tag:blogger.com,1999:blog-8669468Thu, 30 Jul 2015 12:52:31 +0000conferencesblogstravellattice fermionsexperimentarXivgeneral newsperturbation theoryweirdnesspublicitylat2013astronomybook reviewdata analysisgeneral physicsimprovementnobel prizepoliticsstrong couplinganalytical resultscomputingfinanceforecastingquarkssimulationsenergyfittingfortranfunmathematicsneutrinosobituariesscience publishingseminarschemistryevolutiongraphenehopeinterviewslanguageslower dimensionsphilosophypythonrelativitysupersymmetrytadpolestopologyvirusesLife on the latticeThoughts on lattice QCD, particle physics and the world at large.http://latticeqcd.blogspot.com/noreply@blogger.com (Georg v. Hippel)Blogger204125tag:blogger.com,1999:blog-8669468.post-1782869081254270076Sat, 18 Jul 2015 13:19:00 +00002015-07-18T14:19:35.642+01:00conferencesLATTICE 2015, Day FiveIn a marked deviation from the "standard programme" of the lattice conference series, Saturday started off with parallel sessions, one of which featured my own talk.<br /><br />The lunch break was relatively early, therefore, but first we all assembled in the plenary hall for the conference group photo (a new addition to the traditions of the lattice conference), and was followed by afternoon plenary sessions. The first of these was devoted to finite temperature and density, and started with Harvey Meyer giving the review talk on finite-temperature lattice QCD. The thermodynamic properties of QCD are by now relatively well-known: the transition temperature is agreed to be around 155 MeV, chiral symmetry restoration and the deconfinement transition coincide (as well as that can defined in the case of a crossover), and the number of degrees of freedom is compatible with a plasma of quarks and gluons above the transition, but the thermodynamic potentials approach the Stefan-Boltzmann limit only slowly, indicating that there are strong correlations in the medium. Below the transition, the hadron resonance gas model describes the data well. The Columbia plot describing the nature of the transition as a function of the light and strange quark masses is being further solidified: the size of the lower-left hand corner first-order region is being measured, and the nature of the left-hand border (most likely O(4) second-order) is being explored. Beyond these static properties, real-time properties are beginning to be studied through the finite-temperature spectral functions. One interesting point was that there is a difference between the screening masses (spatial correlation lengths) and quasiparticle masses (from the spectral function) in any given channel, which may even tend in opposite directions as functions of the temperature (as seen for the pion channel).<br /><br />Next, Szabolcs Borsanyi spoke about fluctuations of conserved charges at finite temperature and density. While of course the sum of all outcoming conserved charges in a collision must equal the sum of the ingoing ones, when considering a subvolume of the fireball, this can be best described in the grand canonical ensemble, as charges can move into and out of the subvolume. The quark number susceptibilities are then related to the fluctuating phase of the fermionic determinant. The methods being used to avoid the sign problem include Taylor expansions, fugacity expansions and simulations at imaginary chemical potential, all with their own strengths and weaknesses. Fluctuations can be used as a thermometer to measure the freeze-out temperature.<br /><br />Lastly, Luigi Scorzato reviewed the Lefschetz thimble, which may be a way out of the sign problem (e.g. at finite chemical potential). The Lefschetz thimble is a higher-dimensional generalization of the concept of steepest-descent integration, in which the integral of e<sup>S(z)</sup> for complex S(z) is evaluated by finding the stationary points of S and integrating along the curves passing through them along which the imaginary part of S is constant. On such Lefschetz thimbles, a Langevin algorithm can be defined, allowing for a Monte Carlo evaluation of the path integral in terms of Lefschetz thimbles. In quantum-mechanical toy models, this seems to work already, and there appears hope that this might be a way to avoid the sign problem of finite-density QCD.<br /><br />After the coffee break, the last plenary session turned to physics beyond the Standard Model. Daisuke Kadoh reviewed the progress in putting supersymmetry onto the lattice, which is still a difficult problem due to the fact that the finite differences which replace derivatives on a lattice do not respect the Leibniz rule, introducing SUSY-breaking terms when discretizing. The ways past this are either imposing exact lattice supersymmetries or fine-tuning the theory so as to remove the SUSY-breaking in the continuum limit. Some theories in both two and four dimensions have been simulated successfully, including N=1 Super-Yang-Mills theory in four dimensions. Given that there is no evidence for SUSY in nature, lattice SUSY is of interesting especially for the purpose of verifying the ideas of gauge-dravity duality from the Super-Yang-Mills side, and in one and two dimensions, agreement with the predictions from gauge-gravity duality has been found.<br /><br />The final plenary speaker was Anna Hasenfratz, who reviewed Beyond-the-Standard-Model calculations in technicolor-like theories. If the Higgs is to be a composite particle, there must be some spontaneously broken symmetry that keeps it light, either a flavour symmetry (pions) or a scale symmetry (dilaton). There are in fact a number of models that have a light scalar particle, but the extrapolation of these theories is rendered difficult by the fact that this scalar is (and for phenomenologically interesting models would have to be) lighter than the (techni-)pion, and thus the usual formalism of chiral perturbation theory may not work. Many models of strong BSM interactions have been and are being studied using a large number of different methods, with not always conclusive results. A point raised towards the end of the talk was that for theories with a conformal IR fixed-point, universality might be violated (and there are some indications that e.g. Wilson and staggered fermions seem to give qualitatively different behaviour for the beta function in such cases).<br /><br />The conference ended with some well-deserved applause for the organizing team, who really ran the conference very smoothly even in the face of a typhoon. Next year's lattice conference will take place in Southampton (England/UK) from 24th to 30th July 2016. Lattice 2017 will take place in Granada (Spain).<br />http://latticeqcd.blogspot.com/2015/07/lattice-2015-day-five.htmlnoreply@blogger.com (Georg v. Hippel)0tag:blogger.com,1999:blog-8669468.post-1532722304931539399Fri, 17 Jul 2015 13:16:00 +00002015-07-18T14:19:53.519+01:00conferencesLATTICE 2015, Days Three and FourDue to the one-day shift of the entire conference programme relative to other years, Thursday instead of Wednesday was the short day. In the morning, there were parallel sessions. The most remarkable thing to be reported from those (from my point of view) is that MILC are generating a=0.03 fm lattices now, which handily beats the record for the finest lattice spacing; they are observing some problems with the tunnelling of the topological charge at such fine lattices, but appear hopeful that they can be useful.<br /><br />After the lunch break, excursions were offered. I took the trip to Himeji to see Himeji Castle, a very remarkable five-story wooden building that due to its white exterior is also known the "White Heron Castle". During the trip, typhoon Nangka approached, so the rains cut our enjoyment of the castle park a bit short (though seeing koi in a pond with the rain falling into it had a certain special appeal to it, the enjoyment of which I in my Western ignorance suppose might be considered a form of Japanese <i>wabi</i> aesthetics).<br /><br />As the typhoon resolved into a rainstorm, the programme wasn't cancelled or changed, and so today's plenary programme started with a talk on some formal developments in QFT by Mithat Ünsal, who reviewed trans-series, Lefschetz thimbles, and Borel summability as different sides of the same coin. I'm far too ignorant of these more formal field theory topics to do them justice, so I won't try a detailed summary. Essentially, it appears that the expansion of certain theories around the saddle points corresponding to instantons is determined by their expansion around the trivial vacuum, and the ambiguities arising in the Borel resummation of perturbative series when the Borel transform has a pole on the positive real axis can in some way be connected to this phenomenon, which may allow for a way to resolve the ambiguities.<br /><br />Next, Francesco Sannino spoke about the "bright, dark, and safe" sides of the lattice. The bright side referred to the study of visible matter, in particular to the study of technicolor models as a way of implementing the spontaneous breaking of electroweak symmetry, without the need for a fundamental scalar introducing numerous tunable parameters, and with the added benefits of removing the hierarchy problem and the problem of φ<sup>4</sup> triviality. The dark side referred to the study of dark matter in the context of composite dark matter theories, where one should remember that if the visible 5% of the mass of the universe require three gauge groups for their description, the remaining 95% are unlikely to be described by a single dark matter particle and a homogeneous dark energy. The safe side referred to the very current idea of asymptotic safety, which is of interest especially in quantum gravity, but might also apply to some extension of the Standard Model, making it valid at all energy scales.