EXACT SEMICLASSICAL AND NONPERTURBATIVE BPS SPECTRA IN N=2 SUSY QCD

These notes are based on work done in collaboration with Frank Ferrari . We show how to determine the spectra of stable BPS states in N = 2 s upersymmetric SU(2) Yang-Mills theories that are asymptotically free, i.e. without and with N-f = 1, 2, 3 quark hypermultiplets. In all case s: There is a curve of marginal stability diffeomorphic to a circle an d going through all (finite) singular points of moduli space. The BPS spectra are discontinuous across these curves. The strong-coupling spe ctra (inside the curves) contain only those BPS states that can become massless and are responsible for the singularities. Except for N-f = 3, they form a multiplet (with different masses) of the broken global discrete symmetry. Ail other semi-classical BPS states must and do dec ay consistently when crossing the curves. The weak-coupling i.e. semi- classical BPS spectra, contain no magnetic charges larger than one for N-f = 0, 1, 2 and no magnetic charges larger than two for N-f = 3.