Thu·Oct 4, 2007Thursday·October 4, 2007

1. 01*

   Adjoint zero-modes as a tool to understand the Yang-Mills vacuum

   Margarita Garcia Perez🇪🇸 · Antonio Gonzalez-Arroyo🇪🇸 · Alfonso Sastre🇪🇸

   The use of adjoint (quasi) zero-modes of the Dirac operator to probe the Yangs-Mills vacuum has been recently advocated by Gonzalez-Arroyo and Kirchner. The construction relies on the use of the super-symmetric zero mode which, for classical configurations, provides a direct estimate of the gauge action density. In the lattice implementation of this idea, we show how the results improve considerably if the overlap operator is used instead of the Wilson-Dirac one. Before proceeding to the detailed study of Monte Carlo ensembles, we studied here a series of potentially complicated situations which can be encountered. In particular, we study the case of instanton anti-instanton pairs and analyse how the results depend upon separation. The effect of lattice artifacts is also of concern. Indeed, a statistical analysis of zero modes of thermalised SU(2) configurations at beta=2.57 shows a significant fraction having 4N+2 adjoint zero modes, in contradiction with the index theorem. This violation must be associated to the roughness of the lattice configurations. Indeed, we show that this situation occurs for instantons of size of the order of the lattice spacing.

   hep-latPoS(2007)·13 citations
2. 02*

   The SF running coupling with four flavours of staggered quarks

   P.Perez-Rubio🇪🇸 · S.Sint🇮🇪

   In order to study the running coupling in four-flavour QCD, we review the set-up of the Schrödinger functional (SF) with staggered quarks. Staggered quarks require lattices which, in the usual counting, have even spatial lattice extent $L/a$ while the time extent $T/a$ must be odd. Setting $T=L$ is therefore only possible up to $\mathrm{O}(a)$, which introduces different cutoff effects already in the pure gauge theory. We re-define the SF such as to cope with this situation and determine the corresponding classical background field. A perturbative calculation yields the coefficient of the pure gauge $\mathrm{O}(a)$ boundary counterterm to one-loop order.

   hep-latPoS(2007)·8 citations
3. 03*

   B_s and B_d mixing in full lattice QCD using NRQCD b quarks

   Elvira Gamiz🇺🇸 · Christine T.H. Davies🇬🇧 · G. Peter Lepage🇺🇸 · Junko Shigemitsu🇺🇸 · Matthew Wingate🇬🇧

   We give a progress report on studies of B_s and B_d mixing with valence NRQCD b quarks and asqtad light quarks on the MILC configurations including the effect of 2+1 flavours of sea quarks. We explore methods for reducing statistical and systematic errors in the ratio \xi = f_{B_s}\sqrt{B_{B_s}}/f_{B_d}\sqrt{B_{B_d}}.

   hep-latPoS(2007)·10 citations
4. 04*

   Exploring Partially Confined Phases

   Michael C. Ogilvie🇺🇸 · Peter N. Meisinger🇺🇸 · Joyce C. Myers🇺🇸

   Phases of SU(N) gauge theories in which the global Z(N) symmetry breaks spontaneously to a subgroup Z(L) can be realized by adding appropriate Wilson line terms to the gauge action. These phases are partially confining, in the sense that quarks are confined but bound states of L quarks are not. At temperatures large compared to the normal deconfinement temperature, the phase diagram, pressure, string tensions, and 't Hooft loop surface tensions can be calculated analytically. Approximate scaling laws emerge naturally for both string tensions and surface tensions.

   hep-latPoS(2007)·21 citations
5. 05*

   Study of the critical point in lattice QCD at high temperature and density

   Shinji Ejiri🇺🇸

   We propose a method to probe the nature of phase transitions in lattice QCD at finite temperature and density, which is based on the investigation of an effective potential as a function of the average plaquette. We analyze data obtained in a simulation of two-flavor QCD using p4-improved staggered quarks with bare quark mass $m/T=0.4$, and find that a first order phase transition line appears in the high density regime for ${\mu }_q/T\sim 2.5$. The effective potential as a function of the quark number density is also studied. We calculate the chemical potential as a function of the density from the canonical partition function and discuss the existence of the first order phase transition line.