<br /><br />After the coffee break, the traditional experimental talk was given by Toru Iijima of the Belle II collaboration. The Belle II detector is now beginning commissioning at the upcoming SuperKEKB accelerator, which will greatly improved luminosity to allow for precise tests of the Standard Model in the flavour sector. In this, Belle II will be complementary to LHCb, because it will have far lower backgrounds allowing for precision measurements of rare processes, while not being able to access as high energies. Most of the measurements planned at Belle II will require lattice inputs to interpret, so there is a challenge to our community to come up with sufficiently precise and reliable predictions for all required flavour observables. Besides quark flavour physics, Belle II will also search for lepton flavour violation in τ decays, try to improve the phenomenological prediction for (g-2)<sub>μ</sub> by measuring the cross section for e<sup>+</sup>e<sup>-</sup> -> hadrons more precisely, and search for exotic charmonium- and bottomonium-like states.<br /><br />Closely related was the next talk, a review of progress in heavy flavour physics on the lattice given by Carlos Pena. While simulations of relativistic b quarks at the physical mass will become a possibility in the not-too-distant future, for the time being heavy-quark physics is still dominated by the use of effective theories (HQET and NRQCD) and methods based either on appropriate extrapolations from the charm quark mass region, or on the Fermilab formalism, which is sort of in-between. For the leptonic decay constants of heavy-light mesons, there are now results from all formalisms, which generally agree very well with each other, indicating good reliability. For the semileptonic form factors, there has been a lot of development recently, but to obtain precision at the 1% level, good control of all systematics is needed, and this includes the momentum-dependence of the form factors. The z-expansion, and extended versions thereof allowing for simultaneous extrapolation in the pion mass and lattice spacing, has the advantage of allowing for a test of its convergence properties by checking the unitarity bound on its coefficients.<br /><br />After the coffee break, there were parallel sessions again. In the evening, the conference banquet took place. Interestingly, the (excelleent) food was not Japanese, but European (albeit with a slight Japanese twist in seasoning and presentation).<br />http://latticeqcd.blogspot.com/2015/07/lattice-2015-days-three-and-four.htmlnoreply@blogger.com (Georg v. Hippel)0tag:blogger.com,1999:blog-8669468.post-3174432257854640691Wed, 15 Jul 2015 12:47:00 +00002015-07-16T03:05:20.505+01:00conferencesLATTICE 2015, Day TwoHello again from Lattice 2015 in Kobe. Today's first plenary session began with a review talk on hadronic structure calculations on the lattice given by James Zanotti. James did an excellent job summarizing the manifold activities in this core area of lattice QCD, which is also of crucial phenomenological importance given situations such as the proton radius puzzle. It is now generally agreed that excited-state effects are one of the more important issues facing hadron structure calculations, especially in the nucleon sector, and that these (possibly together with finite-volume effects) are likely responsible for the observed discrepancies between theory and experiment for quantities such as the axial charge of the nucleon. Many groups are studying the charges and form factors of the nucleon, and some have moved on to more complicated quantities, such as transverse momentum distributions. Newer ideas in the field include the use of the Feynman-Hellmann theorem to access quantities that are difficult to access through the traditional three-point-over-two-point ratio method, such as form factors at very high momentum transfer, and quantities with disconnected diagrams (such as nucleon strangeness form factors).<br /><br />Next was a review of progress in light flavour physics by Andreas Jüttner, who likewise gave an excellent overview of this also phenomenologically very important core field. Besides the "standard" quantities, such as the leptonic pion and kaon decay constants and the semileptonic K-to-pi form factors, more difficult light-flavour quantities are now being calculated, including the bag parameter B<sub>K</sub> and other quantities related to both Standard Model and BSM neutral kaon mixing, which require the incorporation of long-distance effects, including those from charm quarks. Given the emergence of lattice ensembles at the physical pion mass, the analysis strategies of groups are beginning to change, with the importance of global ChPT fits receding. Nevertheless, the lattice remains important in determining the low-energy constants of Chiral Perturbation Theory. Some groups are also using newer theoretical developments to study quantities once believed to be outside the purview of lattice QCD, such as final-state photon corrections to meson decays, or the timelike pion form factor.<br /><br />After the coffee break, the Ken Wilson Award for Excellence in Lattice Field Theory was announced. The award goes to Stefan Meinel for his substantial and timely contributions to our understanding of the physics of the bottom quark using lattice QCD. In his acceptance talk, Stefan reviewed his recent work on determining |V<sub>ub</sub>|/|V<sub>cb</sub>| from decays of Λ<sub>b</sub> baryons measured by the LHCb collaboration. There has long been a discrepancy between the inclusive and exclusive (from B -> πlν) determinations of V<sub>ub</sub>, which might conceivably be due to a new (BSM) right-handed coupling. Since LHCb measures the decay widths for Λ<sub>b</sub> to both pμν and Λ<sub>c</sub>μν, combining these with lattice determinations of the corresponding Λ<sub>b</sub> form factors allows for a precise determination of |V<sub>ub</sub>|/|V<sub>cb</sub>|. The results agree well with the exclusive determination from B -> πlν, and fully agree with CKM unitarity. There are, however, still other channels (such as b -> sμ<sup>+</sup>μ<sup>-</sup> and b -> cτν) in which there is still potential for new physics, and LHCb measurements are pending.<br /><br />This was followed by a talk by Maxwell T. Hansen (now a postdoc at Mainz) on three-body observables from lattice QCD. The well-known Lüscher method relates two-body scattering amplitudes to the two-body energy levels in a finite volume. The basic steps in the derivation are to express the full momentum-space propagator in terms of a skeleton expansion involving the two-particle irreducible Bethe-Salpeter kernel, to express the difference between the two-particle reducible loops in finite and infinite volume in terms of two-particle cuts, and to reorganize the skeleton expansion by the number of cuts to reveal that the poles of the propagator (i.e. the energy levels) in finite volume are related to the scattering matrix. For three-particle systems, the skeleton expansion becomes more complicated, since there can now be situations involving two-particle interactions and a spectator particle, and intermediate lines can go on-shell between different two-particle interactions. Treating a number of other technical issues such as cusps, Max and collaborators have been able to derive a Lüscher-like formula three-body scattering in the case of scalar particles with a Z<sub>2</sub> symmetry forbidding 2-to-3 couplings. Various generalizations remain to be explored.<br /><br />The day's plenary programme ended with a talk on the Standard Model prediction for direct CP violation in K-> ππ decays by Christopher Kelly. This has been an enormous effort by the RBC/UKQCD collaboration, who have shown that the ΔI=1/2 rule comes from low-energy QCD by way of strong cancellations between the dominant contributions, and have determined ε' from the lattice for the first time. This required the generation of ensembles with an unusual set of boundary conditions (G-parity boundary conditions on the quarks, requiring complex conjugation boundary conditions on the gauge fields) in space to enforce a moving pion ground state, as well as the precise evaluation of difficult disconnected diagrams using low modes and stochastic estimators, and treatment of finite-volume effects in the Lellouch-Lüscher formalism. Putting all of this together with the non-perturbative renormalization (in the RI-sMOM scheme) of ten operators in the electroweak Hamiltonian gives a result which currently still has three times the experimental error, but is systematically improvable, with better-than-experimental precision expected in maybe five years.<br /><br />In the afternoon there were parallel sessions again, and in the evening, the poster session took place. Food ran out early, but it was pleasant to see <a href="http://arxiv.org/abs/1306.1440">free-form smearing</a> begin improved upon and used to very good effect by Randy Lewis, Richard Woloshyn and students.<br />http://latticeqcd.blogspot.com/2015/07/lattice-2015-day-two.htmlnoreply@blogger.com (Georg v. Hippel)0tag:blogger.com,1999:blog-8669468.post-1945513826714968814Tue, 14 Jul 2015 11:30:00 +00002015-07-14T12:30:28.925+01:00conferencestravelLATTICE 2015, Day OneHello from Kobe, where I am attending the Lattice 2015 conference. The trip here was uneventful, as was the jetlag-day.<br /><br />The conference started yesterday evening with a reception in the Kobe Animal Kingdom (there were no animals when we were there, though, with the exception of some fish in a pond and some cats in a cage, but there were lot of plants).