tag:blogger.com,1999:blog-86694682014-12-15T14:12:25.985ZLife on the latticeThoughts on lattice QCD, particle physics and the world at large.Georg v. Hippelhttp://www.blogger.com/profile/15355104409462033075noreply@blogger.comBlogger187125tag:blogger.com,1999:blog-8669468.post-89959268251784986602014-11-21T09:42:00.002Z2014-11-22T23:02:51.949ZScientific Program "Fundamental Parameters of the Standard Model from Lattice QCD"Recent years have seen a significant increase in the overall accuracy of lattice QCD calculations of various hadronic observables. Results for quark and hadron masses, decay constants, form factors, the strong coupling constant and many other quantities are becoming increasingly important for testing the validity of the Standard Model. Prominent examples include calculations of Standard Model parameters, such as quark masses and the strong coupling constant, as well as the determination of CKM matrix elements, which is based on a variety of input quantities from experiment and theory. In order to make lattice QCD calculations more accessible to the entire particle physics community, several initiatives and working groups have sprung up, which collect the available lattice results and produce global averages.<br /><br />We are therefore happy to announce the scientific program "<a href="https://indico.mitp.uni-mainz.de/conferenceDisplay.py?confId=28">Fundamental Parameters of the Standard Model from Lattice QCD</a>" to be held from August 31 to September 11, 2015 at the <a href="http://www.mitp.uni-mainz.de/">Mainz Institute for Theoretical Physics (MITP)</a> at <a href="http://www.uni-mainz.de/">Johannes Gutenberg University Mainz</a>, Germany.<br /><br />This scientific programme is designed to bring together lattice practitioners with members of the phenomenological and experimental communities who are using lattice estimates as input for phenomenological studies. In addition to sharing the expertise among several communities, the aim of the programme is to identify key quantities which allow for tests of the CKM paradigm with greater accuracy and to discuss the procedures in order to arrive at more reliable global estimates.<br /><br />We would like to invite you to consider attending this and to apply through our <a href="https://indico.mitp.uni-mainz.de/conferenceDisplay.py?confId=28">website</a>. After the deadline (March 31, 2015), an admissions committee will evaluate all the applications.<br /><br />Among other benefits. MITP offers all its participants office space and access to computing facilities during their stay. In addition, MITP will cover local housing expenses for accepted participants. The MITP team will arrange the accommodation individually and also book the accommodation for accepted participants.<br /><br />Please do not hesitate to contact us at <a href="mailto:coordinator@mitp.uni-mainz.de?Subject=Scientific Program: Fundamental Parameters of the Standard Model from Lattice QCD">coordinator@mitp.uni-mainz.de</a> if you have any questions.<br /><br />We hope you will be able to join us in Mainz in 2015!<br /><br />With best regards,<br /><br />the organizers:<br /><i>Gilberto Colangelo, Georg von Hippel, Heiko Lacker, Hartmut Wittig</i><br />Georg v. Hippelhttp://www.blogger.com/profile/15355104409462033075noreply@blogger.com2tag:blogger.com,1999:blog-8669468.post-61249583368404465312014-06-30T20:39:00.001+01:002014-06-30T20:48:49.417+01:00LATTICE 2014, Day SixThe last day of the conference started out with a sequence of topical talks. First was Massimo D'Elia speaking about Lattice QCD with purely imaginary sources at zero and non-zero temperature. Contrary to what the name might suggest, an imaginary source is a source term that can be coupled ot the action so as to keep e<sup>-S</sup> real and positive. Examples include an imaginary chemical potential, an imaginary θ term, or an external electromagnetic field with a real magnetic or imaginary electric field strength. Applications include the study of the curvature of the critical line near μ=0 and the nature of the Roberge-Weiss phase transition, and the determination of electric dipole moments and the magnetic properties of nuclear matter.<br /><br />Next was Tilo Wettig introducing the QPACE 2. QPACE now stands for "QCD Parallel Computing Engine" (as there is no more Cell processor involved). Each compute card consists of four Xeon Phi Knights Corner processors linked by a PCI Express bus and a weak CPU, which is only used for booting. The compute cards use a novel patented "brick" concept and employ an innovative kind of water cooling. Each rack has a peak performance of 310 TFlops. To run optimally on this architecture, codes will need some adjustments employing ideas such as site fusing, half-precision gauge fields, and the use of lattice sizes with prime factors of 3 and 5, but with optimal use of the SIMD units, scaling is almost perfect. A future successor, QPACE 3, will use Knights Landing units instead of the Knights Corner ones, and should achieve a peak performance of 1 TFlop per rack.<br /><br />This was followed by Masakiyo Kitazawa speaking about measurements of thermodynamics using the gradient flow. The small-flowtime expansion for the gradient flow allows to define a renormalized energy-momentum tensor in terms of the zero-flowtime limit of two flowed dimension-four operators. This has been applied to obtain results for the trace anomaly and the entropy density, but the difficulty lies in finding a plateau region in flow time where both lattice artifacts and finite-volume effects can be neglected, so as to allow a reliable extrapolation to zero flow time.<br /><br />After the coffee break, Chris Sachrajda reviewed the state of the lattice determination of long-distance effects to flavour-changing processes. As no new physics has been discovered by the LHC so far, precision flavour physics is still the most promising avenue in the search for BSM effects. For some quantities in this area, particularly in the field of Kaon physics, long distance effects are of crucial importance. An example is neutral Kaon mass difference Δm<sub>K</sub>=m<sub>K<sub>L</sub></sub>-m<sub>K<sub>S</sub></sub>; this involves four-volume integrals over the expectation value of matrix elements of electroweak operators between hadronic states, raising the problem of how to prepare such hadronic states in this context. The problem can be solved by taking the time integral over a largeish interval, but placing the creation and annihilation operators well outside of the corresponding four-volume. The relevant correlation functions also contain terms growing exponentially with the time extent T, <br />which can be removed by adding suitably tuned terms to the electroweak Hamiltonian. UV divergences are eliminated the GIM mechanism together with the V-A structure of the electroweak currents. With all these theoretical developments in place, a calculation done at unphysical pion and Kaon masses gives a result for Δm<sub>K</sub> close to the physical value (which may of course still be a fortuitous coincidence), and exhibits an apparent violation of the OZI rule in that the contribution from the disconnected diagram is very significant to the final result. Another example given was the decay K<sub>L</sub>->π<sup>0</sup>l<sup>+</sup>l<sup>-</sup>, for which the long-distance effects are known in χPT, and the question addressed by an exploratory study is whether the lattice can do better. Yet another example are the QED corrections to the pion decay constant, which contain IR divergences requiring a proper Bloch-Nordsieck treatment.<br /><br />After some well-deserved applause for the organizers, the conference closed with the invitation to next year's lattice conference in Kobe, Japan, from 14th to 18th July 2014. The IAC also announced that the 2016 lattice conference will be hosted in Southampton, U.K., in the last week of July 2016.<br /><br />As I had to fly back to Germany in the evening (a lecture having to be given on Monday), the posting of this and the previous day's summaries was delayed a little by travel and subsequent jetlag, but I am sure my readers will be delighted to know that I got home safe and sound, and with all my luggage intact.Georg v. Hippelhttp://www.blogger.com/profile/15355104409462033075noreply@blogger.com0tag:blogger.com,1999:blog-8669468.post-49056533417223796242014-06-30T20:02:00.002+01:002014-06-30T20:48:41.790+01:00LATTICE 2014, Day FiveThe first plenary talk of the morning was by Sasa Prelovsek, who gave the review talk on hadron spectroscopy. In this area, the really hot topic is the nature of the XYZ states, such as the Z<sub>c</sub><sup>+</sup>(3900), which decays into J/ψ π<sup>+</sup>, and thus cannot be a simple quark-antiquark bound state. In order to elucidate this question, the variational method has to be used with a basis of operators containing both one- and two-meson operators as well as possible tetraquark operators, and this then requires the use of all-to-all propagators (with distillation now being the most commonly used approach) as well as a Lüscher-type method to treat the multiparticle states. These added difficulties mean that studies in this area are still a bit rough at the moment, with the physical-pion, large-volume and continuum limits generally not yet taken. For the Z<sub>c</sub><sup>+</sup>, Sasa <i>et al.</i> find a candidate state only when including both two-meson and tetraquark operators in their basis. The more charmonium-like states, such as the X(3872), are better studied, and the X(3872) in particular appears likely to be mostly a DD<sup>*</sup> molecule. The greatest challenges in spectroscopy are the mixing between quarkonia and light hadron states, which is still mostly ignored, and the inclusion of more-than-two particle states, for which the theoretical tools aren't quite there yet.<br /><br />A topical talk on new algorithms for finite-density QCD given by Denes Sexty followed. QCD at finite chemical potential μ suffers from the well-known sign problem; while there are a number of methods to evade it (in particular analytically continuing from imaginary μ and Taylor expansion methods), the newer methods attempt to address it directly. One of these is the complex Langevin method, which responds to the complex action by complexifying the fields and noise term in the Langevin equation (which for gauge links means continuing from SU(N) to SL(N,<b>C</b>) and requires some means of restraining the links from wandering off too far into the unphysical part of the group manifold, e.g. by gauge cooling steps interspersed with the dynamical updates). In the past, this method was hampered by a lack of theoretical understanding and the presence of possibly unphysical runaway trajectories; now, it has been established that for holomorphic actions, the complex Langevin time average does converge to the ensemble average. Unfortunately, the action for QCD with a chemical potential is not holomorphic, but some studies indicate that this case may nevertheless be okay. The other new method to directly address the sign problem is the Lefschetz thimble, which relies on shifting the integration contour for the path integral into the complex plane, and for which simulation algorithms exist in the case of various toy models. For the complex Langevin method, there are now a number of results which look promising.<br /><br />This was followed by another topical talk, Alberto Ramos speaking about the applications of the Wilson flow to scale setting and renormalization. It has long been known that the Wilson flow yields renormalized operators, and besides its use in setting the lattice scale, it is now widely used to define a renormalized coupling, where the renormalization scale is set by μ<sup>2</sup>=1/(8t). To avoid the need for a window where both cut-off and finite-volume effects are small, one can tie the renormalization scale to the volume as μ=1/(cL), however, this means that the boundary conditions become relevant. The errors on the Wilson flow coupling are orders of magnitude smaller than those on the Schrödinger functional coupling, but the SF coupling becomes less noisy at small coupling and thus provides information complementary to that from the WF coupling. Cut-off effects are important for Wilson flow obervables, and tree-level improvement has a big effect there. There is a small-flowtime expansion analogous to the OPE, and a fermionic version of the flow can be used to determine the chiral condensate. All in all, this is a very active field of current research.