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We calculate the light meson spectrum and the light quark masses by lattice QCD simulation, treating all light quarks dynamically and employing the Iwasaki gluon action and the nonperturbatively O(a)-improved Wilson quark action. The calculations are made at the squared lattice spacings at an equal distance a 2 ≃ 0.005, 0.01 and 0.015 fm 2 , and the continuum limit is taken assuming an O(a 2 ) discretization error. The light meson spectrum is consistent with experiment. The up, down and strange quark masses in the MS scheme at 2 GeV are m = (mu + m d )/2 = 3.55 The masses of light quarks are fundamental parameters of QCD. They cannot be measured experimentally since quarks are confined in hadrons. Lattice QCD enables calculations of hadron masses as functions of quark masses, and hence allows a determination of the quark masses from the experimental hadron masses. This approach has been successfully applied, first in quenched QCD [1] and then in N f = 2 QCD where degenerate up (u) and down (d) quarks are treated dynamically [2]. These studies have revealed that the light quark mass values are significantly reduced by dynamical u and d quark effects. In this article, we present our attempt to determine the quark masses in N f = 2 + 1 QCD where the heavier strange (s) quark is also treated dynamically. We wish to examine to what extent the dynamical s quark affects the light quark masses. We determine the quark masses in the continuum limit and estimate all possible systematic errors. We also calculate the prerequisite light meson spectrum. A similar attempt has been made by the MILC Collaboration [3].We adopt the Iwasaki RG gauge action [4] and the clover quark action with the improvement coefficient c SW determined nonperturbatively for the RG action [5]. The choice of the gauge action is made to avoid a first-order phase transition (lattice artifact) observed for the plaquette gauge action [6]. We employed the Wilson quark formalism because we prefer an unambiguous quark-flavor interpretation over the computational ease of the staggered formalism adopted by the MILC collaboration [7].Configurations are generated at three values of the coupling β ≡ 6/g 2 = 2.05, 1.90 and 1.83 corresponding to the squared lattice spacing a 2 ≃ 0.005, 0.01 and 0.015 fm 2 , with the physical volume fixed to about (2.0fm) 3 . At each β, we perform simulations for 10 quark mass combinations using a combined algorithm [8] of the Hybrid Monte Carlo (HMC) for the degenerate u and d quarks and the polynomial Hybrid Monte Calro (PHMC) for the s quark. Table I summarizes the simulation parameters.The meson and quark masses at the simulation points are determined from single exponential correlated χ 2 fits to the correlators P (t)P (0) , V (t)V (0) and A 4 (t)P (0) , where P , V and A µ denote pseudoscalar, vector and nonperturbatively O(a)-improved [9] axialvector current operators, respectively. We use an exponentially smeared source and a point sink, and measurements are made at every 10 HMC trajectories in the Coulomb gauge. For the pse...

The walking technicolor theory attempts to realize electroweak symmetry breaking as the spontaneous chiral symmetry breakdown caused by the gauge dynamics with slowly varying gauge coupling constant and large mass anomalous dimension. Many-flavor QCD theories are candidates owning these features. We focus on the SU(3) gauge theory with ten flavors of massless fermions in the fundamental representation, and compute the gauge coupling constant in the Schrödinger functional scheme. Numerical simulation is performed with OðaÞ-unimproved lattice action, and the continuum limit is taken in linear in lattice spacing. We observe evidence that this theory possesses an infrared fixed point.

We investigate the finite size effect on pseudoscalar meson masses and decay constants using a subset of the "PACS10" configurations which are generated keeping the space-time volumes over (10 fm) 4 in 2+1 flavor QCD at the physical point. We have tried two kinds of analyses, fixing κ values or measured axial Ward identity quark masses. Comparing the results on (5.4 fm) 4 and (10.8 fm) 4 lattices, we have found a sizable finite size effect on the pseudoscalar meson sector in the former analysis: a 2.1(8)%, 4.8(1.6)%, and 0.36(31)% finite size effect on mπ, m ud , and fπ, respectively, on the (5.4 fm) 4 lattice. For the latter analysis, the finite size effect on the pseudoscalar meson decay constants is 0.66(33)% for fπ, 0.26(13)% for fK , and 0.40(32)% for fK /fπ. These values with two-sigma error bars are consistent with the predictions from the full one-loop SU(3) chiral perturbation theory, which are 0.20% for fπ, 0.08% for fK , and 0.13% for fK /fπ. The finite size effect on the pseudoscalar meson masses is hardly detected under the current statistical precision.

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 ␤ϭ5.2, where the lattice spacing is found to be aϭ0.0887(11) fm from the meson mass, on a 20 3 ϫ48 lattice. At each of five sea quark masses corresponding to m PS /m V Ӎ0.8-0.6, we generate 12 000 trajectories using a symmetrically preconditioned Hybrid Monte Carlo algorithm. Finite spatial volume effects are investigated employing 12 3 ϫ48, 16 3 ϫ48 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 m ud MS (2 GeV)ϭ3.223(Ϫ0.069 ϩ0.046) MeV and m s MS (2 GeV)ϭ84.5(Ϫ1.7 ϩ12.0) MeV which are about 20% smaller than in quenched QCD.