<br /><br />Today, the scientific programme began with the first plenary session. After a welcome address by Akira Ukawa, who reminded us of the previous lattice meetings held in Japan and the tremendous progress the field has made in the intervening twelve years, Leonardo Giusti gave the first plenary talk, speaking about recent progress on chiral symmetry breaking. Lattice results have confirmed the proportionality of the square of the pion mass to the quark mass (i.e. the Gell-Mann-Oakes-Renner (GMOR) relation, a hallmark of chiral symmetry breaking) very accurately for a long time. Another relation involving the chiral condensate is the Banks-Casher relation, which relates it to the eigenvalue density of the Dirac operator at zero. It can be shown that the eigenvalue density is renormalizable, and that thus the mode number in a given interval is renormalization-group invariant. Two recent lattice studies, one with twisted-mass fermions and one with O(a)-improved Wilson fermions, confirm the Banks-Casher relation, with the chiral condensates found agreeing very well with those inferred from GMOR. Another relation is the Witten-Veneziano relation, which relates the η' mass to the topological susceptibility, thus explaining how precisely the η' is not a Goldstone boson. The topological charge on the lattice can be defined through the index of the Neuberger operator or through chain of spectral porjectors, but a recently invented and much cheaper definition is through the topological charge density at finite flow time in Lüscher's Wilson flow formalism. The renormalization properties of the Wilson flow allow for a derivation of the universality of the topological susceptibility, and numerical tests using all three definitions indeed agree within errors in the continuum limit. Higher cumulants determined in the Wilson flow formalism agree with large-N<sub>c</sub> predictions in pure Yang-Mills, and the suppression of the topological susceptibility in QCD relative to the pure Yang-Mills case is in line with expectations (which in principle can be considered an <i>a posteriori</i> determination of N<sub>f</sub> in agreement with the value used in simulations).<br /><br />The next speaker was Yu Nakayama, who talked about a related topic, namely the determination of the chiral phase transition in QCD from the conformal bootstrap. The chiral phase transition can be studied in the framework of a Landau effective theory in three dimensions. While the mean-field theory predicts a second-order phase transition in the O(4) universality class, one-loop perturbation theory in 4-ε dimensions predicts a first-order phase transition at ε=1. Making use of the conformal symmetry of the effective theory, one can apply the conformal bootstrap method, which combines an OPE with crossing relations to obtain results for critical exponents, and the results from this method suggest that the phase transition is in fact of second order. This also agrees with many lattice studies, but others disagree. The role of the anomalously broken U(1)<sub>A</sub> symmetry in this analysis appears to be unclear.<br /><br />After the coffee break, Tatsumi Aoyama, a long-time collaborator in the heroic efforts of Kinoshita to calculate the four- and five-loop QED contributions to the electron and muon anomalous moments, gave a plenary talk on the determination of the QED contribution to lepton (g-2). For likely readers of this blog, the importance of (g-2) is unlikely to require an explanation: the current 3σ tension between theory and experiment for (g-2)<sub>μ</sub> is the strongest hint of physics beyond the Standard Model so far, and since the largest uncertainties on the theory side are hadronic, lattice QCD is challenged to either resolve the tension or improve the accuracy of the predictions to the point where the tension becomes an unambiguous, albeit indirect, discovery of new physics. The QED calculations are on the face of it simpler, being straightforward Feynman diagram evaluations. However, the number of Feynman diagrams grows so quickly at higher orders that automated methods are required. In fact, in a first step, the number of Feynman diagrams is reduced by using the Ward-Takahashi identity to relate the vertex diagrams relevant to (g-2) to self-energy diagrams, which are then subjected to an automated renormalization procedure using the Zimmermann forest formula. In a similar way, infrared divergences are subtracted using a more complicated "annotated forest"-formula (there are two kinds of IR subtractions needed, so the subdiagrams in a forest need to be labelled with the kind of subtraction). The resulting UV- and IR-finite integrands are then integrated using VEGAS in Feynman parameter space. In order to maintain the required precision, quadruple-precision floating-point numbers (or an emulation thereof) must be used. Whether these methods could cope with the six-loop QED contribution is not clear, but with the current and projected experimental errors, that contribution will not be required for the foreseeable future, anyway.<br /><br />This was followed by another (g-2)-related plenary, with Taku Izubichi speaking about the determination of anomalous magnetic moments and nucleon electric dipole moments in QCD. In particular the anomalous magnetic moment has become such an active topic recently that the time barely sufficed to review all of the activity in this field, which ranges from different approaches to parameterizing the momentum dependence of the hadronic vacuum polarization, through clever schemes to reduce the noise by subtracting zero-momentum contributions, to new ways of extracting the vacuum polarization through the use of background magnetic fields, as well as simulations of QCD+QED on the lattice. Among the most important problems are finite-volume effects.<br /><br />After the lunch break, there were parallel sessions in the afternoon. I got to chair the first session on hadron structure, which was devoted to determinations of hadronic contributions to (g-2)<sub>μ</sub>.<br /><br />After the coffee break, there were more parallel sessions, another complete one of which was devoted to (g-2) and closely-related topics. A talk deserving to be highlighted was given by Jeremy Green, who spoke about the first direct calculation of the hadronic light-to-light scattering amplitude from lattice QCD.<br />http://latticeqcd.blogspot.com/2015/07/lattice-2015-day-one.htmlnoreply@blogger.com (Georg v. Hippel)0tag:blogger.com,1999:blog-8669468.post-3617778499689342936Fri, 10 Apr 2015 08:19:00 +00002015-04-10T15:59:59.883+01:00conferencesWorkshop "Fundamental Parameters from Lattice QCD" at MITP (upcoming deadline)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 />The scientific programme "<a href="https://indico.mitp.uni-mainz.de/conferenceDisplay.py?confId=28">Fundamental Parameters from Lattice QCD</a>" at the Mainz Institute of Theoretical Physics (<a href="http://www.mitp.uni-mainz.de/">MITP</a>) 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 />The deadline for <a href="https://indico.mitp.uni-mainz.de/confRegistrationFormDisplay.py/display?confId=28" title="Registration form">registration</a> is <b>Wednesday, 15 April 2015</b>. Please register <a href="https://indico.mitp.uni-mainz.de/confRegistrationFormDisplay.py/display?confId=28" title="Register now!">at this link</a>.http://latticeqcd.blogspot.com/2015/03/workshop-fundamental-parameters-from.htmlnoreply@blogger.com (Georg v. Hippel)0tag:blogger.com,1999:blog-8669468.post-6291076893581301399Thu, 12 Mar 2015 17:01:00 +00002015-03-12T17:07:29.688+00:00conferencestravelQNP 2015, Day Five<i>Apologies for the delay in posting this. Travel and jetlag kept me from attending to it earlier.</i><br /><br />The first talk today was by Guy de Teramond, who described applications of light-front superconformal quantum mechanics to hadronic physics. I have to admit that I couldn't fully take in all the details, but as far as I understood an isomorphy between AdS<sup>2</sup> and the conformal group in one dimension can be used to derive a form of the light-front Hamiltonian for mesons from an AdS/QCD correspondence, in which the dilaton field is fixed to be φ(z)=1/2 z<sup>2</sup> by the requirement of conformal invariance, and a similar construction in the superconformal case leads to a light-front Hamiltonian for baryons. A relationship between the Regge trajectories for mesons and baryons can then be interpreted as a form of supersymmetry in this framework.<br /><br />Next was Beatriz Gay Ducati with a review of the pheonomenology of heavy quarks in nuclear matter, a topic where there are still many open issues. The photoproduction of quarkonia on nucleons and nuclei allows to probe the gluon distribution, since the dominant production process is photon-gluon fusion, but to be able to interpret the data, many nuclear matter effects need to be understood.<br /><br />After the coffee break, this was followed by a talk by Hrayr Matevosyan on transverse momentum distributions (TMDs), which are complementary to GPDs in the sense of being obtained by integrating out other variables starting from the full Wigner distributions. Here, again, there are many open issues, such as the Sivers, Collins or Boer-Mulders effects.<br /><br />The next speaker was Raju Venugopalan, who spoke about two outstanding problems in QCD at high parton densities, namely the question of how the systems created in heavy-ion collisions thermalise, and the phenomenon of "the ridge" in proton-nucleus collisions, which would seem to suggest hydrodynamic behaviour in a system that is too small to be understood as a liquid. Both problems may have to do with the structure of the dense initial state, which is theorised to be a colour-glass condensate or "glasma", and the way in which it evolves into a more dilute system.<br /><br />After the lunch break, Sonny Mantry reviewed some recent advances made in applying Soft-Collinear Effective Theory (SCET) to a range of questions in strong-interaction physics. SCET is the effective field theory obtained when QCD fluctuations around a hard particle momentum are considered to be small and a corresponding expansion (analogous to the 1/m expansion in HQET) is made. SCET has been successfully applied to many different problems; an interesting and important one is the problem of relating the "Monte Carlo mass" usually quoted for the top quark to the top quark mass in a more well-defined scheme such as MSbar.<br /><br />The last talk in the plenary programme was a review of the Electron-Ion Collider (EIC) project by Zein-Eddine Meziani. By combining the precision obtainable using an electron beam with the access to the gluon-dominated regime provided by a havy ion beam, as well as the ability to study the nucleon spin using a polarised nucleon beam, the EIC will enable a much more in-depth study of many of the still unresolved questions in QCD, such as the nucleon spin structure and colour distributions. There are currently two competing designs, the eRHIC at Brookhaven, and the MEIC at Jefferson Lab.