<br /><br />After the coffee break, the Ken Wilson Award was announced. The award goes to Gergely Endrődy for <i>significant contributions to our understanding of QCD matter in strong magnetic fields and to QCD thermodynamics</i>. Gergely gave his prize talk on the topic of QCD in magnetic fields, starting from Hofstadter's butterfly, which is a self-similar fractal describing the energy levels accessible to an electron in a crystal (which tries to enforce Bloch waves) in a magnetic field (which tries to enforce Landau levels). The Dirac operator for a free lattice fermion in a magnetic field has a similar structure, which however disappears in the continuum limit, since the magnetic flux through a plaquette scales as a<sup>2</sup>. The quark condensate is related to the Dirac eigenvalues, and hence contains the same self-similar structure, which is washed out by the quark mass, however. When QCD interactions are turned on, these similarly wash out the fractal structure. What is left over is a growth of the quark condensate with the magnetic field at zero temperature ("magnetic catalysis"). At finite temperature, a similar effect was expected from models, but Gergely <i>et al.</i> have shown that in fact the opposite effect happens ("inverse magnetic catalysis").<br /><br />This was followed by Tetsuya Onogi speaking about a hidden exact symmetry of graphene. Graphene, which is the most conductive material known under terrestrial conditions, has a band structure with a Dirac point resembling the dispersion relation for a massless relativistic fermion, with no gap. The symmetry preserving the vanishing of the gap against perturbations can be derived by treating the actual graphene lattice as a staggered version of a coarser hexagonal lattice, where six sites correspond to six internal degrees of freedom (three flavours, two spins), which then reveals a hidden flavour-chiral symmetry.<br /><br />The afternoon saw the last set of parallel sessions. There were two more talks from members of the Mainz group (PhD student Hanno Horch and former postdoc Gregorio Herdoiza, now a Ramón y Cajal Fellow at the Universida Autónoma de Madrid) on work related to (g-2) and the Adler function.Georg v. Hippelhttp://www.blogger.com/profile/15355104409462033075noreply@blogger.com0tag:blogger.com,1999:blog-8669468.post-27313672773842976052014-06-27T02:51:00.000+01:002014-06-30T20:49:12.319+01:00LATTICE 2014, Days Three and FourWednesday was the "short" day as has been customary for many years now. I gave my own talk in the hadron structure session and got a lot less criticism than I expected; apparently it has been widely accepted by now that excited-state effects can be large in nucleon matrix elements even if naively it looks like there aren't any.<br /><br />In the afternoon, there were no organized excursions, so I spent the afternoon in the Metropolitan Museum and took a walk around Central Park and down Fifth Avenue after it closed.<br /><br />Today was started by the first non-lattice talk, given by Anthony Mezzacappa of the CHIMERA collaboration, who spoke about simulating core collapse supernovae to ascertain the mechanism behind these massive stellar explosions. Core collapse supernovae happen when a very massive star has reached the final stage of its life, in which it has an onion-like structure, with a hydrogen envelope around a helium envelope around further layers of increasingly heavy elements around a central iron core which is about the size of the Earth, but so dense as to be about the mass of the Sun. When this central core becomes so compressed that it can no longer keep from collapsing until it reaches nuclear densities (turning into a neutron star or a stellar black hole as a result), the infall of matter is supersonic, but the bounce back is subsonic (because the speed of sound is higher in the denser matter inside), which causes a shockwave to spread that eventually blows the star apart. However, the real story is more complicated than that, because a lot of energy is radiated away in the form of neutrinos, which may cause the shockwave to become weakened and avoid the explosion. The most important question is therefore how the processes occurring in the star cause the shockwave to revive. The simulations to investigate this are become quite large, requiring on the order of 100 Megacore-hours per second of supernova simulated. To fully include all variables would likely require sustained Exaflops, so the problems are usually simplified. Spherical symmetry is a bad assumption apparently, because it leads to no explosion. Azimuthal symmetry gives an explosion, and the generic three-dimensional case is not quite resolved yet.<br /><br />This was followed by a review of BSM physics from the lattice by Yasumichi Aoki. The main idea investigated in this area is walking technicolor, i.e. the search for a technicolor-type gauge theory that has a very slowly running coupling and large mass anomalous dimension in order to permit both the generation of a realistic mass spectrum for the Standard Model fermions and the suppression of flavour-changing neutral currents to a level compatible with experiment. Another problem is to have a light Higgs and no other light unobserved particles. A number of theories under investigation show spectra compatible with this, with the scalar much lighter than the pseudoscalar (as opposed to QCD, where the pion is much ligher than the σ resonance).<br /><br />After the coffee break, we had the experimental talk, by Brendan Casey on the FNAL E989 experiment and the anomalous magnetic moment of the muon. To understand the hadronic contributions much more work is needed, both on the theory side (where the work of my collaborators Anthony Francis and Vera Gülpers received well-deserved praise) and in experiment (where the R-ratio needs to be determined to sub-percent level, and where KLOE will investigate the leading contributions to hadronic light-by-light scattering). The new Fermilab (g-2) experiment is designed specifically to address many of the remaining sources of experimental error on the value (g-2) itself; the effort to get there has been quite impressive, with the pictures showing very nicely what kinds of huge projects even such relatively "small" experiments are.<br /><br />The next talk was Antonin Portelli speaking about electromagnetic and isospin-breaking effects in lattice QCD. While isospin is a reasonably good symmetry of the strong interactions, it is broken at the sub-percent level, and the proton-neutron mass difference is an essential ingredient of the stability of matter. Understanding isospin-breaking effects (both from electromagnetism and from the difference between the up and down quark masses) is therefore a crucial endeavour for lattice theorist in the longer term. A number of collaborations are now simulating QCD+QED dynamically. Since QED does not have a mass gap, it tends to show long autocorrelations in Monte Carlo time; a new HMC Hamiltonian introduced by the BMW collaboration appears to get rid of this effect. The electromagnetic mass differences within the baryon octet are nicely reproduced by now, and the origin of the nucleon mass difference seems to become understood. For some reason, the Ξ<sub>cc</sub> mass difference is also of great interest to phenomenologists, and has also been computed on the lattice.<br /><br />The last plenary of the morning was a review of quark masses by Francesco Sanfilippo. He stressed the importance of ratios of quark masses (where in a mass-independent scheme, the ratio of renormalized masses equals that of the bare ones, avoiding the need for accurate knowledge of renormalization constants), and reviewed a number of methods that have been used to determine heavy quark masses, including the HPQCD method of using moments of current-current correlators, the use of NRQCD with perturbative subtractions and of non-perturbative HQET, as well as the ETMC ratios method. In the light sector, simulations are now done close to the physical point, and the isospin-breaking u-d mass difference is being investigated in a realistic manner.<br /><br />In the afternoon, there were parallel sessions again. Besides some NRQCD talks, incuding a very nice talk on bottomonium spectroscopy using free-form smearing, I attended a number of talks on the gradient flow.<br /><br />In the evening, there was the dinner cruise for those who had bought tickets. I hadn't and, having waived any claim to a left-over free ticket so interested others could attend instead, arranged otherwise for dinner.Georg v. Hippelhttp://www.blogger.com/profile/15355104409462033075noreply@blogger.com0tag:blogger.com,1999:blog-8669468.post-23735369969549601932014-06-25T03:08:00.001+01:002014-06-25T03:08:30.376+01:00LATTICE 2014, Day TwoHello again from New York. The first plenary of the morning was given by Nicolas Garron speaking about K/π physics. After a summary of the most recent updates on the decay constants of the pion and Kaon and their ratio f<sub>K</sub>/f<sub>π</sub>, as well as the zero momentum transfer form factor f<sub>+</sub>(0) (which are increasingly so precise that the question of when the precision was enough was raised from the audience after the talk), he proceeded to discuss the general theory of CP violation in neutral Kaon mixing and the ΔI=1/2 rule in K->ππ decays, and the ways in which lattice calculations are needed to understand these topics. A number of recent updates on the Kaon bag parameter B<sub>K</sub> were summarized, and the renormalization and mixing of the BSM operators entering neutral Kaon mixing (for which Mauro Papinutto showed some impressive results in one of the parallel sessions) were discussed. Finally, RBC/UKQCD now have results on the ΔI=1/2 and ΔI=3/2 amplitudes in K->ππ decays at the physical pion mass, which strongly support the ΔI=1/2 rule at a level compatible with phenomenology.<br /><br />This was followed by a talk on a somewhat related topic, namely Stephan Dürr speaking about the question of whether the validity of χPT extends even to the physical pion mass. Contrary to the often-quoted theorem that the answer to any title with a question mark in it is "no", the answer was "yes" in this case. While the chiral expansion breaks down completely at pion masses of around 500 MeV (where NNLO corrections grow to be larger than the NLO ones), two different analyses (one using staggered, and one using Wilson fermions) that Stephan showed indicate that the NLO low-energy constants can be extracted in a reasonably consistent manner from fits in the range M<sub>π</sub>=135-400 MeV. However, the low-energy constant l<sub>4</sub> showed a significant sensitivity to the range of pion masses used to fit.<br /><br />The last talk before the coffee break was on Multigrid methods for lattice QCD and was given by Andreas Frommer. Multigrid methods have a long history in applied mathematics, where they are used more commonly in the context of finite-element methods (rather than the finite-difference approach used in lattice field theory). The basic ingredients from the applied mathematics point of view are a smooting operation together with restriction and prolongation operations that allow to reduce the size of the problem to a level where it can be solved directly, and then to retrieve the solution of the original problem from this. Interestingly, this was somewhat reinvented in a way tuned to lattice QCD from the physics side, where Lüscher's inexactly deflated SAP-preconditioned GCR that is part of the DD-HMC and openQCD packages forms a two-level multilevel scheme that leads to a great improvement in runtime behaviour as the quark mass is decreased. The Wuppertal applied mathematics group has extended this to a generic multilevel scheme for QCD (where it is found that three levels are even better than two at small quark masses, but four seem not to help appreciably more). From the mathematical side, most of the existing multigrid theory does not apply to QCD, however, so further mathematical research seems required to fully understand why and when these approaches work for QCD.<br /><br />After the coffee break, Raul Briceno spoke about few-body physics. In this area, significant theoretical progress seems to have been made recently and still to be under way, extending Lüscher's finite-volume formalism for scattering phase shifts in various directions.<br /><br />This was followed by a talk on the closely related and somewhat overlapping topic of hadronic interactions by Takeshi Yamazaki, who presented recent results for various scattering lengths and phase shifts, as well as reviewing the alternative HALQCD method, which relies on reconstructing an interaction potential from multi-particle correlators.<br /><br />In the afternoon there were parallel sessions again. I got to chair the session on renormalization from the Schrödinger functional approach, where there has been significant progress on the chirally rotated SF and on studying the mixing of four-quark operators. Another very interesting session later in the afternoon was concerned with the various methods to get at quark-disconnected contributions to hadron structure observables, and some of the results obtained using them.