Quark-gluon vertex from lattice QCD

The quark-gluon vertex in Landau gauge is studied in the quenchedapproximation using the Sheikholeslami-Wohlert (SW) fermion action withmean-field improvement coefficients in the action and for the quark fields. Wesee that the form factor that includes the running coupling is substantiallyenhanced in the infrared, over and above the enhancement arising from theinfrared suppression of the quark propagator alone. We define two differentmomentum subtraction renormalisation schemes -- \MOMT (asymmetric) and \MOMB\(symmetric) -- and determine the running coupling in both schemes. We findLambda_msbar(N_f=0)=300(+150/-180)(55)(30) MeV from the asymmetric scheme. Thisis somewhat higher than other determinations of this quantity, but theuncertainties -- both statistical and systematic -- are large. In the symmetricscheme, statistical noise prevents us from obtaining a meaningful estimate forLambda_msbar.Comment: 33 pages, 12 figures, JHEP3.cls Major restructuring, some discussion of infrared behaviour added, results unchanged. Published versio