first discovery of a particle whose existence was conjectured based on naturalness considerations alone. If not, it would show once again that results based on low statistics should always be taken with great care.

Update: More criticism (by experimentalists) of the claimed discovery can be found on Chad Orzel's blog and on Superweak.

Physics Result of the Year, Anyone?

2006-12-07T17:31:00.000Z

Chad Orzel has a poll about which physics result of 2006 deserves to be called The Physics Result Of The Year. Surprisingly, so far nobody seems interested in nominating their favourite result for this honour. Which is strange, because A) the APS has a list from which you could simply pick one, and B) nobody says you can't nominate your own results if you feel so inclined. This isn't the Nobel Prize, after all.

On the lattice QCD front, I'm going to be both bold and modest at the same time and nominate the recent progress in the debate about the validity of the fourth-root trick for staggered fermions as the result of the year. It isn't really a result, because the debate isn't completely resolved, but there are results now where there was mostly just conjecture before, so this is definitely progress.

Anyway, please go Chad's fine blog and nominate your favourite result of the year. You wouldn't want the public to think physicists have no enthusiasm for their own work, would you?

Hadronic Physics from Lattice QCD

2006-12-05T22:12:00.000Z

As a matter of fact, I have no idea how my small circle of reader is composed with respect to physics expertise or professional position, but I like to pretend that some of my readers are physicists with a genuine interest in, but no real experience with, lattice QCD. It is to these (imagined, and perhaps imaginary) readers that I want to issue a book recommendation, just in time for inclusion on their holiday wishlist.

The book in question is "Hadronic Physics from Lattice QCD", edited by Anthony M. Green, published by World Scientific. The aim of this book is to provide an introduction to lattice QCD for non-specialist readers such as nuclear and particle physicists, and while it cannot replace one of the various introductory testbooks (such as Montvay and Münster or Rothe) as required reading for people interested in pursuing original research in the field, I think it succeeds very well at giving the non-specialist a much better idea of the how and what, the strengths and the limitations, of lattice QCD.

The book is a collection of independent chapters by different authors, each of which focusses on a specific issue of interest that can be studied using lattice QCD.

The first chapter, by Craig McNeile, starts with a basic introduction to lattice QCD and its methods, including a discussion of systematic errors including how they can be reduced via unquenching, improved actions and chiral perturbation theory. He then proceeds to give an overview of the masses of stable mesons and baryons that can be measured accurately, as well as an introduction to the use of maximal entropy methods to determine spectral functions from lattice data, and some of the methods used to incorporate electromagnetic effects and to study unstable particles on the lattice, both of which are rather hard problems.

The second chapter, by Chris Michael, is devoted to a discussion of exotics, or states that are neither conventional mesons nor baryons: glueballs, and their mixing with scalar mesons of the same quantum numbers, hybrid mesons (mesons that contain a gluonic excitation along with a quark-antiquark pair), and hadronic molecules (states consisting of several hadrons bound by their residual strong interactions).

The third chapter, by Gunnar Bali, discusses the quark-antiquark potential, starting from the static quark potential and its relation to Wilson loops, the strong coupling expansion on the lattice, the confining string picture and perturbative calculations of the potential, and going on to discuss some aspects of quark-antiquark and nucleon-nucleon potentials for nonstationary particles.

The fourth chapter, by Rudolf Fiebig and Harald Markum, is concerned with the difficult topic of hadronic interactions in lattice QCD. After describing some of the issues that arise in a 2+1 dimensional "toy" model, they discuss the highly sophisticated techniques that are used to extract information on pion-nucleon, nucleon-nucleon and pion-pion interactions from lattice QCD. This chapter has an appendix which describes aspects of improvement of lattice actions, an important ingredient in any lattice project aiming for precise predictions.

The fifth chapter, by Anthony Green, discusses "bridges" between lattice QCD and nuclear physics, such as nuclear effective field theories and potential models that are founded upon, or at least inspired by, QCD.