<br /><br />In the evening, the poster session took place.<br />Georg v. Hippelhttp://www.blogger.com/profile/15355104409462033075noreply@blogger.com0tag:blogger.com,1999:blog-8669468.post-74274838996299103912014-06-24T03:17:00.001+01:002014-06-24T03:19:29.014+01:00LATTICE 2014, Day OneHello, faithful readers, and a cordial welcome to the annual lattice conference blog, this time form New York, where I arrived two days early in order to beat the jet lag. The jet-lag adjustment days were well-spent in the Metropolitan Museum.<br /><br />The conference started with a reception (a very exclusive event, admission to which was controlled by rather fierce security guards, who at first wouldn't even let us into the building) on Sunday night.<br /><br />Since the plenary talks will be livestreamed at <a href="http://livestream.com/">livestream.com</a> (search for "Lattice2014"), you don't have to rely on my summaries of the talks this time, and in fact I would like to encourage you to cross-check them and post about anything you feel I missed or misrepresented in the comment section (please note that comments are moderated, so it may take a while for yours to turn up).<br /><br />After a brief opening address by the Vice-President of Columbia University, the first plenary talk of the conference was given by Martha Constantinou, who gave a review talk on hadron structure. The most active subfield in this area is nucleon structure, to which accordingly the greater part of her talk was devoted. A crucial quantity there is the axial charge g<sub>A</sub> of the nucleon, which a number of groups have been investigating using a number of methods. (Since I have been involved in the Mainz effort on this front, I am certainly somewhat biased, so take what follows with a grain of salt.) Martha very nicely explained the existing results and discussed the sources of error in detail, but I'm afraid I have to slightly disagree with some of her assessments, in particular regarding excited-state effects (which I believe to be more important) and finite-volume effects (where I think that M<sub>π</sub>L>4 is required to be on the safe side). An interesting development is the Feynman-Hellmann approach, where a term coupling to the current of interest (the axial current in this case) is added to the action, and derivatives of the nucleon mass are taken with respect to the coefficient of that term in order to get at the matrix element of the current; this appears to allow for high statistical precision. Another area of high activity are the nucleon electromagnetic form factors (for which I also believe excited-state effects to be far more important than thought so far). Here, the disconnected contributions relevant for the proton (rather than isovector) form factors are now being computed by some groups, which requires very high statistics (O(100,000) was mentioned) and/or some clever new ideas (like hierarchical probing). For the quark momentum fraction <x>, the importance of excited-state effects is uncontroversial, but the dominant error remains the renormalization. There are also increasingly results for the nucleon spin decomposition, although there are some open problems here, in particular with regards to the gluon angular momentum contributions and the resulting mixing. Beyond the nucleon, first results for hyperon form factors are now available. Further quantities discussed were the pion <x> and the electromagnetic form factors of the ρ meson (there are three of them). Overall, simulations at or near the physical pion mass are now removing the uncertainties from chiral extrapolations (and discretisation effects appear to be small in many nucleonic quantities), so that the confrontation with experiment becomes more acute, requiring full control of all other sources of error.<br /><br />This was followed by another review talk, on heavy flavours, given by Chris Bouchard. The decay constant of the D<sub>s</sub> meson has been the subject of much interest in the past, when a theory-experiment tension seemed to indicate a potential for new physics; that tension has mostly passed, but as a consequence there are now many recent results for f<sub>D<sub>s</sub></sub>, which tend to meet an accuracy target of 1% required to have an impact at the level of experimental precision expected for 2020. For the decay constants of the B and B<sub>s</sub> mesons, there are now results from many different formulations (NRQCD, HQET, Fermilab, heavy HISQ, ratios with heavy twisted mass quarks), which all agree quite well. The extraction of V<sub>cs</sub> from semileptonic decays suggest a small tension with that using f<sub>D<sub>s</sub></sub>, much as there is still some tension between the exclusive and inclusive determinations of V<sub>ub</sub> and V<sub>cb</sub>. In testing for possible new physics, both rare decays (i.e. those that can occur only at the loop level in the Standard Model) and the mixing of neutral heavy-flavour mesons with the antiparticles are of particular relevance. Apparently, a recent calculation of D<sup>0</sup> mixing by ETMC is enough to exclude new physics contributions up to scales as high as thousands of TeV.<br /><br />After the coffee break, Michael Müller-Preussker gave a talk in memory of Pierre van Baal (1955-2013), reviewing recent results on topology on the lattice. Since the topological properties of field configurations are defined in terms of winding numbers of maps between continuous spaces, the definition of topological quantities on the lattice (which is after all discrete) can be ambiguous. Techniques that are used include the direct approach (using a discretisation of the continuum topological charge density and relying on some smoothing operation, such as link smearing, cooling or more recently the gradient flow, to bring the fields close enough to the continuum to make the topology unambiguous), the approach via the Atiyah-Singer index theorem (using the index of a Ginsparg-Wilson Dirac operator to define the topological charge), and the approach via spectral projectors (about which I unfortunately know more or less nothing).<br /><br />The following talk was the review talk on finite-temperature (at vanishing chemical potential) results, which was given by Alexei Bazavov. In keeping with the location of the conference, he showed the Columbia plot before turning to results at the physical point, where the transition is a crossover and the transition temperature hence not so clearly defined. However, when looking for the peak of the chiral susceptibility, the results from different staggered formulations and more recently from domain-wall fermions at the physical pion mass agree quite well. An interesting observation appeared to be that in order for lattice results to match up with hadron resonance gas model predictions, the hadron resonance gas apparently also has to include the "missing states" predicted by quark models, but not observed experimentally. Other results presented included a new method to determine the equation of state using shifted boundary conditions, and numerous new results for the heavy-quark potential and quarkonium spectral functions.<br /><br />In the afternoon there were parallel sessions. I would like to highlight the (first of two) sessions dedicated to lattice results on the anomalous magnetic moment of the muon. There are now a number of different methods of getting at the leading hadronic contribution: by direct determination of the hadronic vacuum polarization, via a mixed-representation approach (where the subtracted vacuum polarization is expressed as an integral over the vector correlator), and from moments of current-current correlators. While in principle all of these process the same information (which is after all encoded in the vector-vector correlation functions), they seem to have different strengths and weaknesses. A first lattice estimate of the systematic error incurred by neglecting disconnected diagrams (whose contribution cannot yet be resolved with the currently available statistics) was presented by Mainz PhD student Vera Gülpers.<br />Georg v. Hippelhttp://www.blogger.com/profile/15355104409462033075noreply@blogger.com0tag:blogger.com,1999:blog-8669468.post-18239950591897402612013-07-01T13:39:00.001+01:002013-07-01T14:30:32.555+01:00LATTICE 2013 - The biggest Lattice conference so farWith 510 registered participants and 476 submitted contributions (not counting the invited plenary talks), LATTICE 2013 is shaping up to be by far the biggest Lattice conference ever (at least so far). While this is of course great news for all lattice people (since it shows the rapid growth of the field) and a great honour for us as organizers, it also means that the parallel programme is under a lot of pressure. We have had to organize additional rooms for parallel sessions and to move some talks to a different topical stream than the one they were submitted under, but in the end there was no way to avoid having to move a few parallel talks to the poster session (which itself is under a lot of pressure given the finite volume of the exhibition hall); if you are one of the authors concerned by such rearrangements, we trust you will understand that there was no other way.<br /><br />Likewise, we hope that all participants will be forgiving of unavoidable clashes between talks that are of equal interest to them. We have taken great efforts to avoid such situations, but given the various additional constraints (such as speakers only being present for part of the week and sessions likely to meet with greater interest having to be put into larger rooms) it is impossible to avoid all potential clashes. The same applies to those speakers whose requests for a rescheduling of their talk to a more convenient time slot could not be fulfilled -- we have tried our best, but there is a limit to the number of times a programme with seven simultaneous parallel streams forming sixty-six parallel sessions can be rearranged to accommodate a single individual.<br /><br />The parallel and poster programme is now finalized and will go to the printers soon. The only changes still possible will be cancellations (which we would greatly regret) and swaps (which should be arranged between the two speakers concerned and communicated to us by email to <b>submission@lattice2013.uni-mainz.de</b>). Any such changes received after Wednesday, 3rd July 2013, will not make it into the printed programme, but will of course be shown in the web version and advertized by flyers, slides and pin-board notices during the conference.<br /><br />Finally, the large number of participants means that some queues at the conference office and at lunchtime will be unavoidable, so a certain amount of patience may be required in these situations. We will try our best to reduce waiting times as much as feasible, but 510 people is quite a lot after all.<br /><br />We look forward to hosting you all in Mainz!Georg v. Hippelhttp://www.blogger.com/profile/15355104409462033075noreply@blogger.com2tag:blogger.com,1999:blog-8669468.post-20991716358182616562013-06-07T21:28:00.000+01:002013-07-01T13:39:50.858+01:00Lattice 2013 - Upcoming deadlineThis is a reminder that the deadline for <a href="https://express.converia.de/frontend/index.php?page_id=1169&do=lps._">abstract submission</a> and <a href="https://express.converia.de/frontend/index.php?page_id=1172">registration</a> for the LATTICE 2013 conference is at 23:59:59 Mainz local time on Saturday, 15th June 2013. Note that due to time zone differences, German midnight might correspond to an earlier time of day for you.<br /><br />Please also note that the booking deadline for most of the hotel pre-reservations we have made will also expire on 15th June -- for further information see <a href="http://www.lattice2013.uni-mainz.de/78_ENG_HTML.php">the conference website</a>. As the pre-reserved rooms may no longer be available after June 15, it might be advisable to make your reservation soon if you wish to take advantage of the special rates arranged for conference participants.Georg v. Hippelhttp://www.blogger.com/profile/15355104409462033075noreply@blogger.com0tag:blogger.com,1999:blog-8669468.post-74830754913205945352013-05-01T11:55:00.003+01:002013-05-01T11:55:57.457+01:00Lattice 2013 - Third CircularAbstract submission for the Lattice 2013 conference, which will be held in Mainz, Germany, from Monday, 29 July 2013, to Saturday, 3 August 2013, is now open. You can follow the "ONLINE REGISTRATION" link from the <a href="http://www.lattice2013.uni-mainz.de/">conference website</a> to submit your abstract, or just follow <a href="https://express.converia.de/frontend/index.php?folder_id=266&ses_id=7cb215a8ab06797245646fa4ec95c5b4">this link</a>.