Infrared-regulated String Theory and Loop Corrections to Coupling Constants

Exact superstring solutions are constructed in 4-D space-time, with positive curvature and non-trivial dilaton and antisymmetric tensor elds. The full spectrum of string excitations is derived as a function of moduli elds T i and the scale 2 = 1=(k + 2) which is induced by the non-zero background elds. The spectrum of string excitations has a non-zero mass gap 2 and in the weak curvature limit (small), 2 plays the role of a well deened infrared regulator, consistent with modular invariance, gauge invariance, supersymmetry and chirality. The eeects of a covariantly constant (chomo)magnetic eld H, as well as additional curvature can be derived exactly up to one string-loop level. Thus, the one-loop corrections to all couplings (gravitational, gauge and Yukawas) are un-ambiguously computed and are nite both in the UltraViolet and the InfraRed regime. These corrections are necessary for quantitative string superuniication predictions at low energies. The one-loop corrections to the couplings are also found to satisfy Infrared Flow Equations. Having in our disposal an exact description which goes beyond the leading order in the 0-expansion or the linearized approximation in the background elds, we nd interesting clues about the physics of string theory in strong gravita-tional and magnetic elds. In particular, the nature of gravitational or magnetic instabilities is studied.