Parameter-Free Calculation of Hadronic Masses from Instantons

We propose a nonperturbative calculation scheme which is based on the semiclassical approximation of QCD and can be used to evaluate quantities of interest in hadronic physics. As a first application, we evaluate the mass of the pion and of the nucleon. These masses are related to a particular combination of Green functions which, in some limit, is dominated by the contribution of very small-sized instantons. The size distribution of these pseudoparticles is determined by the 't Hooft tunneling amplitude formula and therefore our calculation is free from any model parameters. We prove that instanton forces generate a light pion and a nucleon with realistic mass (Mn∼970 MeV). In connection with sum-rule approaches, we discuss the overlap of instantons with pion and nucleon resonances.