PERTURBATIVE AND NONPERTURBATIVE PARTIAL SUPERSYMMETRY BREAKING - N=4-]N=2-]N=1

We show the existence of a supersymmetry breaking mechanism in string theory, where N = 4 supersymmetry is broken spontaneously to N = 2 and N = 1 with moduli-dependent gravitino masses. The spectrum of the spo ntaneously broken theory with lower supersymmetry is in one-to-one cor respondence with the spectrum of the heterotic N = 4 string. The mass splitting of the N = 4 spectrum depends on the compactification moduli as well as the three R-symmetry charges. We also show that, in string theory, chiral theories can be obtained after spontaneous breaking of extended supersymmetry. This was impossible at the level of field the ory. In the large-moduli limit a restoration of the N = 4 supersymmetr y is obtained. As expected the graviphotons and some of the gauge boso ns become massive in N = 1 vacua. At some special points of the moduli space some of the N = 4 states with non-zero winding numbers and with spin 0 and 1/2 become massless chiral superfields of the unbroken N = 1 supersymmetry. Such vacua have a dual type II description, in which there are magnetically charged states with spin 0 and 1/2 that become massless. The heterotic-type II duality suggests some novel non-pertu rbative transitions on the type II side. Such transitions do not seem to have a geometric interpretation, since they relate type II vacua wi th symmetric world-sheet structure to asymmetric ones. The heterotic i nterpretation of such a transition is an ordinary Higgsing of an SU(2) factor. In the case of N = 4 --> N = 2, the perturbative N = 2 prepot ential is determined by the perturbative N = 4 BPS states. This observ ation permits us to suggest a method to determine the exact non-pertur bative prepotential of the effective N = 2 supergravity using the shif ted spectrum of the non-perturbative BPS states of the underlying N = 4 theory. (C) 1997 Elsevier Science B.V.