N. Ukita

Physical Point Simulation in 2+1 Flavor Lattice QCD

We present the results of the physical point simulation in 2+1 flavor lattice QCD with the nonperturbatively O(a)-improved Wilson quark action and the Iwasaki gauge action at {beta} = 1.9 on a 32{sup 3} x 64 lattice. The physical quark masses together with the lattice spacing is determined with m{sub {pi}}, m{sub K} and m{sub {Omega}} as physical inputs. There are two key algorithmic ingredients to make possible the direct simulation at the physical point: One is the mass-preconditioned domain-decomposed HMC algorithm to reduce the computational cost. The other is the reweighting technique to adjust the hopping parameters exactly to the physical point. The physics results include the hadron spectrum, the quark masses and the pseudoscalar meson decay constants. The renormalization factors are nonperturbatively evaluated with the Schroedinger functional method. The results are compared with the previous ones obtained by the chiral extrapolation method.

Precise determination of the strong coupling constant in Nf = 2+1 lattice QCD with the Schrödinger functional scheme

We present an evaluation of the running coupling constant for Nf = 2+1 QCD. The Schrodinger functional scheme is used as the intermediate scheme to carry out non-perturbative running from the low energy region, where physical scale is introduced, to deep in the high energy perturbative region, where conversion to the scheme is safely performed. Possible systematic errors due to the use of perturbation theory occur only in the conversion from three-flavor to four-flavor running coupling constant near the charm mass threshold, where higher order terms beyond 5th order in the β function may not be negligible. For numerical simulations we adopted Iwasaki gauge action and non-perturbatively improved Wilson fermion action with the clover term. Seven renormalization scales are used to cover from low to high energy region and three lattice spacings to take the continuum limit at each scale. A physical scale is introduced from the previous Nf = 2+1 simulation of the CP-PACS/JL-QCD collaboration [1], which covered the up-down quark mass range heavier than mπ ~ 500 MeV. Our final result is = 0.12047(81)(48)(+0−173) and = 239(10)(6)(+0−22) MeV .

SU(2) and SU(3) chiral perturbation theory analyses on baryon masses in 2 + 1 flavor lattice QCD

We investigate the quark mass dependence of baryon masses in 2+1 flavor lattice QCD using SU(3) heavy baryon chiral perturbation theory up to one-loop order. The baryon mass data used for the analyses are obtained for the degenerate up-down quark mass of 3 to 24 MeV and two choices of the strange quark mass around the physical value. We find that the SU(3) chiral expansion fails to describe both the octet and the decuplet baryon data if phenomenological values are employed for the meson-baryon couplings. The SU(2) case is also examined for the nucleon. We observe that higher order terms are controlled only around the physical point. We also evaluate finite size effects using SU(3) heavy baryon chiral perturbation theory, finding small values of order 1% even at the physical point.

Electric and Magnetic Screening Masses at Finite Temperature from Generalized Polyakov-Line Correlations in Two-flavor Lattice QCD

Screenings of the quark-gluon plasma in electric and magnetic sectors are studied on the basis of generalized Polyakov-line correlation functions in lattice QCD simulations with two flavors of improved Wilson quarks. Using the Euclidean-time reflection (R) and the charge conjugation (C), electric and magnetic screening masses are extracted in a gauge-invariant manner. Long distance behavior of the standard Polyakov-line correlation in the quark-gluon plasma is found to be dictated by the magnetic screening. Also, the ratio of the two screening masses agrees with that obtained from the dimensionally-reduced effective field theory and the N=4 supersymmetric Yang-Mills theory.

Heavy-Quark Free Energy, Debye Mass, and Spatial String Tension at Finite Temperature in Two Flavor Lattice QCD with Wilson Quark Action

on a 16 3 × 4 lattice. From the line of constant physics at mPS/mV = 0.65 and 0.80, we extract the heavy-quark free energies, the effective running coupling geff(T) and the Debye screening mass mD(T) for various color channels of heavy quark–quark and quark–anti-quark pairs above the critical temperature. The free energies are well approximated by the screened Coulomb form with the appropriate Casimir factors at high temperature. The magnitude and the temperature dependence of the Debye mass are compared to those of the next-to-leading order thermal perturbation theory and to a phenomenological formula in terms of geff(T). We make a comparison between our results with the Wilson quark action and the previous results with the staggered quark action. The spatial string tension is also studied in the high temperature phase and is compared to the next-to-nextleading order prediction in an effective theory with dimensional reduction.

Thermodynamics of two-flavour lattice QCD with an improved Wilson quark action at non-zero temperature and density

We report the current status of our systematic studies of the QCD thermodynamics by lattice QCD simulations with two flavours of improved Wilson quarks. We evaluate the critical temperature of two-flavour QCD in the chiral limit at a zero chemical potential and show the preliminary result. Also we discuss fluctuations at non-zero temperature and density by calculating the quark number and isospin susceptibilities and their derivatives with respect to the chemical potential.

QCD Thermodynamics at Zero and Finite Densities with Improved Wilson Quarks

The WHOT-QCD Collaboration is pushing forward lattice studies of QCD at finite temperatures and densities using improved Wilson quarks. We first present results on QCD at zero and finite densities with two flavors of degenerate quarks (N_F=2 QCD) adopting the conventional fixed-Nt approach. We then report on the status of a study of N_F=2+1 QCD adopting a fixed-scale approach armed with the T-integration method which we have developed.

SU(2) and SU(3) chiral perturbation theory analyses on meson and baryon masses in 2+1 flavor lattice QCD

We investigate the quark mass dependence of meson and baryon masses obtained from 2+1 flavor dynamical quark simulations performed by the PACS-CS Collaboration. With the use of SU(2) and SU(3) chiral perturbation theories up to NLO, we examine the chiral behavior of the pseudoscalar meson masses and the decay constants in terms of the degenerate up-down quark mass ranging form 3 MeV to 24 MeV and two choices of the strange quark mass around the physical value. We discuss the convergence of the SU(2) and SU(3) chiral expansions and present the results for the low energy constants. We find that the SU(3) expansion is not convergent at NLO for the physical strange quark mass. The chiral behavior of the nucleon mass is also discussed based on the SU(2) heavy baryon chiral perturbation theory up to NNLO. Our results show that the expansion is well behaved only up to m_pi^2 ~ 0.2 GeV^2.