Light hadron spectroscopy with two flavors ofO(a)-improved dynamical quarks

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 $a=0.0887\mathrm{}\left(11\right)\mathrm{}\mathrm{fm}$ from the $\rho$ meson mass, on a ${20}^3\times 48$ lattice. At each of five sea quark masses corresponding to $m_{\mathrm{PS}}{/m}_{\mathrm{V}}\simeq 0.8–0.6,$ we generate 12 000 trajectories using a symmetrically preconditioned Hybrid Monte Carlo algorithm. Finite spatial volume effects are investigated employing ${12}^3\times 48,$ ${16}^3\times 48$ 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 $m_{\mathrm{ud}}^{\overline{\mathrm{MS}}}\left(2\mathrm{}\mathrm{GeV}{)=3.223(}_{-0.069}^{+0.046\mathrm{}}\right)\mathrm{MeV}$ and $m_s^{\overline{\mathrm{MS}}}\left(2\mathrm{}\mathrm{GeV}{)=84.5(}_{-1.7}^{+12.0\mathrm{}}\right)\mathrm{MeV}$ which are about 20% smaller than in quenched QCD.