<br /><br /><b><a href="http://www.lattice2013.uni-mainz.de/39_ENG_HTML.php">Fees</a> and <a href="http://www.lattice2013.uni-mainz.de/55_ENG_HTML.php">Deadlines</a></b><br /><br />The Early Bird conference fee of EUR 330 is still available until Wednesday, 15 May 2013. After this deadline, the fee rises to EUR 400.<br /><br />The fee for an accompanying person is EUR 150.<br /><br />Participants who have been approved for the reduced conference fee are reminded that the reduced fee must be paid by Wednesday, 15 May 2013, and that otherwise the regular fee of EUR 400 will have to be paid.<br /><br />The deadline for both registration and abstract submission is Saturday, 15 June 2013.<br /><br /><b><a href="http://www.lattice2013.uni-mainz.de/37_ENG_HTML.php">Scientific Programme</a></b><br /><br />We are in the course of arranging an interesting and varied plenary programme. <br /><br />For more information on the scientific programme please refer to our <a href="http://www.lattice2013.uni-mainz.de/37_ENG_HTML.php">website</a>, which will be updated regularly.<br /><br /><b><a href="http://www.lattice2013.uni-mainz.de/79_ENG_HTML.php">Travel</a>, <a href="http://www.lattice2013.uni-mainz.de/76_ENG_HTML.php">Visa</a> and <a href="http://www.lattice2013.uni-mainz.de/78_ENG_HTML.php">Accommodation</a></b><br /><br />Mainz is located extremely conveniently for international visitors: Frankfurt Airport (FRA), which is served by over 500 flights each day, is located only 30 minutes from Mainz on a direct local train service.<br /><br />For budget flights from and to many European destinations, the airport Frankfurt-Hahn (HHN, served by Ryanair) is connected to Mainz by a non-stop shuttle bus.<br /><br />For more details on how to get to Mainz, please refer to our <a href="http://www.lattice2013.uni-mainz.de/79_ENG_HTML.php">website</a>.<br /><br />Most participants will not require a visa to enter Germany. If you are unsure whether you might need a visa, please refer to the <a href="http://goo.gl/2n0A2">German Foreign Office website</a> for information.<br /><br />If you require a visa, please let the LOC know as soon as possible by email to <a href="mailto:visaletters@lattice2013.uni-mainz.de?Subject=LATTICE2013 - request for invitation, YOUR NAME">visaletters@lattice2013.uni-mainz.de</a> with the Subject: "LATTICE2013 - request for invitation, YOUR NAME" so that we can issue you with a letter of invitation. Please do not forget to include your postal address, and keep in mind that both the international delivery of letters and the processing of visa applications takes some time.<br /><br />Hotel reservations have to be made directly with the hotel of your choice. Our <a href="http://www.lattice2013.uni-mainz.de/78_ENG_HTML.php">website</a> provides information and links to local hotels offering special rates for the participants of Lattice 2013. Please note that the deadlines for the booking of accommodation vary between the different hotels.<br /><br />All on-campus guest rooms are now fully booked. Another low-cost option for participants with very small budgets is the <a href="http://www.jugendherberge.de/en/hostels/search/portrait/jh.jsp?IDJH=411">Mainz Youth Hostel</a>, which can be found at the bottom of the "Accommodation" section of our <a href="http://www.lattice2013.uni-mainz.de/78_ENG_HTML.php">website</a>.<br /><br /><b><a href="http://www.lattice2013.uni-mainz.de/80_ENG_HTML.php">Venue</a> and Organization</b><br /><br />A <a href="http://www.lattice2013.uni-mainz.de/59_ENG_HTML.php">welcome reception</a> and registration will be held on the evening of Sunday, 28 July 2013, from 6:00 pm to 9:00 pm at the bar/restaurant "<a href="http://maps.google.de/maps?f=q&source=embed&hl=de&geocode=&q=proviantamt+mainz&aq=&sll=49.954362,7.31029&sspn=2.672001,4.345093&ie=UTF8&hq=proviantamt&hnear=Mainz,+Rheinland-Pfalz&t=m&ll=49.999547,8.265109&spn=0.004138,0.010686&z=15&iwloc=A">Proviant-Magazin</a>" in the city centre of Mainz.<br /><br />The conference <a href="http://www.lattice2013.uni-mainz.de/71_ENG_HTML.php">programme</a> starts in the morning of Monday, 29 July 2013, and ends at lunchtime on Saturday, 3 August.<br /><br />The conference will be held on the campus of the <a href="http://www.uni-mainz.de/">University of Mainz</a>, which is conveniently located close to the city centre and can be reached easily using public transportation. A public transportation ticket valid during the conference will be included as part of the name tag.<br /><br />A <a href="http://www.lattice2013.uni-mainz.de/108_ENG_HTML.php">conference desk</a> will be open for registration and enquiries during the entire duration of the conference.<br /><br />Lunch will be served on campus in the university mensa, where a separate seating area for conference participants will be available. Meals will be paid using the mensa card contained in the registration package, which can also be charged and used to pay for snacks at the local cafeteria if so desired.<br /><br />The afternoon of Wednesday, 31 July has been allocated for <a href="http://www.lattice2013.uni-mainz.de/62_ENG_HTML.php">excursions</a>, and you will be requested to select your choice of excursion when registering online. Options include guided tours of Mainz, Frankfurt, and Heidelberg, as well as a wine-tasting trip to the Rheingau, and a tree climbing adventure. Please note that for some excursions, only a limited number of places is available and that these will be allocated on a first-come, first-served basis.<br /><br />The <a href="http://www.lattice2013.uni-mainz.de/61_ENG_HTML.php">conference dinner</a> will take place at 8:00 pm on Thursday, 1 August at the Electoral Palace ("<a href="https://maps.google.de/maps?f=q&source=embed&hl=de&geocode=&q=Kurf%C3%BCrstliches+Schlo%C3%9F,+Peter-Altmeier-Allee,+Mainz&aq=1&oq=kurf%C3%BCrstliches+schloss+&sll=49.954362,7.31029&sspn=2.672001,4.345093&ie=UTF8&hq=Kurf%C3%BCrstliches+Schlo%C3%9F,+Peter-Altmeier-Allee,+Mainz&t=m&ll=50.006967,8.270559&spn=0.005516,0.010686&z=15&iwloc=A">Kurfürstliches Schloss</a>") in Mainz. Details about the location and menu can be found on our website, where you can also find information about Mainz <a href="http://www.lattice2013.uni-mainz.de/138_ENG_HTML.php">restaurants, bars and cafes</a>, as well as further touristic opportunities and <a href="http://www.lattice2013.uni-mainz.de/86_ENG_HTML.php">local attractions</a>.<br /><br /><b>Contact Information</b><br /><br />More information can be found on the <a href="http://www.lattice2013.uni-mainz.de/">conference web site</a> , which is updated regularly.<br /><br />If you need to contact us, please email the <a href="mailto:info@lattice2013.uni-mainz.de">Conference Secretariat</a>.<br /><br /><b>Other Workshops</b><br /><br />Participants of Lattice 2013 might also consider attending the workshop "<a href="http://www.xqcd13.unibe.ch/">Extreme QCD (XQCD)</a>", which will be held in Bern/Switzerland, from 5 to 7 August 2013.<br /><br />Another QCD-related meeting being held in Europe in close temporal proximity to Lattice 2013 will be the workshop on "<a href="http://www.ectstar.eu/node/97">Nucleon Matrix Elements for New-Physics Searches</a>" at the ECT* in Trento/Italy, from 22 to 26 July 2013.<br /><br />We are looking forward to seeing you in Mainz.<br /><br />The Lattice 2013 Local Organizing Committee,<br /><i>Georg von Hippel, Harvey B. Meyer, Owe Philipsen, Lorenz von Smekal,<br />Carsten Urbach, Marc Vanderhaeghen, Marc Wagner, Hartmut Wittig (chair)</i><br />Georg v. Hippelhttp://www.blogger.com/profile/15355104409462033075noreply@blogger.com2tag:blogger.com,1999:blog-8669468.post-35141170654186888672013-04-02T19:24:00.002+01:002013-04-23T12:12:57.933+01:00Lattice 2013 - Second CircularOnline registration for the Lattice 2013 conference, which will be held in <a href="http://en.wikipedia.org/wiki/Mainz">Mainz</a>, Germany, from Monday, 29 July 2013, to Saturday, 3 August 2013 is now open. You can follow the "ONLINE REGISTRATION" link from the <a href="http://www.lattice2013.uni-mainz.de/">conference website</a> to register, or just follow <a href="https://express.converia.de/frontend/index.php?folder_id=266&ses_id=7cb215a8ab06797245646fa4ec95c5b4">this link</a>.<br /><br /><b><a href="http://www.lattice2013.uni-mainz.de/39_ENG_HTML.php">Fees</a> and <a href="http://www.lattice2013.uni-mainz.de/55_ENG_HTML.php">Deadlines</a></b><br /><br />The Early Bird conference fee is EUR 330.<br /><br />The Early Bird Registration deadline is Wednesday, 15 May 2013.<br /><br />After the Early Bird deadline, the fee rises to EUR 400.<br /><br />The fee for an accompanying person is EUR 150.<br /><br />A reduced conference fee of EUR 200 will be available upon application for students and other participants with very limited financial resources. Please email <a href="mailto:financial-support@lattice2013.uni-mainz.de?Subject=LATTICE2013 - reduced fee application, YOUR NAME">financial-support@lattice2013.uni-mainz.de</a> with the Subject: "LATTICE2013 - reduced fee application, YOUR NAME" to apply.<br /><br />The deadline for reduced fee applications is Monday, 15 April 2013, and the reduced fee must be paid before Wednesday, 15 May 2013; otherwise the regular Late fee of EUR 400 will have to be paid.<br /><br />Abstract submission will open on 1 May 2013.<br /><br />The deadline for both registration and abstract submission is Saturday, 15 June 2013.<br /><br /><a href="http://www.lattice2013.uni-mainz.de/78_ENG_HTML.php"><b>Accommodation</b></a><br /><br />Hotel reservations have to be made directly with the hotel of your choice. Our <a href="http://www.lattice2013.uni-mainz.de/78_ENG_HTML.php">web site</a> provides information and links to local hotels offering special rates for the participants of Lattice 2013.<br /><br />Please note that deadlines for the booking of accommodation vary among the different hotels, and that the cheaper hotels tend to have earlier deadlines.<br /><br />A very limited number of guest rooms on campus is available at low cost for participants who have been approved for financial support. Applications for such rooms can be made only after the reduced fee has been paid, and will be filled on a first-come, first-served basis.<br /><br /><b>Visa Requirements</b><br /><br />Germany does not require visas from EU/EEA citizens for stays of any duration or for any purpose. Citizens of Australia, Brazil, Canada, Israel, Japan, New Zealand, South Korea, Taiwan, the US, and some others will also not require a visa for stays of up to 90 days.<br /><br />More information can be found on the website of the <a href="http://www.auswaertiges-amt.de/EN/">German Foreign Office</a>. <br /><br />If you require a visa, please let the LOC know as soon as possible by email to <a href="mailto:visaletters@lattice2013.uni-mainz.de?Subject=LATTICE2013 - request for invitation, YOUR NAME">visaletters@lattice2013.uni-mainz.de</a> with the Subject: "LATTICE2013 - request for invitation, YOUR NAME" so that we can issue you with a letter of invitation.<br /><br />Please remember that both the delivery of the letter by mail from Germany to your country and the processing of your visa application will take some time. <br /><br /><b>Venue and Organization</b><br /><br />On-site registration and a <a href="http://www.lattice2013.uni-mainz.de/59_ENG_HTML.php">welcome reception</a> will be held on the evening of Sunday, 28 July 2013, from 6:00 pm to 9:00 pm at the bar/restaurant "Proviant-Magazin" in the city centre of Mainz.<br /><br />The conference programme starts in the morning of Monday, 29 July 2013, and ends at lunchtime on Saturday, 3 August.<br /><br />The conference will be held on the campus of the <a href="http://www.uni-mainz.de/">University of Mainz</a>, which is conveniently located close to the city centre and can be reached easily using public transportation. A public transportation ticket valid during the conference will be included as part of the name tag.<br /><br />Lunch will be served on campus in the university mensa, where a separate seating area for conference participants will be available. Meals will be paid using the mensa card contained in the registration package, which can also be charged and used to pay for snacks at the local cafeteria if so desired.<br /><br /><a href="http://www.lattice2013.uni-mainz.de/38_ENG_HTML.php"><b>Social Programme</b></a><br /><br />The afternoon of Wednesday, 31 July has been allocated for <a href="http://www.lattice2013.uni-mainz.de/62_ENG_HTML.php">excursions</a>, and you will be requested to select your choice of excursion when registering online.<br /><br />Options include guided tours of Mainz, Frankfurt, and Heidelberg, as well as a wine-tasting trip to the Rheingau, and a tree climbing adventure.<br /><br />Please note that for some excursions, only a limited number of places is available and that these will be allocated on a first-come, first-served basis.