<br /><br />Before the conference closed, Michel Garçon announced that the next conference of the series (QNP 2018) will be held in Japan (either in Tsukuba or in Mito, Ibaraki prefecture). The local organising committee and conference office staff received some well-deserved applause for a very smoothly-run conference, and the scientific part of the conference programme was adjourned.<br /><br />As it was still in the afternoon, I went with some colleagues to visit <a href="http://es.wikipedia.org/wiki/La_Sebastiana">La Sebastiana</a>, the house of <a href="http://en.wikipedia.org/wiki/Pablo_Neruda">Pablo Neruda</a> in Valparaíso, taking one of the city's famous <i>ascensores</i> down (although up might have been more convenient, as the streets get very steep) before walking back to Viña del Mar along the sea coast.<br /><br />The next day, there was an organised excursion to a vineyard in the Casablanca valley, where we got to taste some very good Chilean wines (some of the them matured in traditional clay vats) and liqueurs with a very pleasant lunch.<br /><br />I got to spend another day in Valparaíso before travelling back (a happily uneventful, if again rather long trip).<br />http://latticeqcd.blogspot.com/2015/03/qnp-2015-day-five.htmlnoreply@blogger.com (Georg v. Hippel)0tag:blogger.com,1999:blog-8669468.post-462429565090459560Fri, 06 Mar 2015 12:13:00 +00002015-03-06T12:15:50.731+00:00conferencesQNP 2015, Day FourThe first talk today was a review of experimental results in light-baryon spectroscopy by Volker Credé. While much progress has been made in this field, in particular in the design of so-called complete experiments, which as far as I understand measure multiple observables to unambiguously extract a complete description of the amplitudes for a certain process, there still seem to be surprisingly many unknowns. In particular, the fits to pion photoproduction in doubly-polarised processes seem to disagree strongly between different descriptions (such as MAID).<br /><br />Next was Derek Leinweber with a review of light hadron spectroscopy from the lattice. The <i>de facto</i> standard method in this field is the variational method (GEVP), although there are some notable differences in how precisely different groups apply it (e.g. solving the GEVP at many times and fitting the eigenvalues vs. forming projected correlators with the eigenvectors of the GEVP solved at a single time -- there are proofs of good properties for the former that don't exist for the latter). The way in which the basis of operators for the GEVP is build is also quite different as used by different groups, ranging from simply using different levels of quark field smearing to intricate group-theoretic constructions of multi-site operators. There are also attempts to determine how much information can be extracted from a given set of correlators, e.g. recently by the <a href="http://arxiv.org/abs/1411.6765">Cyprus/Athens group</a> using Monte Carlo simulations to probe the space of fitting parameters (a loosely related older idea based on <a href="http://arxiv.org/abs/0707.2788">evolutionary fits</a> wasn't mentioned).<br /><br />This was followed by a talk by Susan Gardner about testing fundamental symmetries with quarks. While we know that there must be physics beyond the Standard Model (because the SM does not explain dark matter, nor does it provide enough CP violation to explain the observed baryon asymmetry), there is so far no direct evidence of any BSM particle. Low-energy tests of the SM fall into two broad categories: null tests (where the SM predicts an exact null result, as for violations of B-L) and precision tests (where the SM prediction can be calculated to very high accuracy, as for (g-2)<sub>μ</sub>). Null tests play an important role in so far as they can be used to impose a lower limit for the BSM mass scale, but many of them are atomic or nuclear tests, which have complicated theory errors. The currently largest tensions indicating a possible failure of the Standard Model to describe all observations are the proton radius puzzle, and (g-2)<sub>μ</sub>. A possible explanation of either or both of those in terms of a "dark photon" is on the verge of being ruled out, however, since most of the relevant part of the mass/coupling plane has already been excluded by dark photon searches, and the rest of it will soon be (or else the dark photon will be discovered). Other tests in the hadronic sector, which seem to be less advanced so far, are the search for non-(V-A) terms in β-decays, and the search for neutron-antineutron oscillations.<br /><br />After the coffee break and the official conference photo, Isaac Vidaña took the audience on a "half-hour walk through the physics of neutron stars". Neutron stars are both almost-black holes (whose gravitation must be described in General Relativity) and extremely massive nuclei (whose internal dynamics must be described using QCD). Observations of binary pulsars allow to determine the masses of neutron stars, which are found to range up to at least two solar masses. However, the Tolman-Oppenheimer-Volkov equations for the stability of neutron stars lead to a maximum mass for a neutron star that depends on the equation of state of the nuclear medium. The observed masses severely constrain the equation of state and in particular seem to exclude models in which hyperons play an important role; however, it seems to be generally agreed that hyperons must play an important role in neutron stars, leading to a "hyperon puzzle", the solution of which will require an improved understanding of the structure and interactions of hyperons.<br /><br />The last plenary speaker of the day was Stanley Brodsky with the newest developments from light-front holography. The light-front approach, which has in the past been very successful in (1+1)-dimensional QCD, is based on the front form of the Hamiltonian formalism, in which a light-like, rather than a timelike, direction is chosen as the normal defining the Cauchy surfaces on which initial data are specified. In the light-front Hamiltonian approach, the vacuum of QCD is trivial and the Hilbert space can be constructed as a straightforward Fock space. With some additional ansätze taken from AdS/CFT ideas, QCD is reduced to a Schrödinger-like equation for the light-cone wavefunctions, from which observables are extracted. Apparently, all known observations are described perfectly in this approach, but (as for the Dyson-Schwinger or straight AdS/QCD approaches) I do not understand how systematic errors are supposed to be quantified.<br /><br />In the afternoon there were parallel talks. An interesting contribution was given by Mainz PhD student Franziska Hagelstein, who demonstrated how even a very small non-monotonicity in the electric form factor at low Q<sup>2</sup> (where there are no ep scattering data) could explain the difference between the muonic and electronic hydrogen results for the proton radius.<br /><br />The conference banquet took place in the evening at a very nice restaurant, and fun was had over cocktails and an excellent dinner.<br />http://latticeqcd.blogspot.com/2015/03/qnp-2015-day-four.htmlnoreply@blogger.com (Georg v. Hippel)0tag:blogger.com,1999:blog-8669468.post-114106487443998797Thu, 05 Mar 2015 12:27:00 +00002015-03-05T12:27:54.534+00:00conferencesQNP 2015, Day ThreeToday began with a talk by Mikhail Voloshin on QCD sum rules and heavy-quark states. The idea of exploiting quark-hadron duality to link perturbatively calculable current-current correlators to hadronic obervables and extract mesonic decay constants or quark masses is quite old, but has received a boost in recent years with the advent of three- and four-loop perturbative calculations in particularly from Chetyrkin and collaborators, which have also been used in conjunction with lattice results, e.g. by the HPQCD collaboration.<br /><br />A review of hadron spectroscopy at B factories (including LHCb) by Roberto Mussa followed. The charmonium and bottomonium spectra are now measured to great detail, with recent additions being 1D and 3P states, and more states are also being discovered in the heavy-light (where the B<sub>c</sub>(2S) has recently been discovered at ATLAS) and heavy-quark baryon (where the most recent discovery was the Ξ<sub>b</sub>) sectors, and many more transitions being discovered and studied.<br /><br />The next speaker was Raphaël Dupré, who spoke about colour propagation and neutralisation in strongly interacting systems. The idea here appears to be that in hadronisation processes, quarks first loose energy by radiating gluons and thus turn into colourless pre-hadrons, which then bind into hadrons on a longer timescale, and there seems to be experimental evidence supporting this energy-loss model.<br /><br />After the coffee break, Javier Castillo reviewed quarkonium suppression and regeneration in heavy-ion collisions. Quarkonia are generally considered important probes of the quark-gluon plasma, because the production of heavy quark-antiquark pairs is a perturbative process that happens at high energies early in the collision, while their binding is non-perturbative and is expected to be suppressed by Debye screening in the coloured plasma. As a consequence, more tightly bound quarkonia, like the Y(1S), can exist at higher temperatures, while the more lightly bound charmonia or Y(3S) states will "melt" at lower temperatures. However, quarkonia can also be regenerated by thermalised heavy quarks rejoining into quarkonia at the phase boundary. Experimental data support the screening picture, with the J/ψ being more suppressed at the LHC than at STAR (because of the higher temperature), the Y(2S) more suppressed than the Y(1S), and transport models with a negligible regeneration component describing the data well. The regeneration component increases at low p<sub>T</sub>, and the elliptic flow of the charm quarks is inherited by the regenerated J/ψ mesons. Some more difficult to understand effects of the nuclear environment, called Cold Nuclear Matter (CNM) effects are beginning to be seen in the data.