   hep-latPoS(2007)·7 citations
6. 06*

   Infrared exponents of gluon and ghost propagators from Lattice QCD

   O. Oliveira🇵🇹 · P. J. Silva🇵🇹

   The compatibility of the pure power law infrared solution of QCD Dyson-Schwinger equations (DSE) and lattice data for the gluon and ghost propagators in Landau gauge is discussed. For the gluon propagator, the lattice data is compatible with the DSE infrared solution with an exponent $\kappa \sim 0.53$, measured using a technique that suppresses finite volume effects and allows to model these corrections to the lattice data. For the ghost propagator, the lattice data does not seem to follow the infrared DSE power law solution.

   hep-latPoS(2007)·4 citations
7. 07*

   Gauge fixing methods and Gribov copies effects in lattice QCD

   P. J. Silva🇵🇹 · O. Oliveira🇵🇹

   We compare two Landau gauge fixing methods, aiming to find the global maximum of the gauge fixing functional. Moreover, a systematic effect of Gribov copies in the gluon and ghost propagators computed in Landau gauge is presented and discussed.

   hep-latPoS(2007)·8 citations
8. 08*

   New phases of finite temperature gauge theory from an extended action

   Joyce C. Myers🇺🇸 · Michael C. Ogilvie🇺🇸

   We study the behavior of the order parameter, the phase diagram, and the thermodynamics of exotic phases of finite temperature gauge theory. Lattice simulations were performed in SU(3) and SU(4) with an adjoint Polyakov loop term added to the standard Wilson action. In SU(3), the pattern of Z(3) symmetry breaking in the new phase is distinct from the pattern observed in the deconfined phase. In SU(4), the Z(4) symmetry is spontaneously broken down to Z(2), representing a partially-confined phase. The existence of the new phases is confirmed in analytical calculations of the free energy based on high-temperature perturbation theory.

   hep-latPoS(2007)·1 citation
9. 09*

   Pion mass difference from vacuum polarization

   JLQCD Collaboration: E. Shintani🇯🇵 · H. Fukaya🇯🇵 · S. Hashimoto🇯🇵 · H. Matsufuru🇯🇵 · J. Noaki🇯🇵 · T. Onogi🇯🇵 · N. Yamada🇯🇵

   We calculate the electromagnetic contribution to the pion mass difference, $\Delta m_{\pi }^2=m_{{\pi }^{+}}^2-m_{{\pi }^0}^2$, in the chiral limit through the $VV-AA$ type vacuum polarization using Das-Guralnik-Mathur-Low-Young (DGMLY) sum rule. The calculation is made with two-flavors of dynamical overlap fermions on a $16^3\times 32$ lattice at $a\sim$0.12 fm. The exact chiral symmetry of the overlap fermion is essential to control the systematic error in the difference $VV-AA$. We obtain $\Delta m_{\pi }^2=1024(100)\mathrm{Me}{\mathrm{V}}^2$ combining the lattice data with the perturbative contribution in the high momentum region evaluated by the operator product expansion. By analyzing the momentum dependence of the vacuum polarization, we also obtain pion decay constant $f_{\pi }$ and the low-energy constants $L_{10}^r$ in the chiral limit.

   hep-lathep-phPoS(2007)·8 citations
10. 10*

    Non-perturbative renormalization of four-quark operators and B_K with Schroedinger functional scheme in quenched domain-wall QCD

    Yousuke Nakamura · Yusuke Taniguchi · for CP-PACS Collaboration

    We present non-perturbative renormalization factors for $\Delta S=2$ four-quark operators in quenched domain-wall QCD using the Schroedinger functional method. Non-perturbative renormalization factor for $B_K$ is evaluated at hadronic scale. Combined with the non-perturbative RG running obtained by the Alpha collaboration, our result yields renormalization factor which converts lattice bare $B_K$ to the renormalization group invariant one. We apply the renormalization factor to bare $B_K$ previously obtained by the CP-PACS collaboration with the quenched domain-wall QCD(DWQCD). We compare our result with previous ones obtained by perturbative renormalization factors, different renormalization schemes or different quark actions. We also show that chiral symmetry breaking effects in the renormalization factor are numerically small.

    hep-latPoS(2007)·1 citation
11. 11*

    Semileptonic decays of heavy-light pseudoscalar mesons

    Nazario Tantalo🇮🇹

    I discuss the results of a recent quenched lattice calculation of the two independent form factors parametrizing the semileptonic decays between heavy-light pseudoscalar mesons. The differential decay rate of the process B --> D l nu has been calculated at non vanishing momentum transfer both in the case of the light leptons, l=e,mu, and in the case of a non vanishing lepton mass, l=tau.

