Search

We perform a nonperturbative determination of the O(a)-improvement coefficient cSW for the Wilson quark action in three-flavor QCD with the plaquette gauge action. Numerical simulations are carried out in a range of beta=12.0–5.2 on a single lattice size of 83×16 employing the Schrödinger functional setup of lattice QCD. As our main result, we obtain an interpolation formula for cSW and the critical hopping parameter Kc as a function of the bare coupling. This enables us to remove the O(a) scaling violation from physical observables in future numerical simulation in the wide range of beta. Our analysis with a perturbatively modified improvement condition for cSW suggests that finite volume effects in cSW are not large on the 83×16 lattice. We investigate Nf dependence of cSW by additional simulations for Nf=4, 2, and 0 at beta=9.6. As a preparatory step for this study, we also determine cSW in two-flavor QCD at beta=5.2. At this beta, several groups have carried out large-scale calculations of the hadron spectrum, while no systematic determination of cSW has been performed.

A lattice QCD calculation of the B meson decay constant is presented. In order to investigate the scaling violation associated with the heavy quark, parallel simulations are carried out employing both Wilson and the O(a)-improved clover actions for the heavy quark. The discretization errors due to the large b quark mass are estimated in a systematic way with the aid of the non-relativistic interpretation approach of El-Khadra, Kronfeld and Mackenzie. As our best value from the quenched simulations at beta=5.9,6.1 and 6.3 we obtain fB=163±16 MeV and fBs=175±18 MeV in the continuum limit where the error includes both statistical and systematic uncertainties.

type:text, A lattice QCD calculation of the B meson decay constant is presented. In order to investigate the scaling violation associated with the heavy quark, parallel simulations are carried out employing both Wilson and the O(a)-improved clover actions for the heavy quark. The discretization errors due to the large b quark mass are estimated in a systematic way with the aid of the non-relativistic interpretation approach of El-Khadra, Kronfeld and Mackenzie. As our best value from the quenched simulations at beta=5.9,6.1 and 6.3 we obtain fB=163±16 MeV and fBs=175±18 MeV in the continuum limit where the error includes both statistical and systematic uncertainties.

We present a polynomial hybrid Monte Carlo (PHMC) algorithm for lattice QCD with odd numbers of flavors of O(a)-improved Wilson quark action. The algorithm makes use of the non-Hermitian Chebyshev polynomial to approximate the inverse square root of the fermion matrix required for an odd number of flavors. The systematic error from the polynomial approximation is removed by a noisy Metropolis test for which a new method is developed. Investigating the property of our PHMC algorithm in the Nf=2 QCD case, we find that it is as efficient as the conventional HMC algorithm for a moderately large lattice size (163×48) with intermediate quark masses (mPS/mV∼0.7–0.8). We test our odd-flavor algorithm through extensive simulations of two-flavor QCD treated as an Nf=1+1 system, and comparing the results with those of the established algorithms for Nf=2 QCD. These tests establish that our PHMC algorithm works on a moderately large lattice size with intermediate quark masses (163×48,mPS/mV∼0.7–0.8). Finally we experiment with the (2+1)-flavor QCD simulation on small lattices (43×8 and 83×16), and confirm the agreement of our results with those obtained with the R algorithm and extrapolated to a zero molecular dynamics step size.

We propose an exact simulation algorithm for lattice QCD with dynamical Kogut–Susskind fermion in which the Nf-flavor fermion operator is defined as the Nf/4th root of the Kogut–Susskind (KS) fermion operator. The algorithm is an extension of the Polynomial Hybrid Monte Carlo (PHMC) algorithm to KS fermions. The fractional power of the KS fermion operator is approximated with a Hermitian Chebyshev polynomial, with which we can construct an algorithm for any number of flavors. The error which arises from the approximation is corrected by the Kennedy–Kuti noisy Metropolis test. Numerical simulations are performed for the two-flavor case for several lattice parameters in order to confirm the validity and the practical feasibility of the algorithm. In particular tests on a 16 4 lattice with a quark mass corresponding to mPS/mVnot, vert, similar0.68 are successfully accomplished. We conclude that our algorithm provides an attractive exact method for dynamical QCD simulations with KS fermions.

We have determined non-perturbative values of theO(a) improvement coefficient csw in three-flavor dynamical QCD for the RG improved as well as the plaquette gauge actions, using the Schro¨dinger functional scheme. Results are compared with 1-loop estimates at weak gauge coupling.