<br /><br />Next was Zoltan Fodor with a talk about Lattice QCD results at zero and finite temperature from the BMW collaboration. By simulating QCD+QED with 1+1+1+1 flavours of dynamical quarks, BMW have been able to determine the isospin splitting of the nucleon and other baryonic systems. This work, which appears set to become a cover story in "Science", had to overcome a number of serious obstacles, in particular long-range autocorrelations (which could cured by a Fourier-accelerated HMC variant) and power-law finite-volume effects (which had to be fitted to results obtained at a range of volumes) introduced by the massless photon. In the finite-temperature regime, the crossover temperature is now generally agreed to be around 150-160 MeV, but the position and even existence of the critical endpoint is still contentious (and any existing results are not yet continuum-extrapolated in any case).<br /><br />After the lunch break, Yiota Foka gave an overview of heavy-ion results from RHIC and the LHC. The elliptic flow is still found to be in agreement with perfect hydrodynamics, but people are now also studying higher harmonics, as well as the interplay between jets and flow, which provide important constraints on the physics of the quark-gluon plasma. At the LHC, it has been found that it is the mass, and not the valence quark content, that drives the flow behaviour of hadrons, as the φ meson has the same flow behaviour as the proton.<br /><br />The next speaker was Carl Gagliardi, who reviewed results in nucleon structure from high-energy polarised proton-proton collisions. Proton-proton scattering is complementary to DIS in that it gives access to the gluonic degrees of freedom which are invisible to electrons, and RHIC has a programme of polarised proton collisions to explore the spin structure of the nucleon. Without the RHIC data, the gluon polarisation ΔG is almost unconstrained, but with the RHIC data, it is seen to be clearly positive and contribute about 0.2 to the proton spin. Using W production, it is possible to separate polarised quark and antiquark distributions, and there is more to come in the near future.<br /><br />The last plenary speaker of the day was Craig Roberts, who reviewed the pion and nucleon structure from the point of view of the Dyson-Schwinger equations approach. In this approach, the pion is closely linked to the quark mass function, which comes out of a quark gap equation and describes how the running quark mass at high energies turns into a much larger constituent quark mass at low energies. Landau-gauge gluons also become massive at low energies, and confinement is explained as the splitting of poles into pairs of conjugate complex poles giving an exponentially damped behaviour of the position space propagator. While this approach seems to be able to readily explain every single known experimental result, I do not understand how the systematic errors from the truncation of the infinite tower of DSEs are supposed to be controlled or quantified.<br /><br />After the coffee break, there were parallel sessions. An interesting parallel talk was given by Johan Bijnens, who has determined the leading logarithms for the nucleon mass (and some other systems) to rather high orders (which also for effective theories can be done using only one-loop integrals from a consistency argument by Weinberg).<br /><br />http://latticeqcd.blogspot.com/2015/03/qnp-2015-day-three.htmlnoreply@blogger.com (Georg v. Hippel)0tag:blogger.com,1999:blog-8669468.post-3084179203009592898Wed, 04 Mar 2015 12:05:00 +00002015-03-04T12:06:26.803+00:00conferencesQNP 2015, Day TwoHello again from Valparaíso. Today's first speaker was Johan Bijnens with a review of recent results from chiral perturbation theory in the mesonic sector, including recent results for charged pion polarisabilities and for finite-volume corrections to lattice measurements. To allow others to perform their own calculations for their own specific needs (which might include technicolor-like theories, which will generally have different patterns of chiral symmetry breaking, but otherwise work just the same way), Bijnens & Co. have recently published CHIRON, a general two-loop mesonic χPT package. The leading logarithms have been determined to high orders, and it has been found that the speed of convergence depends both on the observable and on whether the leading-order or physical pion decay constant is used.<br /><br />Next was Boris Grube, who presented some recent results from light-meson spectroscopy. The light mesons are generally expected to be some kind of superpositions of quark-model states, hybrids, glueballs, tetraquark and molecular states, as may be compatible with their quantum numbers in each case. The most complex sector is the 0<sup>++</sup> sector of f<sub>0</sub> mesons, in which the lightest glueball state should lie. While the γγ width of the f<sub>0</sub>(1500) appears to be compatible with zero, which would agree with the expectations for a glueball, whereas the f<sub>0</sub>(1710) has a photonic width more in agreement with being an s-sbar state, in J/ψ -> γ (ηη), which as a gluon-rich process should couple strongly to glueball resonances, little or no f<sub>0</sub>(1500) is seen, whereas a glueball nature for the f<sub>0</sub>(1710) would be supported by these results. New data to come from GlueX, and later from PANDA, should help to clarify things.<br /><br />The next speaker was Paul Sorensen with a talk on the search for the critical point in the QCD phase diagram. The quark-gluon plasma at RHIC is not only a man-made system that is over 300 times hotter than the centre of the Sun, it is also the most perfect fluid known, as it close to saturates the viscosity bound η/s > 1/(4π). Studying it experimentally is quite difficult, however, since one must extrapolate back to a small initial fireball, or "little bang", from correlations between thousands of particle tracks in a detector, not entirely dissimilar from the situation in cosmology, where the properties of the hot big bang (and previous stages) are inferred from angular correlations in the cosmic microwave background. Beam energy scans find indications that the phase transition becomes first-order at higher densities, which would indicate the existence of a critical endpoint, but more statistics and more intermediate energies are needed.<br /><br />After the coffee break, François-Xavier Girod spoke about Generalised Parton Distributions (GPDs) and deep exclusive processes. GPDs, which reduce to form factors and to parton distributions upon integrating out the unneeded variables in each case, correspond to a three-dimensional image of the nucleon performed in the longitudinal momentum fraction and the transverse impact parameter, and their moments are related to matrix elements of the energy-momentum tensor. Experimentally, they are probed using deeply virtual Compton scattering (DVCS); the 12 GeV upgrade at Jefferson Lab will increase the coverage in both Bjørken-x and Q<sup>2</sup>, and the planned electron-ion collider is expected to allow probing the sea and gluon GPDs as well.<br /><br />After the lunch break, there were parallel sessions. I chaired the parallel session on lattice and other perturbative methods, with presentations of lattice results by Eigo Shintani and Tereza Mendes, as well as a number of AdS/QCD-related results by various others.<br />http://latticeqcd.blogspot.com/2015/03/qnp-2015-day-two.htmlnoreply@blogger.com (Georg v. Hippel)0tag:blogger.com,1999:blog-8669468.post-5268534792935554920Tue, 03 Mar 2015 14:38:00 +00002015-03-03T14:38:23.203+00:00conferencestravelQNP 2015, Day OneHello from Valparaíso, where I continue this year's hectic conference circuit at the 7th International Conference on Quarks and Nuclear Physics (QNP 2015). Except for some minor inconveniences and misunderstandings, the long trip to Valparaíso (via Madrid and Santiago de Chile) went quite smoothly, and so far, I have found Chile a country of bright sunlight and extraordinarily helpful and friendly people.<br /><br />The first speaker of the conference was Emanuele Nocera, who reviewed nucleon and nuclear parton distributions. The study of parton distributions become necessary because hadrons are really composed not simply of valence quarks, as the quark model would have it, but of an indefinite number of (sea) quarks, antiquarks and gluons, any of which can contribute to the overall momentum and spin of the hadron. In an operator product expansion framework, hadronic scattering amplitudes can then be factorised into Wilson coefficients containing short-distance (perturbative) physics and parton distribution functions containing long-distance (non-perturbative) physics. The evolution of the parton distribution functions (PDFs) with the momentum scale is given by the DGLAP equations containing the perturbatively accessible splitting functions. The PDFs are subject to a number of theoretical constraints, of which the sum rules for the total hadronic momentum and valence quark content are the most prominent. For nuclei, on can assume that a similar factorisation as for hadrons still holds, and that the nuclear PDFs are linear combinations of nucleon PDFs modified by multiplication with a binding factor; however, nuclei exhibit correlations between nucleons, which are not well-described in such an approach. Combining all available data from different sources, global fits to PDFs can be performed using either a standard χ<sup>2</sup> fit with a suitable model, or a neural network description. There are far more and better data on nucleon than nuclear PDFs, and for nucleons the amount and quality of the data also differs between unpolarised and polarised PDFs, which are needed to elucidate the "proton spin puzzle".