The Critical Mass of Wilson Fermions: A Comparison of Perturbative and   Monte Carlo Results
  We calculate the critical value of the hopping parameter, $\kappa_c$, in Lattice QCD with Wilson fermions, to two loops in perturbation theory.   This quantity is an additive renormalization; as such, it is characterized not only by the standard caveats regarding the asymptotic nature of perturbative results, but also by a linear divergence in the lattice spacing. Consequently, our calculation tests rather stringently the limits of applicability of perturbation theory.   We compare our results to non perturbative evaluations of $\kappa_c$ coming from Monte Carlo simulations.   Finally, we apply a tadpole improvement technique on our results; this shifts them quite favourably towards the non perturbative values. 
E. Follana and H. Panagopoulos (University of Cyprus, Nicosia)