THE STRONG-COUPLING SPECTRUM OF THE SEIBERG-WITTEN THEORY

We carefully study the global structure of the solution of the N = 2 s upersymmetric pure Yang-Mills theory with gauge group SU(2) obtained b y Seiberg and Witten. We exploit its Z(2) symmetry and describe the cu rve in moduli space where BPS states can become unstable, separating t he strong-coupling from the weak-coupling region. This allows us to ob tain the spectrum of stable BPS states in the strong-coupling region: we prove that only the two particles responsible for the singularities of the solution (the magnetic monopole and the dyon of unit electric charge) are present in this region. Our method also permits us to very easily obtain the weak-coupling spectrum, without using semiclassical methods, We discuss how the BPS states disintegrate when crossing the border from the weak- to the strong-coupling region.