<br /><br />The <a href="http://www.lattice2013.uni-mainz.de/61_ENG_HTML.php">conference dinner</a> will take place at 8:00 pm on Thursday, 1 August at the Electoral Palace ("Kurfürstliches Schloss") in Mainz.<br /><br />For other touristic opportunities and local attractions, please check our <a href="http://www.lattice2013.uni-mainz.de/86_ENG_HTML.php">web site</a>.<br /><br /><b>Contact Information</b><br /><br />More information can be found on the <a href="http://www.lattice2013.uni-mainz.de/">conference web site</a>, which will be updated regularly.<br /><br />If you need to contact us, please email the <a href="mailto:info@lattice2013.uni-mainz.de">Conference Secretariat</a>.<br /><br /><b>Other Workshops</b><br /><br />Participants of Lattice 2013 might also consider attending the workshop "<a href="http://www.xqcd13.unibe.ch/">Extreme QCD (XQCD)</a>", which will be held in Bern/Switzerland, fom 5 - 7 August 2013.<br /><br />Another QCD-related meeting being held in Europe in close temporal proximity to Lattice 2013 will be the workshop "<a href="http://www.ectstar.eu/node/97">Nucleon Matrix Elements for New-Physics Searches</a>" at the ECT* in Trento, Italy, from 22 to 26 July 2013.<br /><br />We are looking forward to seeing you in Mainz.<br /><br />The Lattice 2013 Local Organizing Committee,<br /><i>Georg von Hippel, Harvey B. Meyer, Owe Philipsen, Lorenz von Smekal,<br />Carsten Urbach, Marc Vanderhaeghen, Marc Wagner, Hartmut Wittig (chair)</i><br />Georg v. Hippelhttp://www.blogger.com/profile/15355104409462033075noreply@blogger.com0tag:blogger.com,1999:blog-8669468.post-89121358966089980002013-01-31T14:03:00.001Z2013-02-01T11:12:08.585ZLattice 2013 - First CircularLattice 2013, the 31st International Symposium on Lattice Field Theory will be held in <a href="http://en.wikipedia.org/wiki/Mainz">Mainz</a>, Germany, from Monday, 29 July 2013, to Saturday, 3 August 2013.<br /><br />The conference will be held on the campus of the <a href="http://www.uni-mainz.de/eng/index.php">University of Mainz</a>, which is conveniently located close to the city centre and can be reached easily using public transportation. All plenary and parallel sessions will take place in the same <a href="http://www.lattice2013.uni-mainz.de/116_ENG_HTML.php">building</a>.<br /><br />Registration and <a href="http://www.lattice2013.uni-mainz.de/59_ENG_HTML.php">reception</a> will be held on the evening of Sunday, 28 July 2013, from 6:00 pm to 9:00 pm at the bar/restaurant "<a href="http://www.proviant-magazin.de/">Proviant-Magazin</a>" in the city centre of Mainz.<br /><br />The conference programme starts in the morning of Monday, 29 July 2013, and ends at lunchtime on Saturday, 3 August.<br /><br />Mainz is <a href="http://www.lattice2013.uni-mainz.de/79_ENG_HTML.php">conveniently located</a> and can be reached in 30 minutes by a direct <a href="http://en.wikipedia.org/wiki/S8_%28Rhine-Main_S-Bahn%29">local train</a> from <a href="http://www.frankfurt-airport.com/content/frankfurt_airport/en.html">Frankfurt airport</a>.<br /><br />More information can be found on the <a href="http://www.lattice2013.uni-mainz.de/">conference web site</a>, which will be updated regularly. If you need to contact us, please email the <a href="mailto:info@lattice2013.uni-mainz.de">Conference Secretariat</a>.<br /><br /><b><a href="http://www.lattice2013.uni-mainz.de/55_ENG_HTML.php">Important Deadlines</a></b><br /><br />The Early Bird Registration deadline is Wednesday, 15 May 2013.<br />The Registration and Abstract Submission deadline is Saturday, 15 June 2013.<br /><br />Registration and abstract submission will proceed via the conference web page, and an announcement will be made when these features become available.<br /><br />Hotel reservations have to be made directly with the hotel of your choice. Our web site will provide information and links to local hotels offering special rates for the participants of Lattice 2013. Deadlines for the booking of accommodation vary among the different hotels. Full details will be provided when the list of hotels is online.<br /><br /><b>Programme</b><br /><br />The programme will include plenary talks, parallel talks and a poster session on the following topics:<br /><ul><li> Algorithms and machines</li><li> Applications beyond QCD</li><li> Chiral symmetry</li><li> Hadron spectroscopy and interactions</li><li> Hadron structure</li><li> Nonzero temperature and density</li><li> Standard model parameters and renormalization</li><li> Theoretical developments</li><li> Vacuum structure and confinement</li><li> Weak decays and matrix elements</li></ul><br /><b>International Advisory Committee</b><br /><br />Gert Aarts (Swansea)<br />Sinya Aoki (U. Tsukuba and Kyoto U.)<br />Norman Christ (Columbia U.)<br />William Detmold (MIT)<br />Zoltan Fodor (Wuppertal)<br />Philippe de Forcrand (ETH Zürich and CERN)<br />Margarita Garcia Perez (IFT Madrid)<br />Anna Hasenfratz (U. Colorado, Boulder)<br />James Hetrick (U. Pacific)<br />Andreas Jüttner (U. Southampton)<br />David Kaplan (U. Washington)<br />Andreas Kronfeld (Fermilab)<br />Weonjong Lee (Seoul National U.)<br />Derek Leinweber (U. Adelaide)<br />Nilmani Mathur (Tata Institute Mumbai)<br />Robert Mawhinney (Columbia U.)<br />Tereza Mendes (U. São Paulo)<br />Shigemi Ohta (KEK / Sokendai / RBRC)<br />Tetsuya Onogi (Osaka)<br />Kostas Originos (William & Mary / Jefferson Lab)<br />Dru Renner (Jefferson Lab)<br />Kari Rummukainen (Helsinki)<br />Sinead Ryan (Trinity College Dublin)<br />Stefan Schaefer (CERN)<br />Stephen Sharpe (U. Washington)<br />Cecilia Tarantino (U. Roma Tre)<br /><br /><b><a href="http://www.lattice2013.uni-mainz.de/157_ENG_HTML.php">Financial Support</a></b><br /><br />A reduced conference fee will be available for a limited number of students, and for participants with very limited financial resources.<br /><br />Applicants should send their application (with a brief motivation) by email to <a href="mailto:financial-support@lattice2013.uni-mainz.de?Subject="LATTICE2013 - reduced fee application, YOUR NAME">financial-support@lattice2013.uni-mainz.de</a> with Subject: "LATTICE2013 - reduced fee application, YOUR NAME". Student applicants should provide proof that they are registered as students during the year 2013 or academic year 2012-2013 and a statement of support by their supervisor, as well as the title of any presentation they intend to give.<br /><br />The deadline for reduced fee applications is 15 April 2013. Payment of the reduced fee must be received on 15 May 2013 at the latest. After that date the regular late fee has to be paid.<br /><br />Please note that the reduced fee does not include expenses for travel and accommodation, and that the available number of reduced-fee places is limited; not all applications may be successful.<br /><br /><b><a href="http://www.lattice2013.uni-mainz.de/79_ENG_HTML.php">Travel Information</a></b><br /><br /><a href="http://www.frankfurt-airport.com/content/frankfurt_airport/en.html">Frankfurt Airport</a> (FRA), which is served by over 500 flights each day, is located only 30 minutes from Mainz on a direct <a href="http://en.wikipedia.org/wiki/S8_%28Rhine-Main_S-Bahn%29">local train service</a>.<br /><br />For cheap connections from and to many European destinations, <a href="http://www.hahn-airport.de/Default.aspx?menu=passengers_visitors&cc=en">Hahn airport</a> (HHN, served by <a href="http://www.ryanair.com/en">Ryanair</a>) is connected to Mainz by a non-stop <a href="http://hahn.orn-online.de/busfahrplan_mainz.htm">shuttle bus</a>.<br /><br />Mainz also has excellent railway and motorway <a href="http://maps.google.com/maps?daddr=Mainz,+Germany&hl=en&sll=49.992862,8.247253&sspn=0.220717,0.676346&geocode=FZ7U-gId1dd9ACl5E98zLpG9RzGgG9sQ1dQiBA&dirflg=w&doflg=ptm&mra=ltm&t=m&z=11">connectivity</a>.<br /><br />A local transportation ticket valid for the duration of the conference in all buses and trams in Mainz and the neighbouring town of Wiesbaden, is included in the registration pack. The registration pack also includes maps of the city and the university campus.<br /><br /><b><a href="http://www.lattice2013.uni-mainz.de/86_ENG_HTML.php">Excursions and Sightseeing</a></b><br /><br />Situated near the UNESCO world heritage site <a href="http://en.wikipedia.org/wiki/Upper_Middle_Rhine_Valley">Upper Middle Rhine Valley</a>, Mainz is in its origins a Roman city, which has been an episcopal see since 746 and since 1950 is the capital of the state of <a href="http://en.wikipedia.org/wiki/Rhineland-Palatinate">Rhineland-Palatinate</a>.<br /><br />The city is located on the bank of the Rhine. The neighbouring cities of <a href="http://en.wikipedia.org/wiki/Frankfurt">Frankfurt</a> and <a href="http://en.wikipedia.org/wiki/Wiesbaden">Wiesbaden</a> are easily accessible via public transportation. The climate is among the warmest and driest in Germany, with average temperatures in July around 24 <sup>o</sup>C (75 <sup>o</sup>F) and a low chance of precipitation. The surrounding area is a <a href="http://en.wikipedia.org/wiki/Rheinhessen_%28wine_region%29">wine region</a> which is particularly renowned for producing excellent Rieslings.<br /><br />Mainz by itself offers plenty of opportunities for sightseeing, including the <a href="http://en.wikipedia.org/wiki/Mainz_Cathedral">Cathedral</a> with its 10th century bronze gate, the church of <a href="http://en.wikipedia.org/wiki/St._Stephen%27s_Church,_Mainz">St Stephan</a> with its exquisite set of windows designed by Marc Chagall, as well as a number of <a href="http://en.wikipedia.org/wiki/Drususstein">Roman remains</a>.<br /><br />Museums in Mainz include the <a href="http://www.gutenberg-museum.de/index.php?id=29&L=1">Gutenberg-Museum</a> for the history of printing, and the <a href="http://web.rgzm.de/1.html?&L=1">Romano-Germanic Central Museum</a> with its impressive collection of archaeological finds from the Roman and early medieval periods.<br /><br />Beyond Mainz, there is Frankfurt with its world-famous museums, among them the <a href="http://www.staedelmuseum.de/sm/index.php?StoryID=1190&websiteLang=en">Städel</a>, which houses one of Europe's prime art collections including works by Vermeer, Botticelli, Duerer, Monet and Picasso, and the <a href="http://www.senckenberg.de/root/index.php?page_id=5256">Senckenberg</a> museum of natural history with its outstanding collection of dinosaur skeletons. The <a href="http://www.alte-oper.de/">Alte Oper</a> (old opera house) is a first-class concert venue.<br /><br />The afternoon of Wednesday, 31 July has been allocated for <a href="http://www.lattice2013.uni-mainz.de/62_ENG_HTML.php">excursions</a>, and we are currently in the process of organizing a selection of options.<br /><br /><b>Other QCD Workshops</b><br /><br />Participants of Lattice 2013 might also consider attending the workshop "<a href="http://www.xqcd13.unibe.ch/">Extreme QCD (xQCD)</a>", which will be held in Bern/Switzerland, from 5 to 7 August 2013.<br /><br />Another QCD-related meeting being held in Europe in close temporal proximity to Lattice 2013 will be the workshop "<a href="http://www.ectstar.eu/Meetings/ConfsWksAndCollMeetings/ThisYearConfsWksAndCollMeetings/thisyearconfswksandcollmeetings.htm">Nucleon Matrix Elements for New-Physics Searches</a>" at the ECT* in Trento, Italy, from 22 to 26 July 2013.<br /><br />We are looking forward to seeing you in Mainz.<br /><br />The Lattice 2013 Local Organizing Committee,<br /><i>Georg von Hippel, Harvey B. Meyer, Owe Philipsen, Lorenz von Smekal,<br />Carsten Urbach, Marc Vanderhaeghen, Marc Wagner, Hartmut Wittig (chair)</i>Georg v. Hippelhttp://www.blogger.com/profile/15355104409462033075noreply@blogger.com0tag:blogger.com,1999:blog-8669468.post-56190106293253192982012-12-18T18:12:00.001Z2013-01-31T16:22:19.857ZWorkshop linksThe week of July 22-26, 2013, i.e. the week immediately prior to the Lattice 2013 conference, there is a workshop on "<a href="http://www.ectstar.eu/Meetings/ConfsWksAndCollMeetings/ThisYearConfsWksAndCollMeetings/thisyearconfswksandcollmeetings.htm">Nucleon Matrix Elements for New-Physics Searches</a>" scheduled at the <a href="http://www.ectstar.eu/">ECT*</a>, the organizers of which have requested that Lattice 2013 participants be made aware of it so as to avoid the potential for other scheduling conflicts.<br /><br />Another interesting upcoming event should be the school/workshop "<a href="http://www.ifsc.usp.br/~lattice/lqcdschool-iip/">New Horizons in Lattice Field Theory</a>", which will be held March 13-27, 2013, in <a href="http://en.wikipedia.org/wiki/Natal,_Rio_Grande_do_Norte">Natal (Brazil)</a>. With lectures by Mike Creutz, Owe Philipsen, Chris Sachrajda, Steve Sharpe, and Rainer Sommer, this ought to be a highly instructive school for students wishing to study lattice topics in the tropics.