Quark-gluon vertex from lattice QCD

The quark-gluon vertex in Landau gauge is studied in the quenched approximation using the Sheikholeslami-Wohlert (SW) fermion action with mean-field improvement coefficients in the action and for the quark fields. We see that the form factor that includes the running coupling is substantially enhanced in the infrared, over and above the enhancement arising from the infrared suppression of the quark propagator alone. We define two different momentum subtraction renormalisation schemes – \MOMT (asymmetric) and \MOMB
(symmetric) – and determine the running coupling in both schemes. We find Lambda_msbar(N_f=0)=300(+150/-180)(55)(30) MeV from the asymmetric scheme. This is somewhat higher than other determinations of this quantity, but the uncertainties – both statistical and systematic – are large. In the symmetric scheme, statistical noise prevents us from obtaining a meaningful estimate for Lambda_msbar.