<br /><br />Next was the first lattice talk of the meeting, given by Huey-Wen Lin, who gave a review of the progress in lattice studies of nucleon structure. I think Huey-Wen gave a very nice example by comparing the computational and algorithmic progress with that in videogames (I'm not an expert there, but I think the examples shown were screenshots of Nethack versus some modern first-person shooter), and went on to explain the importance of controlling all systematic errors, in particular excited-state effects, before reviewing recent results on the tensor, scalar and axial charges and the electromagnetic form factors of the nucleon. As an outlook towards the current frontier, she presented the inclusion of disconnected diagrams and a new idea of obtaining PDFs from the lattice more directly rather than through their moments.<br /><br />The next speaker was Robert D. McKeown with a review of JLab's Nuclear Science Programme. The CEBAF accelerator has been upgraded to 12 GeV, and a number of experiments (GlueX to search for gluonic excitations, MOLLER to study parity violation in Møller scattering, and SoLID to study SIDIS and PVDIS) are ready to be launched. A number of the planned experiments will be active in areas that I know are also under investigation by experimental colleagues in Mainz, such as a search for the "dark photon" and a study of the running of the Weinberg angle. Longer-term plans at JLab include the design of an electron-ion collider.<br /><br />After a rather nice lunch, Tomofumi Nagae spoke about the hadron physics programme an J-PARC. In spite of major setbacks by the big earthquake and a later radiation accident, progress is being made. A search for the Θ<sup>+</sup> pentaquark did not find a signal (which I personally do not find surprising, since the whole pentaquark episode is probably of more immediate long-term interest to historians and sociologists of science than to particle physicists), but could not completely exclude all of the discovery claims.<br /><br />This was followed by a take by Jonathan Miller of the MINERνA collaboration presenting their programme of probing nuclei with neutrinos. Major complications include the limited knowledge of the incoming neutrino flux and the fact that final-state interactions on the nuclear side may lead to one process mimicking another one, making the modelling in event generators a key ingredient of understanding the data.<br /><br />Next was a talk about short-range correlations in nuclei by Or Henn. Nucleons subject to short-range correlations must have high relative momenta, but a low center-of-mass momentum. The experimental studies are based on kicking a proton out of a nucleus with an electron, such that both the momentum transfer (from the incoming and outgoing electron) and the final momentum of the proton are known, and looking for a nucleon with a momentum close to minus the difference between those two (which must be the initial momentum of the knocked-out proton) coming out. The astonishing result is that at high momenta, neutron-proton pairs dominate (meaning that protons, being the minority, have a much larger chance of having high momenta) and are linked by a tensor force. Similar results are known from other two-component Fermi systems, such as ultracold atomic gases (which are of course many, many orders of magnitude less dense than nuclei).<br /><br />After the coffee break, Heinz Clement spoke about dibaryons, specifically about the recently discovered d<sup>*</sup>(2380) resonance, which taking all experimental results into account may be interpreted as a ΔΔ bound state<br /><br />The last talk of the day was by André Walker-Loud, who reviewed the study of nucleon-nucleon interactions and nuclear structure on the lattice, starting with a very nice review of the motivations behind such studies, namely the facts that big-bang nucleosynthesis is very strongly dependent on the deuterium binding energy and the proton-neutron mass difference, and this fine-tuning problem needs to be understood from first principles. Besides, currently the best chance for discovering BSM physics seems once more to lie with low-energy high-precision experiments, and dark matter searches require good knowledge of nuclear structure to control their systematics. Scattering phase shifts are being studied through the Lüscher formula. Current state-of-the-art studies of bound multi-hadron systems are related to dibaryons, in particular the question of the existence of the H-dibaryon at the physical pion mass (note that the dineutron, certainly unbound in the real world, becomes bound at heavy enough pion masses), and three- and four-nucleon systems are beginning to become treatable, although the signal-to-noise problem gets worse as more baryons are added to a correlation function, and the number of contractions grows rapidly. Going beyond masses and binding energies, the new California Lattice Collaboration (CalLat) has preliminary results for hadronic parity violation in the two-nucleon system, albeit at a pion mass of 800 MeV.<br />http://latticeqcd.blogspot.com/2015/03/qnp-2015-day-one.htmlnoreply@blogger.com (Georg v. Hippel)0tag:blogger.com,1999:blog-8669468.post-3506938512717239565Fri, 27 Feb 2015 10:01:00 +00002015-02-27T10:01:06.061+00:00travelBack from MumbaiOn Saturday, my last day in Mumbai, a group of colleagues rented a car with a driver to take a trip to Sanjay Gandhi National Park and visit the <a href="http://en.wikipedia.org/wiki/Kanheri_caves">Kanheri caves</a>, a Buddhist site consisting of a large number of rather simple monastic cells and some worship and assembly halls with ornate reliefs and inscriptions, all carved out out of solid rock (some of the cell entrances seem to have been restored using steel-reinforced concrete, though).<br /><br />On the way back, we stopped at <a href="http://en.wikipedia.org/wiki/Mani_Bhavan">Mani Bhavan</a>, where Mahatma Gandhi lived from 1917 to 1934, and which is now a museum dedicated to his live and legacy.<br /><br />In the night, I flew back to Frankfurt, where the temperature was much lower than in Mumbai; in fact, on Monday there was snow.<br />http://latticeqcd.blogspot.com/2015/02/back-from-mumbai.htmlnoreply@blogger.com (Georg v. Hippel)0tag:blogger.com,1999:blog-8669468.post-509818626180283347Fri, 20 Feb 2015 12:02:00 +00002015-02-20T12:02:43.470+00:00conferencesPerspectives and Challenges in Lattice Gauge Theory, Day FiveToday's programme started with a talk by Santanu Mondal on baryons in the sextet gauge model, which is a technicolor-style SU(3) gauge theory with a doublet of technifermions in the sextet (two index symmetric) representation, and a minimal candidate for a technicolor-like model with an IR almost-fixed point. Using staggered fermions, he found that when setting the scale by putting the technipion's decay constant to the value derived from identifying the Higgs vacuum expectation value as the technicondensate, the baryons had masses in excess of 3 TeV, heavy enough to not yet have been discovered by the LHC, but to be within reach of the next run. However, the anomaly cancellation condition when embedding the theory into the Standard Model of the electroweak interactions requires charge assignments such that the lightest technibaryon (which would be a stable particle) would have a fractional electrical charge of 1/2, and while the cosmological relic density can be made small enough to evade detection, the technibaryons produced by the cosmic rays in the Earth's atmosphere should have been able to accumulate (there currently appear to be no specific experimental exclusions for charge-1/2 particles though).<br /><br />Next was Nilmani Mathur speaking about mixed action simulations using overlap valence quarks on the MILC HISQ ensembles (which include the radiative <a href="http://arxiv.org/abs/0812.0503">corrections</a> to the lattice gluon action from the quarks). Tuning the charm quark mass via the kinetic rather than rest mass of charmonium, the right charmonium hyperfine splitting is found, as well as generally correct charmonium spectra. Heavy-quark baryons (up to and including the Ω<sub>ccc</sub>) have also been simulated, with results in good agreement with experimental ones where the latter exist. The mixed-action effects appear to be mild small in mixed-action χPT, and only half as large as those for domain-wall valence fermions on an asqtad sea.<br /><br />In a brief note, Gunnar Bali encouraged the participants of the workshop to seek out opportunities for Indo-German research collaboration, of which there are still only a limited number of instances.<br /><br />After the tea break, there were two more theoretical talks, both of them set in the framework of Hamiltonian lattice gauge theory: Indrakshi Raychowdhury presented a loop formulation of SU(2) lattice gauge theory based on the prepotential formalism, where both the gauge links and their conjugate electrical fields are constructed from harmonic oscillator variables living on the sites using the Schwinger construction. By some ingenious rearrangements in terms of "fusion variables", a representation of the perturbative series for Hamiltonian lattice gauge theory purely in terms of integer-valued quantum numbers in a geometric-combinatorial construction was derived.<br /><br />Lastly, Sreeraj T.P. presented a derivation of an analogy between the Gauss constraint in Hamiltonian lattice gauge theory and the condition of equal "angular impulses" in the SU(2) x SU(2) description of the SO(4) symmetry of the Coulomb potential to derive a description of the Hilbert space of SU(2) lattice gauge theory in terms of hydrogen atom (n,l,m) variables located on the plaquettes subject only to the global constraint of vanishing total angular momentum, from where a variational ansatz for the ground state can be constructed.<br /><br />The workshop closed with some well-deserved applause for the organisers and all of the supporting technical and administrative staff, who have ensured that this workshop ran very smoothly indeed. Another excellent lunch (I understand that our lunches have been a kind of culinary journey through India, starting out in the north on Monday and ending in Kerala today) concluded the very interesting workshop.<br /><br />I will keep the small subset of my readers whom it may interest updated about my impressions from an excursion planned for tomorrow and my trip back.