ar X iv : 0 80 7 . 00 30 v 1 [ he p - la t ] 1 J ul 2 00 8 A specific lattice artefact in non - perturbative renormalization of operators

We discuss a specific cut-off effect which appears in applying the nonperturbative RI/MOM scheme to compute the renormalization constants. To illustrate the problem a Dirac operator satisfying the Ginsparg-Wilson relation is used, but the arguments are more general. We propose a simple modification of the method which gets rid of the corresponding discretization error. Applying this to full-QCD simulations done at a = 0.13 fm with the Fixed Point action we find that the renormalization constants are strongly distorted by the artefacts discussed. We consider also the role of global gauge transformations, a freedom which still remains after the conventional gauge fixing procedure is applied.