2-loop additive mass renormalization with clover fermions and Symanzik   improved gluons
  We calculate the critical value of the hopping parameter, kappa_c, in Lattice QCD, up to two loops in perturbation theory. We employ the Sheikholeslami-Wohlert (clover) improved action for Wilson fermions and the Symanzik improved gluon action for 4- and 6-link loops.   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.   Our results are polynomial in c_{SW} (clover parameter) and cover a wide range of values for the Symanzik coefficients c_i. Furthermore, the dependence on the number of colors N and the number of fermionic flavors N_f is shown explicitly. In order to compare our results to non perturbative evaluations of kappa_c coming from Monte Carlo simulations, we employ an improved perturbation theory method applied to improved actions. 
A. Skouroupathis, M. Constantinou and H. Panagopoulos