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At the precision reached in current lattice QCD calculations, electromagnetic effects are becoming numerically relevant. We will present preliminary results for electromagnetic corrections to light hadron masses, based on simulations in which a $mathrm{U}(1)$ degree of freedom is superimposed on $N_f=2+1$ QCD configurations from the BMW collaboration.
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For some time, the MILC Collaboration has been studying electromagnetic effects on light mesons. These calculations use fully dynamical QCD, but only quenched photons, which suffices to NLO in XPT. That is, the sea quarks are electrically neutral, while the valence quarks carry charge. For the photons we use the non-compact formalism. We have new results with lattice spacing as small as 0.045 fm and a large range of volumes. We consider how well chiral perturbation theory describes these results and the implications for light quark masses
We present details of simulations for the light hadron spectrum in quenched QCD carried out on the CP-PACS parallel computer. Simulations are made with the Wilson quark action and the plaquette gauge action on 32^3x56 - 64^3x112 lattices at four lattice spacings (a approx 0.1-0.05 fm) and the spatial extent of 3 fm. Hadronic observables are calculated at five quark masses (m_{PS}/m_V approx 0.75 - 0.4), assuming the u and d quarks being degenerate but treating the s quark separately. We find that the presence of quenched chiral singularities is supported from an analysis of the pseudoscalar meson data. We take m_pi, m_rho and m_K (or m_phi) as input. After chiral and continuum extrapolations, the agreement of the calculated mass spectrum with experiment is at a 10% level. In comparison with the statistical accuracy of 1-3% and systematic errors of at most 1.7% we have achieved, this demonstrates a failure of the quenched approximation for the hadron spectrum: the meson hyperfine splitting is too small, and the octet masses and the decuplet mass splittings are both smaller than experiment. Light quark masses are calculated using two definitions: the conventional one and the one based on the axial-vector Ward identity. The two results converge toward the continuum limit, yielding m_{ud}=4.29(14)^{+0.51}_{-0.79} MeV. The s quark mass depends on the strange hadron mass chosen for input: m_s = 113.8(2.3)^{+5.8}_{-2.9} MeV from m_K and m_s = 142.3(5.8)^{+22.0}_{-0} MeV from m_phi, indicating again a failure of the quenched approximation. We obtain Lambda_{bar{MS}}^{(0)}= 219.5(5.4) MeV. An O(10%) deviation from experiment is observed in the pseudoscalar meson decay constants.
The aim of the GRAL project is to simulate full QCD with standard Wilson fermions at light quark masses on small to medium-sized lattices and to obtain infinite-volume results by extrapolation. In order to establish the functional form of the volume dependence we study systematically the finite-size effects in the light hadron spectrum. We give an update on the status of the GRAL project and show that our simulation data for the light hadron masses depend exponentially on the lattice size.
CP-PACS and JLQCD collaborations are carrying out a joint project of the 2+1 flavor full QCD simulation. Gauge configurations are generated for the non-perturbatively $O(a)$-improved Wilson quark action and the Iwasaki gauge action using PHMC algorithm at three lattice spacings, $asim 0.076$, 0.010 and 0.122 fm, with a fixed physical volume $(2.0 fm)^3$. We present analysis for the light meson spectrum and quark masses in the continuum limit, which are determined using data obtained from the simulations at the two coarser lattices. Our simulations reproduce experimental values of meson masses. The ud and strange quark masses turn out to be $m_{ud}^{bar{MS}}(mu=2 GeV)=3.34(23) MeV$ and $m_s^{bar{MS}}(mu=2 GeV)=86.7(5.9) MeV$. We also show preliminary results at our finest lattice spacing for which simulations are still being continued.
We present the final results of the CP-PACS calculation of the light hadron spectrum and quark masses with two flavors of dynamical quarks. Simulations are made with a renormalization-group improved gauge action and a mean-field improved clover quark action for sea quark masses corresponding to $m_{rm PS}/m_{rm V} approx 0.8$--0.6 and the lattice spacing $a=0.22$--0.11 fm. For the meson spectrum in the continuum limit a clearly improved agreement with experiment is observed compared to the quenched case, demonstrating the importance of sea quark effects. For light quark masses we obtain $m_{ud}^{bar{MS}}(2GeV)=3.44^{+0.14}_{-0.22}$ MeV and $m_s^{bar{MS}}(2GeV)=88^{+4}_{-6}$ MeV ($K$-input) and $m_s^{bar{MS}}(2GeV)=90^{+5}_{-11}$ MeV ($phi$-input), which are reduced by about 25% compared to the values in quenched QCD.
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