Numerical study of lattice Landau gauge QCD and the Gribov copy problem

Abstract

The infrared properties of lattice Landau gauge QCD of SU(3) are studied by measuring gluon propagator, ghost propagator, QCD running coupling and Kugo-Ojima parameter of $\beta=6.0, 16^4,24^4,32^4$ and $\beta=6.4, 32^4, 48^4, 56^4$ lattices. By the larger lattice measurements, we observe that the runnning coupling measured by the product of the gluon dressing function and the ghost dressing function squared rescaled to the perturbative QCD results near the highest lattice momentum has the maximum of about 2.2 at around $q=0.5$ GeV/c, and behaves either approaching constant or even decreasing as $q$ approaches zero. The magnitude of the Kugo-Ojima parameter is getting larger but staying around -0.83 in contrast to the expected value -1 in the continuum theory. We observe, however, there is an exceptional sample which has larger magnitude of the Kugo-Ojima parameter and stronger infrared singularity of the ghost propagator. The reflection positivity of the 1-d Fourier transform of the gluon propagator of the exceptional sample is manifestly violated. Gribov noise problem was studied by performing the fundamental modular gauge (FMG) fixing with use of the parallel tempering method of $\beta=2.2, 16^4$ SU(2) configurations. Findings are that the gluon propagator almost does not suffer noises, but the Kugo-Ojima parameter and the ghost propagator in the FMG becomes $\sim 5$% less in the infrared region than those suffering noises. It is expected that these qualitative aspects seen in SU(2) will reflect in the infrared properties of SU(3) QCD as well.