Discontinuous BPS spectra in $N = 2$ gauge theory

We consider the spectrum of BPS saturated states in $N = 2$ gauge theories in four dimensions. This spectrum may be discontinuous across real codimension one submanifolds of marginal stability in the moduli space of vacua. An example, which can be treated with semiclassical methods in the weak coupling limit, is the decay of quark-soliton bound states. For a quark and a soliton of electric-magnetic charge vectors $Q$ and $Q^\prime$ respectively, we find that as the manifold of marginal stability is crossed, the number of soliton states changes by a factor of $2^{Q \cdot Q^\prime}$, where the dot denotes the symplectic product.