The Critical Hopping Parameter in O(a) improved Lattice QCD

We calculate the critical value of the hopping parameter, κc, in O(a) improved Lattice QCD, to two loops in perturbation theory. We employ the Sheikholeslami-Wohlert (clover) improved action for Wilson fermions. The quantity which we study is a typical case of a vacuum expectation value resulting in an additive renormalization; as such, it is characterized by a power (linear) divergence in the lattice spacing, and its calculation lies at the limits of applicability of perturbation theory. The dependence of our results on the number of colors N , the number of fermionic flavors Nf , and the clover parameter cSW, is shown explicitly. We compare our results to non perturbative evaluations of κc coming from Monte Carlo simulations.