<br />http://latticeqcd.blogspot.com/2015/02/perspectives-and-challenges-in-lattice_20.htmlnoreply@blogger.com (Georg v. Hippel)0tag:blogger.com,1999:blog-8669468.post-541500898025329140Thu, 19 Feb 2015 18:32:00 +00002015-03-04T17:30:32.282+00:00conferencesPerspectives and Challenges in Lattice Gauge Theory, Day FourToday was dedicated to topics and issues related to finite temperature and density. The first speaker of the morning was Prasad Hegde, who talked about the QCD phase diagram. While the general shape of the Columbia plot seems to be fairly well-established, there is now a lot of controversy over the details. For example, the two-flavour chiral limit seems to be well-described by either the O(4) or O(2) universality class, it isn't currently possible to exclude that it might be Z(2), and while the three-flavour transition appears to be known to be Z(2), simulations with staggered and Wilson quarks give disagreeing results for its features. Another topic that gets a lot of attention is the question of U(1)<sub>A</sub> restoration; of course, U(1)<sub>A</sub> is broken by the axial anomaly, which arises from the path integral measure and is present at all temperatures, so it cannot be expected to be restored in the same sense that chiral symmetry is, but it might be that as the temperature gets larger, the influence of the anomaly on the Dirac eigenvalue spectrum gets outvoted by the temporal boundary conditions, so that the symmetry violation might disappear from the correlation functions of interest. However, numerical studies using domain-wall fermions suggest that this is not the case. Finally, the equation of state can be obtained from stout or HISQ smearing with very similar results and appears well-described by a hadron resonance gas at low T, and to match reasonably well to perturbation theory at high T.<br /><br />The next speaker was Saumen Datta speaking on studies of the QCD plasma using lattice correlators. While the short time extent of finite-temperature lattices makes it hard to say much about the spectrum without the use of techniques such as the Maximum Entropy Method, correlators in the spatial directions can be readily used to obtain screening masses. Studies of the spectral function of bottomonium in the Fermilab formalism suggest that the Y(1S) survives up to at least twice the critical temperature.<br /><br />Sorendu Gupta spoke next about the equation of state in dense QCD. Using the Taylor expansion (which was apparently first invented in the 14th-15th century by the Indian mathematician <a href="http://en.wikipedia.org/wiki/Madhava_of_Sangamagrama">Madhava</a>) method together with Padé approximants to reconstruct the function from the truncated series, it is found that the statistical errors on the reconstruction blow up as one nears the suspected critical point. This can be understood as a specific instance of the "no-free-lunch theorem", because a direct simulation (were it possible) would suffer from critical slowing down as the critical point is approached, which would likewise lead to large statistical errors from a fixed number of configurations.<br /><br />The last talk before lunch was Bastian Brandt with an investigation of an alternative formulation of pure gauge theory using auxiliary bosonic fields in an attempt to render the QCD action amenable to a dual description that might allow to avoid the sign problem at finite baryon chemical potential. The alternative formulation appears to describe exactly the same physics as the standard Wilson gauge action at least for SU(2) in 3D, and in 2D and/or in certain limits, its a continuum limit is in fact known to be Yang-Mills theory. However, when fermions are introduced, the dual formulation still suffers from a sign problem, but it is hoped that any trick that might avoid this sign problem would then also avoid the finite-μ one.<br /><br />After lunch, there were two non-lattice talks. The first one was given by Gautam Mandal, who spoke about thermalisation in integrable models and conformal field theories. In CFTs, it can be shown that for certain initial states, the expectation value of an operator equilibrates to a certain "thermal" expectation value, and a generalisation to integrable models, where the "thermal" density operator includes chemical potentials for all (infinitely many) conserved charges, can also be given.<br /><br />The last talk of the day was a very lively presentation of the fluid-gravity correspondence by Shiraz Minwalla, who described how gravity in Anti-deSitter space asymptotically goes over to Navier-Stokes hydrodynamics in some sense.<br /><br />In the evening, the conference banquet took place on the roof terrace of a very nice restaurant serving very good European-inspired cuisine and Indian red wine (also rather nice -- apparently the art of winemaking has recently been adapted to the Indian climate, e.g. the growing season is during the cool season, and this seems to work quite well).<br />http://latticeqcd.blogspot.com/2015/02/perspectives-and-challenges-in-lattice_19.htmlnoreply@blogger.com (Georg v. Hippel)0tag:blogger.com,1999:blog-8669468.post-1376696135463803266Wed, 18 Feb 2015 16:07:00 +00002015-02-18T16:09:24.494+00:00conferencesPerspectives and Challenges in Lattice Gauge Theory, Day ThreeToday's first talk was given by Rainer Sommer, who presented two effective field theories for heavy quarks. The first one was non-perturbatively matched HQET, which has been the subject of a long-running effort by the ALPHA collaboration. This programme is now reaping its first dividends in the form of very reliable fully non-perturbative results for B physics observables. Currently, the form factors for B->πlν decays, which are very important for determining the CKM matrix element V<sub>ub</sub> (currently subject to some significant tension between inclusive and exclusive determinations) are in the final stages of analysis. The other effective theory was QCD with N<sub>f</sub><6 flavours -- which is of course technically an effective theory where the heavy quarks have been integrated out! Rainer presented a new factorisation formula that relates the mass of a light hadron in the theory with a heavy quark to that of the same hadron in a theory in which the heavy quark is massless by a factor dependent on the hadron and a universal perturbative factor. The factorisation formula has been tested for gluonic observables in the pure gauge theory matched to the two-flavour theory.<br /><br />After tea, we had a session focussed on algorithms and machines. The first speaker was Andreas Frommer speaking about multigrid solvers for the Dirac equation in lattice QCD. A multigrid solver consists of a smoother and a coarse-grid correction. For the smoother for the Dirac equation, the Schwartz Alternating Procedure (SAP) is a natural choice, whereas for the coarse-grid correction, aggregate-based interpolation (essentially the same idea as Lüscher-style inexact deflation) can be used. The resulting multigrid algorithm is very similar to the domain-decomposed algorithm used in the DD-HMC and openQCD codes, but generalises to more than two levels, which may lead to better performance. Applications to the overlap operator were presented.<br /><br />Next, Stephan Solbrig presented the QPACE2 project, which aims to build a supercomputer based on Intel Knight's Corner (Xeon Phi) cards as processors, where each node consists of four Xeon Phis linked to each other, a weak host CPU used only for booting, and to an Infiniband card via a PCIe switch. The whole system uses hot water cooling, building on experience gathered in the iDataCool project. The 512bit wide registes of the Xeon Phi necessitate several programming tricks such as site fusing to make optimal use of computing resources; the resulting code seems to scale almost perfectly as long as there are sufficient numbers of domains to keep all nodes busy. An interesting side note was that apparently there are extremophile bacteria that thrive in the copper pipes of water-cooled computer clusters.<br /><br />Pushan Majumdar rounded off the session with a talk about QCD on GPUs. The special programming model of GPUs (small amount of memory per core, restrictions on branching, CPU/GPU data transfer as a bottleneck) makes programming GPUs challenging. The OpenACC compiler standard, which aims to offload the burden of dealing with GPU particulars onto the compiler vendor, may offer a possibility to easily port OpenMP-based code written for CPUs on GPUs, and Pushan showed some worked examples of Fortran 90 OpenMP code adapted for OpenACC.<br /><br />After lunch, I had to retire to my room for a little (let me hasten to add that the truly excellent lunch provided by the extremely hospitable TIFR is definitely absolutely blameless in this), and thus missed the afternoon's first two talks, catching only the end of Jyotirmoy Maiti's talk about exploring the spectrum of the pure SU(3) gauge theory using the Wilson flow.<br /><br />Gunnar Bali closed the day's proceeding with a very nice colloquium talk for a larger scientific audience, summarising the Standard Model and lattice QCD in an accessible manner for non-experts before proceeding to present recent results on the sea quark content and spin structure of the proton.<br /><br />http://latticeqcd.blogspot.com/2015/02/perspectives-and-challenges-in-lattice_18.htmlnoreply@blogger.com (Georg v. Hippel)0tag:blogger.com,1999:blog-8669468.post-7097988891138360580Tue, 17 Feb 2015 15:44:00 +00002015-02-17T15:44:25.937+00:00conferencesPerspectives and Challenges in Lattice Gauge Theory, Day TwoToday's first session started with a talk by Wolfgang Söldner, who reviewed the new CLS simulations using 2+1 flavours of dynamical fermions with open boundary conditions in the time direction to avoid the freezing of topology at small lattice spacing. Besides the new kind of boundary conditions, these simulations use a number of novel tricks, such as twisted mass reweighting, to make the simulations more stable at light pion masses. First studies of the topology and of the scale setting look promising, and there will likely be some interesting first physics results at the lattice conference in Kobe.