Infrared features of the Landau gauge QCD

The infrared features of Landau gauge QCD are studied by lattice simulation of beta=6.0,16(4),24(4),32(4) and beta=6.4,32(4),48(4). We adopt two definitions of the gauge field, (1) U linear and (2) logU, and measured the gluon propagator and ghost propagator. The infrared singularity of the gluon propagator is less than that of the tree level result, but the gluon propagator at 0 momentum remains finite. The infrared singularity of the ghost propagator is stronger than the tree level. The QCD running coupling measured by using the gluon propagator and the ghost propagator has a maximum alpha(s)(p)similar or equal to1 at around p=0.5 GeV and decreases as p approaches 0. The data are analyzed with use of the formula of the principle of minimal sensitivity and the effective charge method, and by the contour-improved perturbation method, which suggest the necessity of the resummation of the perturbation series in the infrared region together with the existence of the infrared fixed point. The Kugo-Ojima parameter is about -0.8 in contrast with the theoretically expected value of -1. The color off-diagonal part of the ghost propagator in the Landau gauge is consistent with zero, and its fluctuation can be parametrized as a constant/(qa)(4).