Dyon electric charge and fermion fractionalization in N=2 gauge theory

A first principles calculation of the quantum corrections to the electric charge of a dyon in an N=2 gauge theory with arbitrary gauge group is presented. These corrections arise from the fermion fields via the mechanism of fermion fractionalization. For a dyon whose magnetic charge is a non-simple co-root, the correction is a discontinuous function on the moduli space of vacua and the discontinuities occur precisely on co-dimension-1 curves on which the dyon decays. In this way, the complete spectrum of dyons at weak coupling is found for a theory with any gauge group. It is shown how this spectrum is consistent with the semi-classical monodromies. (orig.)