<br /><br />After the tea break, Asit Kumar De talked about lattice gauge theory with equivariant gauge fixing. This is an attempt to evade the Neuberger 0/0 problem with BRST invariance on a lattice by leaving a subgroup of the gauge group unfixed. As a result, on gets four-ghost interactions in the gauge fixed action (this seems to be a general feature of theories trying to extend BRST symmetry; the Curci-Ferrari model for massive gauge fields also has such an interaction).<br /><br />This was followed Mughda Sarkar speaking about simulations of the gauge-fixed compact U(1) gauge theory. Apparently, the added parameters of the gauge fixing part appear to allow for changing the nature of the phase transition between strong and weak coupling from first to second order, although I didn't quite understand how that is compatible with the idea of having all gauge-invariant quantities be unaffected by the gauge fixing.<br /><br />After lunch, we had an excursion to the island of <a href="http://en.wikipedia.org/wiki/Elephanta_Caves">Elephanta</a>, where there are some great temples carved out of the rock. Today was a festival of Shiva, so admission was free (otherwise the price structure is quite interesting: र10 for Indians, र250 for foreigners), and there were many people on the island and in the caves. The site is certainly well worth the visit, although many of the statues have been damaged quite severely in the past.<br />http://latticeqcd.blogspot.com/2015/02/perspectives-and-challenges-in-lattice_17.htmlnoreply@blogger.com (Georg v. Hippel)0tag:blogger.com,1999:blog-8669468.post-1830169565229952867Mon, 16 Feb 2015 13:20:00 +00002015-02-16T13:21:32.583+00:00conferencestravelPerspectives and Challenges in Lattice Gauge Theory, Day OneHello from Mumbai, where I'm attending the workshop "Perspectives and Challenges in Lattice Gauge Theory" at the Tata Institute for Fundamental Research. I arrived on Sunday at an early hour, and had some opportunity to see some of the sights of Mumbai while trying to get acclimatized and jetlag-free.<br /><br />Today was the first day of the workshop, which started with a talk by Gergely Endrődi on the magnetic response of isospin-asymmetric QCD matter. This is relevant both for heavy-ion collisions and for the astrophysics of neutron stars, where in both cases strong magnetic fields interact with nuclear matter that has more neutrons than protons. From analytical calculations it is known that free quarks would form a paramagnetic state of matter, whereas pions would yield diamagnetism. As QCD matter at low energies should be mostly a hadron gas, and at high temperatures a quark-gluon plasma, the expectation would be that the behaviour of QCD at zero chemical potential changes from diamagnetic to paramagnetic as the temperature increases. On the other hand, at zero temperature and non-zero isospin chemical potential, at small isospin chemical potential the magnetic susceptibility vanishes (by the "Silver Blaze" effect), before suddenly going negative from pion condensation when the chemical potential exceeds half the pion mass, and again going positive as the chemical potential is increased further. <a href="http://arxiv.org/abs/1406.0269"">Lattice</a> <a href="http://arxiv.org/abs/1407.1216">simulations</a> confirm this overall picture, although the susceptibility remains finite at μ<sub>I</sub>=1/2 m<sub>π</sub> since the pions already start to melt rather than to condense into a superconductor).<br /><br />After the coffee break, it was my turn to talk about recent work we have done at Mainz regarding the importance of excited-state effects on nucleon form factors. Briefly summarised, the splitting to the first excited state (nucleon+pion P-wave, or nucleon+2 pions S-wave) gets very small in the chiral regime, but the errors on the nucleon two- and three-point functions grow exponentially as the source-sink separation is increased, making it very hard to find a Euclidean time region of both clean ground-state signal and reasonable statistical precision. Treating the excited states using different methods (summation method and explicit two-state fits) yields indications hinting that the current discrepancy between the nucleon charge radius obtained from lattice simulations and experiment may be due mostly to excited-state effects.<br /><br />This was followed by Andreas Schäfer speaking about much more ambitious hadron structure observables, namely Transverse Momentum Distributions (TMDs), Parton Distribution Functions (PDFs) and Generalised Parton Distributions (GPDs). Knowledge of these is important to clarify systematics for some of the LHC measurements, so lattice results could certainly have a huge impact here, but the necessary calculations appear quite involved.<br /><br />After the lunch break, Stefan Dürr reviewed some of the newer inhabitants of the <a href="http://latticeqcd.blogspot.com/2011/02/whats-new-at-fermion-zoo.html">fermion zoo</a>, namely firstly the Brillouin fermions obtained by replacing the standard discretisation of the Laplacian in the Wilson action with its Brillouin discretisation, and the symmetric derivative with its isotropic alternative, and secondly the staggered Wilson fermions of Adams (Adams fermions). In particular for heavier quark masses, the Brillouin fermions seem to do much better than standard Wilson fermions, including by giving a much more continuum-like dispersion relation.<br /><br />After a more technical talk on simulating the Gross-Neveu model with Boriçi-Creutz fermions by Jinshu Goswami, Kalman Szabo gave a colloquium for a more general audience explaining the origin of mass from QCD, electromagnetism and the Higgs effect (which is roughly the order of importance for ordinary matter), and how to determine the proton-neutron mass difference (which is after all of great anthropic significance, since an even slightly smaller value would leave hydrogen atoms unstable under inverse β-decay, whereas a somewhat larger value would create too much of a bottleneck in the creation of heavier elements) on the lattice. The <a href="http://arxiv.org/abs/1406.4088">lattice results</a> are certainly impressive both in terms of the theoretical and computational effort needed to obtain them and in the accuracy with which they reproduce the experimentally-known situation.<br />http://latticeqcd.blogspot.com/2015/02/perspectives-and-challenges-in-lattice.htmlnoreply@blogger.com (Georg v. Hippel)0tag:blogger.com,1999:blog-8669468.post-7375574243517801457Mon, 19 Jan 2015 16:20:00 +00002015-01-19T16:20:50.353+00:00conferencesUpcoming conference/workshop deadlinesThis is just a short reminder of some upcoming deadlines for conferences/workshops in the organization of which I am in some way involved.<br /><br />Abstract submission for <a href="https://indico.cern.ch/event/304663/">QNP 2015</a> closes on <b>6th February 2015</b>, and registration closes on 27th February 2015. Visit <a href="https://indico.cern.ch/event/304663/call-for-abstracts/">this link</a> to submit and abstract, and <a href="https://indico.cern.ch/event/304663/registration/register#/register">this link</a> to register.<br /><br />Applications for the Scientific Programme "<a href="http://indico.mitp.uni-mainz.de/conferenceDisplay.py?confId=28">Fundamental Parameters from Lattice QCD</a>" at MITP close on <b>31st March 2015</b>. Visit <a href="http://indico.mitp.uni-mainz.de/confRegistrationFormDisplay.py/display?confId=28">this link</a> to apply.<br />http://latticeqcd.blogspot.com/2015/01/upcoming-conferenceworkshop-deadlines.htmlnoreply@blogger.com (Georg v. Hippel)0tag:blogger.com,1999:blog-8669468.post-8995926825178498660Fri, 21 Nov 2014 09:42:00 +00002015-01-19T16:22:10.674+00:00conferencesScientific 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 />http://latticeqcd.blogspot.com/2014/11/scientific-program-fundamental.htmlnoreply@blogger.com (Georg v. Hippel)2tag:blogger.com,1999:blog-8669468.post-6124958336840446531Mon, 30 Jun 2014 19:39:00 +00002014-06-30T20:48:49.417+01:00conferencestravelLATTICE 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.http://latticeqcd.blogspot.com/2014/06/lattice-2014-day-six.htmlnoreply@blogger.com (Georg v. Hippel)0tag:blogger.com,1999:blog-8669468.post-4905653341722379624Mon, 30 Jun 2014 19:02:00 +00002014-06-30T20:48:41.790+01:00conferencesLATTICE 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.http://latticeqcd.blogspot.com/2014/06/lattice-2014-day-five.htmlnoreply@blogger.com (Georg v. Hippel)0tag:blogger.com,1999:blog-8669468.post-2731367277384297605Fri, 27 Jun 2014 01:51:00 +00002014-06-30T20:49:12.319+01:00conferencesLATTICE 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.http://latticeqcd.blogspot.com/2014/06/lattice-2014-days-three-and-four.htmlnoreply@blogger.com (Georg v. Hippel)0tag:blogger.com,1999:blog-8669468.post-2373536996954960193Wed, 25 Jun 2014 02:08:00 +00002014-06-25T03:08:30.376+01:00conferencesLATTICE 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 />http://latticeqcd.blogspot.com/2014/06/lattice-2014-day-two.htmlnoreply@blogger.com (Georg v. Hippel)0tag:blogger.com,1999:blog-8669468.post-7427483899629910391Tue, 24 Jun 2014 02:17:00 +00002014-06-24T03:19:29.014+01:00conferencestravelLATTICE 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 />http://latticeqcd.blogspot.com/2014/06/lattice-2014-day-one.htmlnoreply@blogger.com (Georg v. Hippel)0tag:blogger.com,1999:blog-8669468.post-1823995059189740261Mon, 01 Jul 2013 12:39:00 +00002013-07-01T14:30:32.555+01:00conferenceslat2013LATTICE 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!http://latticeqcd.blogspot.com/2013/07/lattice-2013-biggest-lattice-conference.htmlnoreply@blogger.com (Georg v. Hippel)2tag:blogger.com,1999:blog-8669468.post-2099171635818261656Fri, 07 Jun 2013 20:28:00 +00002013-07-01T13:39:50.858+01:00conferenceslat2013Lattice 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.http://latticeqcd.blogspot.com/2013/06/lattice-2013-upcoming-deadline.htmlnoreply@blogger.com (Georg v. Hippel)0