All chapters have extensive bibliographies that should function as excellent starting places for readers who wish to learn more about the subject.

The arXiv is changing

2006-12-04T19:04:00.000Z

Via Urbano Franca: The arXiv preprint archive is changing the way it labels papers with effect from 1st January 2007. The familiar arch-ive/YYMMNNN identifiers like hep-lat/0411026 will be gone (although they will be retained for old papers), and new identifiers of the form YYMM.NNNN will take their place. The stated reason for this is that the math archive is getting dangerously close to 1000 submissions a month, which would break the existing indentifier system. The new identifiers will no longer be assigned on an archive-by-archive basis; including the archive will be done as in 0701.1234 [hep-lat]. The new system is expected to be good for a number of years, and after that five-digit identifiers YYMM.NNNNN will be needed.

This change appears to be orthogonal to the other announced big change in the physics arXiv, although it is possible that the latter is considered redundant now. A slightly more open information policy on the part of the arXiv might be nice from time to time, but I suspect they are afraid that more openness might offer more inroads to cranks and crackpots, so I kind of understand their policy of semi-secret decision-making. Still, I think it probably couldn't hurt too much if they sent out informative emails to registered authors from time to time.

More on modern Fortran

2006-11-28T20:48:00.000Z

From the echo on my earlier post about why we use Fortran for number crunching applications, I gather that many people still associate Fortran with the worst features of the now mostly obsolete FORTRAN 77 standard (fixed source form, implicit typing) and are mostly unaware of the great strides the development of the Fortran standard has made in the past 30 (sic!) years. So I feel that this might be a good opportunity to talk a little about the advanced features that make Fortran 95 so convenient for developping computational physics applications. [Note that in the following Fortran 95 will be referred to simply as "Fortran" for the sake of brevity.]

To start with, there are Fortran's superior array features which greatly facilitate working with vectors, matrices and tensors: Being able to write vector addition as


a = b + c

instead of some kind of for-loop is a great boon in terms of code legibility. Having the sum, product, dot_product and matmul intrinsic functions available to perform common mathematical operations on arrays using a (hopefully) efficient vendor-supplied math library also helps.

But where Fortran's array features really shine is when it comes to defining elemental functions and subroutines, which save a huge amount of coding. An elemental function or subroutine is one which is defined with scalar dummy arguments, but which fulfils certain technical conditions that allow it to be called with an array passed as the actual argument. When called in this way, an elemental function is evaluated on each element of the passed array argument, and the results are assembled into an array of the same shape. Most of the mathematical functions Fortran provides as intrinsics are elemental. So one can do something like


Pi = 4.*atan(1.)
x = (/ (i*Pi/n,i=0,n) /) ! array constructor with implied do-loop
y = sin(x)               ! an elemental assignment operation

to load y(i) with sin(x(i)) for all i. And better yet, user-defined functions can also be elementary, so you only ever need to write the overloaded operators for your automatically-differentiating spinor type as scalars, and Fortran will take care of dealing with the arrays of those spinors that occur in your code.

Next in usefulness and importance comes the already mentioned support for overloading operators and intrinsic functions on user-defined types. Again, this provides a lot of convenience in terms of maintaining a coding style that stays as close to standard mathematical notation as possible, and of keeping the gory details of complex type operations (such as multiplying two automatically-differentiating spinors) transparent to the programmer/user on the next higher level. The ability to have public and private procedures and variables in modules also helps with this kind of encapsulation.

And that isn't all: namelist I/O provides a cheap kind of configuration files; selected_int_kind and selected_real_kind allow testing whether the system provides sufficient range/precision at compile time; the forall construct allows some measure of parallelization on parallel processors where the compiler supports it.

The next incarnation of the Fortran standard, Fortran 2003, exists only as a standard document at this time, although compiler vendors are beginning to add features from Fortran 2003 to their compilers in a piecewise fashion. Major new features include object orientation (single inheritance, polymorphism and deferred binding) and C interoperability (so you can call C system libraries from Fortran programs, and/or call your Fortran matrix-crunching code from a C application).

