Steven Gottlieb

Scaling studies of QCD with the dynamical highly improved staggered quark action

We study the lattice spacing dependence, or scaling, of physical quantities using the highly improved staggered quark (HISQ) action introduced by the HPQCD/UKQCD collaboration, comparing our results to similar simulations with the asqtad fermion action. Results are based on calculations with lattice spacings approximately 0.15, 0.12 and 0.09 fm, using four flavors of dynamical HISQ quarks. The strange and charm quark masses are near their physical values, and the light-quark mass is set to 0.2 times the strange-quark mass. We look at the lattice spacing dependence of hadron masses, pseudoscalar meson decay constants, and the topological susceptibility. In addition to the commonly used determination of the lattice spacing through the static quark potential, we examine a determination proposed by the HPQCD collaboration that uses the decay constant of a fictitious “unmixed ss̄” pseudoscalar meson. We find that the lattice artifacts in the HISQ simulations are much smaller than those in the asqtad simulations at the same lattice spacings and quark masses. PACS numbers: 12.38.Gc,14.20.Dh

Quarkonium mass splittings in three-flavor lattice QCD

We report on calculations of the charmonium and bottomonium spectrum in lattice QCD. We use ensembles of gauge fields with three flavors of sea quarks, simulated with the asqtad improved action for staggered fermions. For the heavy quarks we employ the Fermilab interpretation of the clover action for Wilson fermions. These calculations provide a test of lattice QCD, including the theory of discretization errors for heavy quarks. We provide, therefore, a careful discussion of the results in light of the heavy-quark effective Lagrangian. By and large, we find that the computed results are in agreement with experiment, once parametric and discretization errors are taken into account.

Parallel zero-copy algorithms for fast Fourier transform and conjugate gradient using MPI datatypes

Many parallel applications need to communicate noncontiguous data. Most applications manually copy (pack/unpack) data before communications even though MPI allows a zero-copy specification. In this work, we study two complex use-cases: (1) Fast Fourier Transformation where we express a local memory transpose as part of the datatype, and (2) a conjugate gradient solver with a checkerboard layout that requires multiple nested datatypes. We demonstrate significant speedups up to a factor of 3.8 and 18%, respectively, in both cases. Our work can be used as a template to utilize datatypes for application developers. For MPI implementers, we show two practically relevant access patterns that deserve special optimization.

Tuning Fermilab heavy quarks in 2+1 flavor lattice QCD with application to hyperfine splittings

We report the nonperturbative tuning of parameters - {kappa}{sub c}, {kappa}{sub b}, and {kappa}{sub crit} - that are related to the bare heavy-quark mass in the Fermilab action. This requires the computation of the masses of D{sub s}{sup (*)} and B{sub s}{sup (*)} mesons comprised of a Fermilab heavy quark and a staggered light quark. Additionally, we report the hyperfine splittings for D{sub s}{sup (*)} and B{sub s}{sup (*)} mesons as a cross-check of our simulation and analysis methods. We find a splitting of 145 {+-} 15 MeV for the D{sub s} system and 40 {+-} 9 MeV for the B{sub s} system. These are in good agreement with the Particle Data Group average values of 143.9 {+-} 0.4 MeV and 46.1 {+-} 1.5 MeV, respectively. The calculations are carried out with the MILC 2+1 flavor gauge configurations at three lattice spacings a {approx} 0.15, 0.12, and 0.09 fm.

Topological susceptibility with the asqtad action

Chiral perturbation theory predicts that in quantum chromodynamics (QCD), light dynamical quarks suppress the gauge-field topological susceptibility of the vacuum. The degree of suppression depends on quark multiplicity and masses. It provides a strong consistency test for fermion formulations in lattice QCD. Such tests are especially important for staggered fermion formulations that lack a full chiral symmetry and use the fourth-root procedure to achieve the desired number of sea quarks. Over the past few years we have measured the topological susceptibility on a large database of 18 gauge-field ensembles, generated in the presence of 2+1 flavors of dynamical asqtad quarks with up and down quark masses ranging from 0.05 to 1 in units of the strange quark mass and lattice spacings ranging from 0.045 fm to 0.12 fm. Our study also includes three quenched ensembles with lattice spacings ranging from 0.06 to 0.12 fm. We construct the topological susceptibility from the integrated point-to-point correlator of the discretized topological charge density FF-tilde. To reduce its variance, we model the asymptotic tail of the correlator. The continuum extrapolation of our results for the topological susceptibility agrees nicely at small quark mass with the predictions of lowest-order SU(3) chiral perturbation theory, thus lendingmorexa0» support to the validity of the fourth-root procedure.«xa0less

Lattice determination of heavy-light decay constants

We report on the MILC Collaborations calculation of

QCD thermodynamics with nonzero chemical potential at Nt=6 and effects from heavy quarks

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Thermodynamics of lattice QCD with two light quark flavours on A 16{sup 3} x 8 lattice II.

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Topological susceptibility with the improved Asqtad action

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Charmonium with three flavors of dynamical quarks

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