    hep-latPoS(2007)·3 citations
12. 12*

    Dynamical Simulations with Highly Improved Staggered Quarks

    K. Y. Wong🇬🇧 · R. M. Woloshyn🇨🇦

    It is well established that lattice artifacts can be suppressed substantially by the use of SU(3)-projected smeared links in the fermion action. An example is the Highly Improved Staggered Quark action where the ASQ-like effective links are constructed from reunitarized Fat7 links. A general procedure is presented for computing the derivative of the fermion action with respect to the base links (fermion force) - a key component in dynamical simulations using molecular dynamics evolution. The method is iterative and can be applied to actions with arbitrary levels of smearing and reunitarization. The cost of calculating the fermion force is determined for the ASQ action and the HISQ action. Test results show that calculating the HISQ force is about two times more expensive than the ASQ force.

    hep-latPoS(2007)·9 citations
13. 13*

    Transport and spectral functions in high-temperature QCD

    Gert Aarts (Swansea University)🇬🇧

    The current status of transport coefficients in relativistic field theories at high temperature is reviewed. I contrast weak coupling results obtained using kinetic theory/diagrammatic techniques with strong coupling results obtained using gauge/gravity duality, and describe the recent developments in extracting transport coefficients and spectral functions from lattice QCD simulations. The fate of quarkonium at high temperature as seen from the lattice is briefly mentioned as well.

    hep-lathep-phnucl-thPoS(2007)·25 citations
14. 14*

    B Semileptonic Decays at High Recoil Momentum

    C. T. H. Davies🇬🇧 · E. Follana🇬🇧 · G. P. Lepage🇺🇸 · J. Shigemitsu🇺🇸 · K. Y. Wong🇬🇧

    We explore the possibility of studying $B\to \pi l\nu$ semileptonic decays at large recoil momentum. Our methods include the use of a random-wall source for the pion to reduce statistical errors, and different smearing functions are used for the B meson to improve the overlap with the ground state. We observe, in general, a factor of 3-4 improvement in the signal-to-noise ratio in correlation functions if random-wall propagators are used.

    hep-latPoS(2007)·9 citations
15. 15*

    Universality and Scaling at the chiral transition in two-flavor QCD at finite temperature

    Tereza Mendes🇧🇷

    The order of the phase transition in finite-temperature QCD with two degenerate light quarks is still an open problem and corresponds to the last question mark in the zero-density phase diagram of QCD. We argue that establishing the nature of the transition in this case is also a crucial test for numerical simulations of lattice QCD, allowing precise estimates of possible systematic errors related e.g. to the choice of fermion-simulation algorithm or of discretized formulation for fermions.

    hep-latPoS(2007)·6 citations
16. 16*

    On the conformal anomaly of k-strings

    Pietro Giudice🇮🇹 · Ferdinando Gliozzi🇮🇹 · Stefano Lottini🇮🇹

    We discuss the long distance behaviour of the flux tube associated to baryon vertices and argue that, if the gauge system admits stable k-strings, the conformal field theory describing this string in the IR has conformal anomaly c=(d-2)\sigma_k/\sigma, where \sigma_k is the k-string tension and \sigma that of the fundamental representation. We check this result in a 3D Z_4 gauge model at finite temperature, where a string effect directly related to "c" can be clearly identified.

    hep-latPoS(2007)·1 citation
17. 17*

    Interactive Visualization Package for 4D Lattice Field Theories

    Ivan Hip🇭🇷

    Recent interest in exploring local vacuum structure of QCD through the properties of the eigenmodes of the lattice Dirac operators rises again the challenge to visualize four-dimensional objects and structures which appear in lattice field theories. In spite of complex and powerful commercial visualization software packages on the market, there are reasons to develop Interactive Visualization Package (IVP). We believe that an apprehension of the complex structures is possible only through the interactive approach, with the user being able to manipulate data representations and slices through the lattice in real-time. Further insight should also be gained by an interactive parallel examination of different physical quantities, e.g. eigenmode density with topological charge or action densities. Finally, thanks to constantly falling hardware prices, IVP makes it possible to use almost any Linux PC as a visualization tool for research in lattice field theory.

    hep-latPoS(2007)·1 citation

* Reconstructed cohort: no mailing for this day survives in the archive. Papers are grouped by their submission times and arXiv's announcement cut-off, assuming announcement without delay; positions follow identifier order. Validated at ~91% exact-day agreement against the archived era.