INFRARED BEHAVIOR OF CLOSED SUPERSTRINGS IN STRONG MAGNETIC AND GRAVITATIONAL-FIELDS

A large class of four-dimensional supersymmetric ground states of clos ed superstrings with a non-zero mass gap are constructed. For such gro und states we rum on chrome-magnetic fields as well as curvature, The exact spectrum as function of the chrome-magnetic fields and curvature is derived. We examine the behavior of the spectrum, and find that th ere is a maximal value for the magnetic field H-max similar to M(planc k)(2). At this value all states that couple to the magnetic field beco me infinitely massive and decouple. We also find tachyonic instabiliti es for strong background fields of the order O(mu M(planck)) where mu is the mass gap of the theory, Unlike the field theory case, we find t hat such ground states become stable again for magnetic fields of the order O(M(planck)(2)). The implications of these results are discussed .