<br /><br />A propos schools, the slides of the INT Summer School on Lattice QCD for Nuclear Physics are <a href="http://www.int.washington.edu/PROGRAMS/12-2c/Lectures.html">up on the web</a>, along with videos of the lectures, providing another excellent educational resource on lattice QCD.Georg v. Hippelhttp://www.blogger.com/profile/15355104409462033075noreply@blogger.com0tag:blogger.com,1999:blog-8669468.post-22248546313545920532012-06-30T09:49:00.002+01:002012-06-30T09:52:45.341+01:00Lattice 2012, Day FiveHello for a final time from Cairns. The first plenary session of the morning had a somewhat reduced occupation number, as is usual the morning after the banquet. The first speaker was Maria Paola Lombardo, who spoke about high-temperature QCD on the lattice. Finite-T results are still being dominated by the staggered results, although there is a noticeable discrepancy in the equation of state between HISQ and stout-smeared quarks, and Wilson simulations are beginning to catch up. There are still many open issues in this field, including the fate of the U(1)<usb>A</sub> symmetry at high temperature and the effects of a θ term and of magnetic fields. On the other hand, quarkonium suppression is predicted well by the lattice, and for fluctuations the lattice measurements and hard thermal loop calculations meet up at around 200 MeV.<br /><br />The second talk was on strategies for finite chemical potential by Gert Aarts. At finite chemical potential, the fermionic determinant is complex, which precludes a simple probability interpretation, rendering ordinary Markov Chain-based Monte Carlo simulations impossible (the "sign problem"). Replacing the complex determinant by its absolute value, a technique known as phase quenching, leads to poor overlap and the so-called "Silver Blaze" problem, i.e. that extreme cancellations of highly oscillatory integrands are required to get the correct behaviour. It is therefore of interest to study models that have no sign problem, and these include two-colour QCD, and QCD with the gauge group G<sub>2</sub> (one of the exceptional simple Lie groups). For real-world QCD, which does have a sign problem, there are a number of approaches to avoiding it: some groups simulate at zero chemical potential and measure susceptibilities to perform a Taylor expansion in μ, others use an imaginary chemical potential (where the fermion determinant is real) and try to analytically continue to real μ. A completely different approach is given by complex Langevin dynamics, where all field variables are complexified and subjected to Langevin evolution. This method seems to work well in resolving the Silver Blaze problem for many models; however, it is known to sometimes converge to the wrong limit, so further theoretical work is certainly needed.<br /><br />The second plenary began with a talk by Kim Splittorff about chiral dynamics with Wilson fermions. Here there are two competing scenarios for approaching vanishing quark mass, the Aoki phase and the Sharpe-Singleton scenario, where in the latter case the pion mass never vanishes. In the quenched case, only the Aoki phase exists, but in unquenched simulations both scenarios have been observed. In Wilson chiral perturbation theory, it turns out that the sign of a given combination of low-energy constants parametrising the breaking of chiral symmetry by the Wilson term decides which scenario occurs. The eigenvalue density of the Dirac operator can also be determined analytically using Wilson χPT in the &epsilom;-regime, and the analytical results agree with simulations, finding an <i>a/V<sup>1/2</sup></i> scaling for the lowest eigenvalue.<br /><br />Next was Masanori Hanada speaking about Monte Carlo approaches to string/M theory. Via the AdS/CFT correspondence, supergravity/string theories can be related to Super-Yang-Mills theories. In some regimes, the string theory is easier to calculate with, and hence string calculations can be used to make statements about some aspects of gauge theories. In other regimes, which apparently are of particular interest to string theorists, the SYM theory is easier to work with, and hence lattice simulations can be used to make predictions about aspects of string theory. In particular, a specific kind of Chern-Simons theory with matter (the ABJM theory) may apparently be the definition of M theory, the elusive unifying description of string theory. There also seems to be the possibility that simulations of certain zero-dimensional models may contain the key to why there are three spatial dimensions and the Universe is expanding.<br /><br />After this, the Ken Wilson Lattice Award 2012 was announced: it goes to Blum et al. for their <a href="http://arxiv.org/abs/1206.5142">paper</a> on K->ππ decays.<br /><br />Then an invitation was given to a summer school in Brazil, and finally your correspondent could invite the conference participants to Mainz for next year.<br /><br />After the lunch break, there were parallel sessions, and after the coffee break, there was a final plenary session. The first speaker of the latter was Peter Boyle presenting the BlueGene/Q system. Lattice QCD presents a special design challenge to a designer of HPC systems, since in order to achieve scalability it requires that the network bandwidth and the memory bandwidth be about equal and closely matched to the FPU speed. With input from lattice physicists, this was realised in the BG/Q system. As a result, the BG/Q has been able to scale to unprecedented performances, smashing the Petaflop barrier by achieving 3.07 PFlop/s sustained performance, while being the most energy efficient computer in the world.<br /><br />After this, Gilberto Colangelo presented the FLAG-2 group and its work. FLAG-2 has moved beyond FLAG by also including physicists from the US and Japan, and by broadening its mandate to include also heavy-quark observables and α<sub>s</sub>. FLAG-2 expects to publish a review of results published up to the end of 2012 in early 2013, and every two years thereafter. End users will always be reminded to cite not just the FLAG review, but also the original paper(s).<br /><br />The last plenary talk was given by Tom Blum, who spoke about the anomalous magnetic moment of the muon. The 3.5σ tension (which is about two times the size of the electroweak corrections) between current theory and experiment is one of the biggest hints of BSM physics that exists so far. However, progress is hindered by the theoretical uncertainties, the leading contribution to which is the uncertainty on the hadronic effects. The leading hadronic effect is the hadronic vacuum polarisation, on which much work is being done, including by the Mainz group and ETMC, with updated and improved results presented at this conference. Tom Blum presented another avenue towards improving the precision of the lattice predictions by using all-mode-averaging. The next-largest contribution is hadronic light-by-light scattering, which naively would be an infeasible O(V<sup>2</sup>) calculation, but which can be attacked using simulations of QCD+QED with muons. This is particularly important, since reducing the error on this contribution to 10% would increase the tension (assuming the means remained the same) to the 5σ (="discovery") level.<br /><br />After the last plenary, Derek Leinweber spoke a few closing words and the lattice community scattered again, to reconvene next year in Mainz.<br /><br />This ends our coverage of Lattice 2012. I will be putting up a summary of what I learned from Cairns for organising Lattice 2013 in Mainz later, and I will keep you updated on the preparations for Lattice 2013 as it approaches.<br /><br />Georg v. Hippelhttp://www.blogger.com/profile/15355104409462033075noreply@blogger.com0tag:blogger.com,1999:blog-8669468.post-60333555206289340422012-06-29T06:54:00.004+01:002012-06-29T06:56:05.995+01:00Lattice 2012, Days Three and FourApologies for the late update. Last night I was too tired (or tipsy, your guess) to blog.<br /><br />Wednesday was the customary short day; there were plenary talks in the morning and excursions in the afternoon. Having already had a look at the wonders of the Great Barrier Reef in better weather before the conference, I decided to go to the zoo. In case that sounds kind of boring, let me tell you that the Cairns Tropical Zoo hosts some rather impressive animals; the saltwater crocodiles in particular are scarily big (one of them was known to eat cattle before he got captured), and the many birds and lizards are just very different from anything on the Northern hemisphere (and there were koalas and kangaroos, too).<br /><br />Thursday started with another experimental talk, presented by Justine Serrano of LHCb, who spoke about the many flavour physics observations made by that collaboration. Highlights included pushing the bounds for the branching ratio B<sub>s</sub>->μμ very close to the Standard Model prediction (this is an observable for which most of the uncertainty actually comes from lattice QCD predictions of f<sub>B<sub>s</sub></sub>) as well as observing the decay B->πμμ for the first time (this is the rarest B decay ever observed). New measurements of φ<sub>s</sub> from B<sub>s</sub>->J/ψφ and B<sub>s</sub>->J/ψππ are compatible with zero, and the parameter space for many new physics models has already now been tightly constrained by LHCb. There is some tension in the (poorly known) UT angle γ and in the isospin asymmetry in B->Kμμ and K->K<sup>*</sup>μμ, but the latter discrepancy seems most likely to be a fluctuation that will go away with more data. LHCb has also made the most precise measurements of B spectroscopy so far. With an upgrade intended to improve the acquisition rate to 10-20 times ahead, LHCb will certainly continue to impress in the future.<br /><br />The next speaker was Cecilia Tarantino talking about the theoretical side of flavour physics. Here one of the most pressing issues is the inclusive-exclusive discrepancy in V<sub>ub</sub> and V<sub>cb</sub>, where in each case the inclusive and exclusive measurements differ by more than 2σ. A unitarity triangle analysis favours the exclusive value for V<sub>ub</sub> and the inclusive value for V<sub>cb</sub>; in each case more precise lattice input for the exclusive determination is needed along with more experimental data for the inclusive one. Another tension that arises in the UT fit is coming from the branching ratio BR(B->τν); this cannot be explained in the 2-doublet Higgs model of type II, but more elaborate 2-doublet Higgs models might still explain it. Since D mixing is now entering the stage, we might become sensitive to different potential new physics, since the charm is an up-type quark; the f<sub>D<sub>s</sub></sub> puzzle, on the other hand, has now been resolved: the lattice values went up and the experiments came down.<br /><br />The second plenary opened with a talk by Huey-Wen Lin on hadron structure from the lattice, where there are a number of open puzzles, some of most pressing ones of which are the nucleon charge radii and the axial charge of the nucleon. It is likely that many systematic effects contribute here, including excited states effects, which can be overcome by using the summation method or by explicitly including excited states in fits.<br /><br />This was followed by a talk by Ross Young about nucleon strangeness measurements and their impact on dark matter searches. The theoretical uncertainties of dark matter searches are dominated by the uncertainties of the nucleon sigma terms, in particular the strange sigma term. These can analysed both directly from an analysis of nucleon three-point functions, or indirectly via the Feynman-Hellmann theorem. Modern estimates of the nucleon strangeness (and their errors) are much lower than those of ten years ago, and lattice QCD can contribute significantly to reducing the uncertainties of searches for the stuff than makes up one quarter of the Universe, but of which so far we somewhat embarrassingly no idea what it actually is.<br /><br />The last plenary talk of the morning was given by Walter Freeman, who spoke about determining electromagnetic sea effects on hadron polarisabilities by reweighting. He compared various approaches to reducing the noise of stochatic estimators for reweighting factors, finding that neither projecting out the low modes nor introducing intermediate reweighting steps helped for this case, but that looking at derivatives of the reweighting factors instead and performing a hopping parameter expansion did help.<br /><br />In the afternoon there were parallel sessions. Mainz graduate student Vera Gülpers gave a very nice talk on measuring the scalar form factor of the pion. My own talk was just an update on the ongoing radiative improvement of NRQCD, so actually not terribly exciting.