Quark propagator at finite temperature and finite momentum in quenched lattice QCD

We present an analysis of the quark spectral function above and below the critical temperature for deconfinement performed at zero and nonzero momentum in quenched lattice QCD using clover improved Wilson fermions in Landau gauge. It is found that the temporal quark correlation function in the deconfined phase near the critical temperature is well reproduced by a two-pole ansatz for the spectral function. This indicates that excitation modes of the quark field have small decay rates. The bare quark mass and momentum dependence of the spectral function is analyzed with this ansatz. In the chiral limit we find that the quark spectral function has two collective modes corresponding to the normal and plasmino excitations in the high-temperature limit. Over a rather wide temperature range in the deconfined phase the pole mass of these modes at zero momentum, which corresponds to the thermal mass of the quark, is approximately proportional to temperature. With increasing bare quark masses the plasmino mode gradually disappears, and the spectral function is dominated by a single pole. We also discuss quasiparticle properties of heavy quarks in the deconfined phase. In the confined phase, it is found that the pole ansatz for the spectral function fails completely.