And after that, the future Fortran 2008 standard is expected to include co-arrays to support parallel and distributed computing, a bitfield type, a Fortran-specific macro preprocessor, among other things.

Advanced programmers keen to learn about Fortran 2003's new features may want to have a look at the Fortran 95/2003 book by Cohen, Metcalf and Reid. This is the successor to Metcalf and Reid's Fortran 90/95 book, which is still probably the best reference to modern Fortran, as most of the Fortran 2003 features aren't available on most platforms yet, whereas standard Fortran 95 is very portable.

For beginning programmers, or people who have never worked with any flavour of Fortran before, the book by Chapman (which I haven't read personally) may be a better idea from the reviews I've seen, but the reviews also indicate that it is not useful as a reference, so you may have to get the Metcalf et.al. book(s) anyway.

Lattice 2006 proceedings available

2006-11-28T20:23:00.000Z

The proceedings for the Lattice 2006 conference are now available at Proceedings of Science, so you can read the written versions of the talks and get your own idea about most of what was being said and discussed in Tucson, if you weren't there, except for all the bits that had to be left out due to the fairly strict (for an online publication) page limits.

One year on the lattice

2006-11-22T20:30:00.000Z

I just noticed that I let our first anniversary as the world's first and only lattice QCD group blog slip by unnoticed last week. Yes, it has been over a year now since I joined the blogosphere and took over the role of Lead Contributor to "Life on the Lattice" from Matt!
So far, I have thoroughly enjoyed the experience, and I would not hesitate to suggest it to any other lattice field theorist (or any other kind of physicist) who might be thinking about ways to connect to a wider audience.

Lattice Forecast for 2056

2006-11-21T22:02:00.000Z

Via Cosmic Variance and BioCurious: New Scientist has some well-known scientist forecast where science will be in 50 years.

A lot of the predictions are of the kind that people made 50 years ago for today: AIs more intelligent than people, permanent colonies on other planets, immortality drugs, contact with alien civilisations. They haven't come true in the past 50 years, and (exponential growth laws notwithstanding) I see no reason why they should come true in the next 50 years. The other kind of prediction seems much more likely to come true: detection of gravity waves, important discoveries at the LHC, significant progress in neuroscience, solutions for all of the Millennium problems, a firm understanding of dark matter and dark energy, a means to grow human organs in vitro, working quantum computers. And of course, just like nobody 50 years ago predicted the internet or the role of mobile phones in today's world, we should really expect that something completely unexpected will become the leading technology in 50 years.

What really irks me, though, is that there is no forecast from a lattice field theorist. After all, lattice QCD has made huge progress over the past decade, but apparently it isn't sexy enough for New Scientist these days. So here I am going to contribute my own 50-year forecast:

Over the next few decades, parallel computing will make huge advances, with machines that make today's TOP500 look feeble by comparison becoming readily affordable even to smaller academic institutions. As a consequence, large-scale simulations using dynamical chiral fermions will become feasible and will once and for all lay to rest any remaining scepticism regarding the reliability of lattice simulation results.

Predictions of "gold-plated" quantities will achieve accuracies of better than 0.1%, outshining the precision of the experimental results. If the limits of the Standard Model are at all accessible, discrepancies between accurate lattice predictions and experimental results in the heavy quark sector will be a very likely mode of discovering these limits, and will hint at what comes beyond. The use of lattice QCD simulations of nuclear structure and processes will become commonplace, providing a first principles foundation for nuclear physics and largely replacing the nuclear models used today.

On the theoretical side, the discovery of an exact gauge dual to quantum gravity will allow the study of quantum gravity using Monte Carlo simulations of lattice gauge theory, leading to significant quantitative insights into the earliest moments of the universe and the nature of black holes.