<br /><br />In the evening there was the conference banquet, which was very good; however, the waiting staff took the slightly strange decision to serve the chicken or vegetarian entree and the meat or fish main course to people based on whether they were seated on even or odd seats (I have no idea whether this might be an Australian custom, though).<br /><br />Georg v. Hippelhttp://www.blogger.com/profile/15355104409462033075noreply@blogger.com0tag:blogger.com,1999:blog-8669468.post-67163660853358274572012-06-26T13:02:00.000+01:002012-06-27T10:35:06.096+01:00Lattice 2012, Day TwoHello again from Cairns. The first plenary of the second day began with a talk by Joel Giedt on technicolor-related theories on the lattice. Since two of the main theoretical problems facing the Standard Model, namely the hierarchy problem and the triviality problem, are related to the existence of a fundamental scalar, a clean solution to those problems might be to assume that no fundamental Higgs field exists and chiral symmetry is instead broken by a vacuum condensate of some new fermion fields interacting under some new "technicolor" gauge interaction. In order for such a fermion condensate to be able to give masses not just to the W and Z bosons, but also to the Standard Model fermions, there must be some interaction ("extended technicolor") mediating four-fermion interactions between the new and SM fermions, and in order for the resulting fermion masses to not be unreasonably suppressed, the technicolor theory must be slow-running ("walking") or conformal with an IR fixed point. Possible candidates for such models include QCD with N<sub>f</sub>=12 flavours, or with adjoint fermions. It appears that different groups studying these models are so far obtaining results that are impossible to reconcile with each other, so the picture still seems to be fairly confused.<br /><br />Next was the traditional experimental talk, delivered by Geoffrey Taylor of ATLAS. As we all know, the LHC is running admirably and has delivered an unprecedented luminosity, which has allowed the "rediscovery" of the Standard Model to be performed very rapidly. No signs of BSM physics have been found so far, but exclusion limits on many SUSY particles, Kaluza-Klein modes and assorted exotics have reached the 1 TeV-scale, and large regions of the parameter space of many SUSY models have been ruled out. Also, the Standard Model Higgs has been ruled out above a mass of 130 GeV, but there is a tantalizing excess of events across multiple channels in the 120-130 GeV range. If this excess is the Higgs, an excess above SM expectations in the γγ channel might suggest that this is either not the SM Higgs, or that there are new particles mediating the Higgs decays. Of course there wasn't going to be any big reveal from experiments at the lattice conference -- that will be reserved (assuming there is anything to reveal already) for ICHEP: the presentation of the results from CERN will be live-streamed on 4th July 2012. Until then the bets as to the next Nobel Prize are still open ...<br /><br />The second plenary started after the coffee break with Norman Christ speaking about kaon mixing and K->2π decays on the lattice. These are very hard observables to treat, but working at (almost) physical quark masses and with a chiral fermion formulation helps significantly; the use of non-perturbative renormalisation and extensions to the Lüscher formula also contributed to make the recent results that were shown possible.<br /><br />This was followed by a talk by Takumi Doi presenting the work of the HALQCD collaboration on nuclear physics from lattice QCD. HALQCD measure Bethe-Salpeter amplitudes on the lattice and infer a non-local potential from them, which can then be expanded into local interactions. Besides nucleon-nucleon interactions, they have also studied hyperon-nucleon potentials and three-nucleon forces. A new contraction algorithm has helped them to significantly reduce the computational effort for these multi-quark correlators.<br /><br />The last plenary talk was given by Marco Panero who spoke about Large-N gauge theories on the lattice. In the limit of an infinite number of colours and vanishing coupling (such that the 't Hooft coupling λ=g<sup>2</sup>N remains finite), gauge theories are known to simplify significantly -- perturbatively, only the planar diagrams without dynamical fermion loops survive, with all other classes of diagrams suppressed by some power of 1/N. Non-perturbatively, numerical studies at N>3 suggest that the large-N limit is approached smoothly, with many thermodynamic observables showing only a trivial N-dependence.<br /><br />In the afternoon there were parallel talks, and after that the poster session (Australian snacks are tasty, and Australian wines drink nicely). Certainly one of the prettiest posters was the one of Benjamin Jäger and Thomas Rae (both from Mainz) who presented the proposal and first tests of an anisotropic smearing method designed to improve signal-to-noise ratio for hadron with non-vanishing momentum.<br /><br />Georg v. Hippelhttp://www.blogger.com/profile/15355104409462033075noreply@blogger.com2tag:blogger.com,1999:blog-8669468.post-68176073320309658792012-06-25T12:47:00.001+01:002012-06-25T12:48:46.581+01:00Lattice 2012, Day OneHello from Lattice 2012 in Cairns, Queensland, Australia (the tropical "down under"). I suppose this year we will have particularly many readers on this blog, since so many people couldn't make the long trip; I will try not to disappoint them too much.<br /><br />Having had a couple of days to get over the jetlag and the acclimatization to the tropical climate here in Cairns, as well as to recover from the 32+ hour trip, I was quite ready for the conference to start. The reception last night was pleasant, and the staff are doing a great job keeping everything well-organised.<br /><br />Today, the first session (after the Welcome by Derek Leinweber) was started by Stefan Schaefer, who spoke about prospects and challenges of dynamical fermion simulations. Over the last few years, the parameters of what would be considered a typical dynamical simulation have been steadfastly approaching to the physical point in the pion mass while increasingly larger and finer lattices are being studied. This progress has been made possible not just by Moore's law and increases in parallelism, but also and even more significantly by algorithmic improvements in the MD integrators used in HMC simulations, the solvers and preconditioners used in solving the Dirac equation (such as local deflation), and the treatment of the fermion determinant (e.g. the Hasenbusch trick or the DD-HMC), all of which are to some extent interrelated (in particular Stefan pointed out that a good frequency splitting in the determinant reduces force fluctuations, thereby aiding Omelyan-type integrators by making the difference between the shadow Hamiltonian and the real one more constant). One major issue confronting dynamical simulations at fine lattice spacings is the slowing down of the topological charge as the continuum limit is approached and the topological sectors emerge, leading to potentially very long autocorrelation times. One possible solution to this problem is to simulate using open boundary conditions in time, as proposed by Martin Lüscher and now implemented in the openQCD program, and first results demonstrating the absence of the problem in this setup were shown. I suppose it remains to be seen how the effects of the open boundary conditions on hadronic correlators can be handled (they are probably quite suppressed in the central region for large enough time extent).<br /><br />Next was Jo Dudek talking about spectroscopy, with a focus on resonances and more qualitative statements rather then on precision physics with stable states. This is an area in which a number of experiments (including glueX, COMPASS and BES-III) are interested, but in which theory is still ahead of experiment, in particular as far as the search for hybrids is concerned; exotic hybrids in particular would present a "smoking gun" evidence of gluonic excitations in an experiment, but have not yet been seen. The work of the HadSpec collaboration, which Jo mainly presented, relies on the "distillation" approach for building correlation functions, and on the variational method with an operator basis constructed from quark bilinears with some covariant derivatives added in and the resulting operators put into definite continuum irreps and subduced to the corresponding lattice irreps. The results then allow to identify the continuum spin from which a given lattice state (at least predominantly) came on the basis of the generalised eigenvectors going with it. Moreover, it is possible to identify likely hybrids as presumably mainly containing a chromomagnetic excitation in addition to their quark model content, and to make some phenomenological statements about excitation energies and quark model identifications. The advantages of the distillation approach were demonstrated in the example of the η/η' system, where the disconnected parts are much less noisy in this way then with other approaches.<br /><br />After the coffee break, Daniel Mohler continued the topic of resonances with his talk reviewing methods and results for determining resonance parameters. Besides the now widely-used Lüscher method, he explained the histogram method (which at least I had not yet heard of) and reviewed a study comparing the two. In addition, recent results for a number of resonances including the ρ, the Kπ, Dπ and D<sup>*</sup>π channels, were reviewed, and some even compared to experiment (which seemed to agree unexpectedly well given the limitations of the lattice results). As Daniel summarised, this is an area that is still in its infancy, but making good progress, even though a firm theoretical basis for treating the inelastic case appears to be lacking.<br /><br />The next speaker was Taku Izubushi, who spoke about QCD+QED on the lattice. Isospin symmetry is broken not just by the different up and down quark masses, but also by electromagnetic effects, which need to be treated in order to go beyond the isospin limit. Another reason for being interested in QED effects is that the hadronic contributions to the anomalous magnetic moment of the muon are the source of the dominant theoretical uncertainty for this precision observable, in which there is some persistent tension between SM predictions and experiment, and that the next-to-leading hadronic contribution involves the hadronic light-by-light scattering amplitude, which can probably only be computed in a QCD+QED simulation of some sort. By adding quenched non-compact QED fields onto an existing lattice ensemble and reweighting the individual configurations accordingly, it is now possible to simulate QCD+QED, and this has been used to determine the electromagnetic effects on masses and decay constants; the difference of the up and down quark masses has also been determined, along with its effects on the nucleon mass difference.<br /><br />The last plenary speaker was Tatsu Misumi with a talk about new fermion discretisations. He summarised the recent developments in this field by demonstrating some of the connections between the different recent proposals of new fermion actions, including what he called "flavored mass" (which includes the staggered overlap fermions of Adams), the "central branch" (Wilson fermions without the on-site term) and the "flavored chemical potential" (minimally doubled fermions) formalisms. In particular the Adams case of the "flavored mass" formalism was shown to possess attractive features, such as reducing the numerical cost for overlap fermions and the taste breaking effects for staggered fermions, while exactly preserving hypercubic symmetry (which is broken e.g. for the minimally doubled fermions).<br /><br />After the lunch break (let it be noted that eating out in Cairns [perhaps generally in Australia? -- I wouldn't know] is rather expensive) there were parallel sessions. After the last of those, I had a slightly heated discussion about the one and only truly correct way to automate lattice perturbation theory (my sincere apologies to anyone offended by the raised voices -- it was all settled peacefully in the end, possibly just in time before the Convention Centre staff would have thrown us out of the building to lock up).