SOLVING THE DECOMPACTIFICATION PROBLEM IN STRING THEORY

We investigate heterotic ground states in four dimensions in which N = 4 supersymmetry is spontaneously broken to N = 2. The N = 4 supersymm etry is restored at a decompactification limit corresponding to m(3/2) --> 0. We calculate the full moduli-dependent threshold corrections a nd confirm that they are suppressed in the decompactification limit, a s expected from the restoration of N = 4 supersymmetry. This should be contrasted with the behaviour of the standard N = 2 ground states, wh ere the couplings blow up linearly with the volume of the decompactify ing manifold. This mechanism provides a solution to the decompactifica tion problem for the gauge coupling constants. We also discuss how the mechanism can be implemented in ground states with lower supersymmetr y.