We perform a nonperturbative determination of the O(a)-improvement coefficient cSW and the critical hopping parameter kappac for Nf=3, 2, and 0 flavor QCD with the (RG) renormalization-group-improved gauge action using the Schrödinger functional method. In order to interpolate cSW and kappac as a function of the bare coupling, a wide range of beta from the weak coupling region to the moderately strong coupling points used in large-scale simulations is studied. Corrections at finite lattice size of O(a/L) turned out to be large for the RG-improved gauge action, and hence we make the determination at a size fixed in physical units using a modified improvement condition. This enables us to avoid O(a) scaling violations which would remain in physical observables if cSW determined for a fixed lattice size L/a is used in numerical simulations.

We present a lattice QCD calculation of the heavy quark expansion parameters µpi2 and µG2 for heavy-light mesons and heavy-light-light baryons. The calculation is carried out on a 203×48 lattice at beta=6.0 in the quenched approximation, using the lattice NRQCD action for heavy quarks. We obtain the parameters µpi2 and µG2 in two different methods: a direct calculation of the matrix elements and an indirect calculation through the mass spectrum, and confirm that both the methods give consistent results. We also discuss an application to the lifetime ratios.

We explore sea quark effects in the light hadron mass spectrum in a simulation of two-flavor QCD using the nonperturbatively O(a)-improved Wilson fermion action. In order to identify finite-size effects, light meson masses are measured on 123 × 48, 163 × 48 and 203 × 48 lattices with a 0.1 fm. On the largest lattice, where the finite-size effect is negligible, we find a significant increase of the strange vector meson mass compared to the quenched approximation. We also investigate the quark mass dependence of pseudoscalar meson masses and decay constants and test the consistency with (partially quenched) chiral perturbation theory.

We present a high statistics study of the light hadron spectrum and quark masses in QCD with two flavors of dynamical quarks. Numerical simulations are carried out using the plaquette gauge action and the O(a)-improved Wilson quark action at beta=5.2, where the lattice spacing is found to be a=0.0887(11) fm from the rho meson mass, on a 203×48 lattice. At each of five sea quark masses corresponding to mPS/mV~=0.8–0.6, we generate 12 000 trajectories using a symmetrically preconditioned Hybrid Monte Carlo algorithm. Finite spatial volume effects are investigated employing 123×48, 163×48 lattices. We also perform a set of simulations in quenched QCD with the same lattice actions at a similar lattice spacing to those for the full QCD runs. In the meson sector we find clear evidence of sea quark effects. The J parameter increases for lighter sea quark masses, and the full QCD meson masses are systematically closer to experiment than in quenched QCD. Careful finite-size studies are made to ascertain that these are not due to finite-size effects. Evidence of sea quark effects is less clear in the baryon sector due to larger finite-size effects. We also calculate light quark masses and find mud[overline MS](2 GeV)=3.223(-0.069+0.046) MeV and ms[overline MS](2 GeV)=84.5(-1.7+12.0) MeV which are about 20(null)maller than in quenched QCD.

We test the one-loop chiral perturbation theory formula on unquenched lattice data of pseudoscalar meson decay constants. The chiral extrapolation including the effect of the chiral logarithm is attempted and its uncertainty is discussed.

We present preliminary results of a new lattice computation of hadronic matrix elements of baryon number violating operators which appear in the low-energy effective Lagrangian of (SUSY-)Grand Unified Theories. The contribution of irrelevant form factor which has caused an underestimate of the matrix elements in previous studies is subtracted in this calculation. Our results are 24 times larger than the most conservative values often employed in phenomenological analyses of nucleon decay with specific GUT models.

CP-PACS and JLQCD Collaborations are carrying out a joint project of the 2+1 flavor full QCD with the RG-improved gauge action and the non-perturbatively -improved Wilson quark action. This simulation removes quenching effects of all three light quarks, which is the last major uncertainty in lattice QCD. In this report we present our results for the light meson spectrum and quark masses on a 203×40 lattice at the lattice spacing a0.10 fm.

A lattice QCD calculation of the B meson decay constant is presented. In order to investigate the scaling violation associated with the heavy quark, parallel simulations are carried out employing both Wilson and the O(a)-improved clover actions for the heavy quark. The discretization errors due to the large b quark mass are estimated in a systematic way with the aid of the non-relativistic interpretation approach of El-Khadra, Kronfeld and Mackenzie. As our best value from the quenched simulations at beta=5.9,6.1 and 6.3 we obtain fB=163±16 MeV and fBs=175±18 MeV in the continuum limit where the error includes both statistical and systematic uncertainties.