<br />Georg v. Hippelhttp://www.blogger.com/profile/15355104409462033075noreply@blogger.com0tag:blogger.com,1999:blog-8669468.post-17558901723617064022012-04-12T15:45:00.001+01:002012-04-12T15:47:34.576+01:00Recent progress regarding the rooting procedureThe fourth-root trick for the staggered determinant has long been controversial. Most recently, the debate has been rekindled by a series of papers by Mike Creutz, in which he argues that the rooting procedure fails in specific ways. While some of the arguments have been refuted by members of the staggered community, criticisms related to the question whether the rooted staggered theory can describe the axial anomaly correctly remain important. A direct physical probe of the axial anomaly is given by the η'-η splitting. Unfortunately, the determination of this splitting requires the evaluation of disconnected contributions to the η' correlator, which are very noisy and cannot be measured with sufficient precision to make a clear statement at the current time. In his recent <a href="http://arxiv.org/abs/1203.2560">paper</a>, Stephan Dürr approaches the question of the correctness of the rooting procedure from the angle of a theory in which sufficient statistics can be readily obtained, namely the Schwinger model.<br /><br />The Schwinger model is simply QED in 1+1 spacetime dimensions, as far as its action is concerned. Its physics is, however, radically different from that of QED in 3+1 dimensions, since firstly there is neither spin nor a physical gauge boson degree of freedom in 1+1d, and secondly the 1-dimensional Coulomb potential is linear and hence confining. The Schwinger model therefore has a spectrum similar to that of QCD, with a mass gap and meson degrees of freedom (note that there are neither baryons nor "photoballs" due to the abelian nature of the interaction [although there aren't any glueballs in 1+1d QCD either due to the absence of the gauge boson as a degree of freedom]), and can therefore serve as a laboratory for ideas in QCD. The basic meson η of the Schwinger model, which Schwinger demonstrated to have a mass squared of <i>m<sup>2</sup>=e<sup>2</sup>/π</i> (where <i>e</i> is the dimensionful gauge coupling in 1+1d), in particular, is an analogue of the η' in QCD, since its mass is mainly due to the axial anomaly.<br /><br />The Schwinger model is much easier to simulate than QCD both because two dimensions are easier than four, and also because it turns out that reweighting works very well in two dimensions where the fermionic determinant can be evaluated exactly due to its comparably small size, so that one can generate quenched ensembles and include the fermionic determinant via reweighting. In particular the latter feature allows the generation of huge statistics (80,000 configurations in this case). Dürr employs an algorithm incorporating the introduction of instantons and antiinstantons as well as parity transformations to optimise the sampling of topological sectors. The resulting ensembles are then used to simulate the <i>N<sub>f</sub>=1(2)</i> Schwinger model via reweighting with the rooted (unrooted) staggered fermion determinant. The latter is correct by construction; testing the former is the motivation for the study.<br /><br />Using all-to-all propagators and U(1)-projected triply APE-smeared gauge links, Dürr is able to show the validity of the staggered index theorem with impressive precision. Turning to the meson spectrum, he finds that the connected part of the η has the same mass as the <i>N<sub>f</sub>=2</i> π meson up to cut-off effects, so that the mass of the physical η in the chiral limit comes entirely from the disconnected part. The ratio of the disconnected to the connected Green's functions for the η approaches the correct limiting value expected if the rooting trick works correctly. After a continuum and chiral extrapolation, he finds that the mass of the <i>N<sub>f</sub>=1</i> η meson agrees with Schwinger's analytical result.<br /><br />This paper provides a very interesting study that adds to the empirical support for the correctness of the rooting procedure for staggered quarks. Of course it remains to see if this result will carry over to QCD, but I'd be honestly surprised if it didn;t. An analytical construction demonstrating the correctness of the rooted staggered formalism would of course be very welcome. Perhaps some of the recent results regarding the connection between staggered and overlap fermions will point the way in that regard.Georg v. Hippelhttp://www.blogger.com/profile/15355104409462033075noreply@blogger.com0tag:blogger.com,1999:blog-8669468.post-61567224261993673962011-12-20T15:16:00.000Z2011-12-20T15:29:03.062ZLattice 2013 in MainzIf 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.Georg v. Hippelhttp://www.blogger.com/profile/15355104409462033075noreply@blogger.com0tag:blogger.com,1999:blog-8669468.post-73987973763650413962011-09-21T10:10:00.000+01:002011-09-21T10:26:38.820+01:00Virus-related thingsViruses 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 <a href="http://www.bbc.co.uk/news/world-asia-pacific-14997307">spread to China from Pakistan</a>. 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.<br /><br />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 <a href="http://www.theregister.co.uk/2011/09/19/beast_exploits_paypal_ssl/">hijack an encrypted connection</a> and, e.g., steal from the user's online banking or PayPal account.<br /><br />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 <a href="http://www.ll.mit.edu/news/DRACO.html">developed this substance</a> 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.Georg v. Hippelhttp://www.blogger.com/profile/15355104409462033075noreply@blogger.com1tag:blogger.com,1999:blog-8669468.post-7988373837172020732011-07-17T03:15:00.001+01:002011-07-17T03:15:51.893+01:00Lattice 2011, Day SixThe 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.<br /><br />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.<br /><br />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<sub>e.m.</sub> and new NLO results for (g-2), which however exclude the light-by-light contribution.<br /><br />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<sub>A</sub> of the nucleon.<br /><br />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).<br /><br />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.<br /><br />After this, the conference closed with a round of well-deserved applause for the Local Organising Committee.Georg v. Hippelhttp://www.blogger.com/profile/15355104409462033075noreply@blogger.com0tag:blogger.com,1999:blog-8669468.post-10050910678287970032011-07-17T03:14:00.000+01:002012-01-16T18:06:06.047ZLattice 2011, Day FiveSorry for the delayed update; I was too tired to blog last night.<br /><br />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.<br /><br />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.<br /><br />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.<br /><br />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)<sub>μ</sub>, φ<sub>B<sub>s</sub></sub>, and the branching ratio B<sub>s</sub> --> μ<sup>+</sup>μ<sup>-</sup>. The LHCb experiment should be able to clarify the situation soon.<br /><br />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.<br /><br />The plenary session closed with the invitation to LATTICE 2012 to be held in Cairns, Australia, from 24th to 29th June 2012.<br /><br />In the afternoon there were parallel sessions one last time (this included my own talk in the last possible slot).Georg v. Hippelhttp://www.blogger.com/profile/15355104409462033075noreply@blogger.com1tag:blogger.com,1999:blog-8669468.post-49715695336473635602011-07-15T06:29:00.001+01:002011-07-15T06:29:48.977+01:00Lattice 2011, Days Three and FourWednesday 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.<br /><br />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 10<sup>14</sup> 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.<br /><br />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.<br /><br />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.<br /><br />Next was a talk about U(1)<sub>A</sub> 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<sub>f</sub>)<sub>L</sub>xSU(N<sub>f</sub>)<sub>R</sub> 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)<sub>A</sub> remains broken above the critical temperature.<br /><br />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).<br /><br />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.Georg v. Hippelhttp://www.blogger.com/profile/15355104409462033075noreply@blogger.com0tag:blogger.com,1999:blog-8669468.post-33913540091039489452011-07-13T16:15:00.000+01:002011-07-13T16:18:29.655+01:00Lattice 2011, Day TwoHello again from Squaw Valley.<br /><br />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.<br /><br />The second plenary talk was the traditional experimental talk, delivered by Adam Martin from Fermilab. With 1 fb<sup>-1</sup> 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<sup>-1</sup>, 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".<br /><br />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<sub>c</sub>, N<sub>f</sub>, 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.<br /><br />In the next talk, Daniel Nogradi spoke about a specific model that has particular phenomenological appeal, namely the SU(3) theory with N<sub>f</sub>=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).<br /><br />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 <a href="http://arxiv.org/abs/1103.4818">paper</a> on the anomalous magnetic moment of the muon.<br /><br />In the afternoon, there were parallel sessions, and in the evening, the poster session took place.Georg v. Hippelhttp://www.blogger.com/profile/15355104409462033075noreply@blogger.com0tag:blogger.com,1999:blog-8669468.post-45031691746146444692011-07-12T06:25:00.000+01:002011-07-13T16:19:43.158+01:00Lattice 2011, Day OneHello from The Village at Squaw Valley, where I am at the Lattice 2011 conference.<br /> 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.<br /><br />The welcome was in a different style from the usual speeches -- we were shown a short <a href="http://dl.dropbox.com/u/18065445/FinalFinalEdit1.mp4">movie</a> 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.<br /><br />After that, the first plenary session started with a talk by Eigo Shintani on the determination of α<sub>s</sub> from lattice QCD. In fact, currently lattice determinations are dominating the world average for α<sub>s</sub>(M<sub>Z</sub><sup>2</sup>), 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<sub>s</sub> 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).<br /><br />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.<br /><br />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.<br /><br />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 <i>N</i>=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.<br /><br />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.<br /><br />The last plenary speaker of the day was Paul Rakow, who spoke about flavour-blindness and the pattern of flavour breaking in N<sub>f</sub>=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.<br /><br />In the afternoon there were parallel sessions.Georg v. Hippelhttp://www.blogger.com/profile/15355104409462033075noreply@blogger.com2tag:blogger.com,1999:blog-8669468.post-33304239222066448472011-02-11T16:07:00.000Z2011-02-11T16:09:22.339ZWhat's new at the fermion zoo?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<sub>f</sub>α<sub>s</sub>a<sup>2</sup>) corrections, and Iwasaki), but for fermions there is a veritable zoo.<br /><br />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:<br /><ul><br /><li><strong>Wilson fermions</strong> 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(a<sup>2</sup>). 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.</li><br /><li><strong>twisted-mass fermions</strong> are close relatives of Wilson fermions, consisting of a doublet of unimproved Wilson fermions with a twisted mass term of the form τ<sub>3</sub>γ<sup>5</sup>; 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.</li><br /><li><strong>staggered fermions</strong> 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<sub>f</sub>=4 degenerate flavours), and issue that has been surrounded by some controversy. The actions in current use are the asqtad and HISQ actions.</li><br /><li><strong>overlap fermions</strong> 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 <em>very</em> expensive to simulate, and thus are not in widespread use yet.</li><br /><li><strong>domain-wall fermions</strong> 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.</li><br /></ul><br />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.<br /><br />One is the family of <strong>minimally doubled fermions</strong>, 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<sub>f</sub>=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.<br /><br />The other new fermion family are the <strong>staggered overlap fermions</strong> 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.<br /><br />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.Georg v. Hippelhttp://www.blogger.com/profile/15355104409462033075noreply@blogger.com1