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We report on a study of the light quark spectrum using an improved gauge action and both Kogut-Susskind and Naik quark actions. We have studied six different lattice spacings, corresponding to plaquette couplings ranging from 6.8 to 7.9, with five to six quark masses per coupling. We compare the two quark actions in terms of the spectrum and restoration of flavor symmetry. We also compare these results with those from the conventional action.
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We investigate effects of action improvement on the light hadron spectrum and the static quark potential in two-flavor QCD for $a^{-1} approx 1$ GeV and $m_{PS}/m_V = 0.7-0.9$. We compare a renormalization group improved action with the plaquette action for gluons, and the SW-clover action with the Wilson action for quarks. We find a significant improvement in the hadron spectrum by improving the quark action, while the gluon improvement is crucial for a rotationally invariant static potential. We also explore the region of light quark masses corresponding to $m_{PS}/m_V geq 0.4$ on a 2.7 fm lattice using the improved gauge and quark action. A flattening of the potential is not observed up to 2 fm.
We use improved lattice actions for glue, light quarks and heavy quarks for which we use lattice NRQCD to compute hadron masses. Our results are in good agreement with experiment, except for charmed hadrons. It seems that charmed quar ks are not well approximated as heavy quarks nor as light quarks.
We present first results from a simulation of quenched overlap fermions with improved gauge field action. Among the quantities we study are the spectral properties of the overlap operator, the chiral condensate and topological charge, quark and hadron masses, and selected nucleon matrix elements. To make contact with continuum physics, we compute the renormalization constants of quark bilinear operators in perturbation theory and beyond.
We describe a calculation of heavy-light decay constants including virtual quark loop effects. We have generated dynamical gauge configurations at three $beta$ values using two flavors of Kogut-Susskind quarks with a range of masses. These are analyzed with a Wilson valence quark action. Preliminary results based on a ``fat-link clover valence quark action are also reported. Results from the two methods differ by 30 to 50 MeV, which is presumably due to significant - but as yet unobserved - lattice spacing dependence in one or both of the approaches.
We present a summary of results of the joint CP-PACS and JLQCD project toward a 2+1 flavor full QCD simulation with the O(a)-improved Wilson quark formalism and the Iwasaki gauge action. Configurations were generated during 2002-2005 at three lattice spacings, a~0.076, 0.100 and 0.122 fm, keeping the physical volume constant at (2.0fm)^3. Up and down quark masses are taken in the range m_{PS}/m_V~0.6-0.78. We have completed the analysis for the light meson spectrum and quark masses in the continuum limit using the full configuration set. The predicted meson masses reproduce experimental values in the continuum limit at a 1% level. The average up and down, and strange quark masses turn out to be m_{ud}^{bar{MS}}(mu=2 GeV)=3.50(14)({}^{+26}_{-15}) MeV and m_s^{bar{MS}}(mu=2 GeV)=91.8(3.9)({}^{+6.8}_{-4.1}) MeV. We discuss our future strategy toward definitive results on hadron spectroscopy with the Wilson-clover formalism.
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