GAUGE COUPLING RUNNING IN MINIMAL SU(3)XSU(2)XU(1) SUPERSTRING UNIFICATION

We study the evolution of the gauge coupling constants in string unification schemes in which the light spectrum below the compactification scale is exactly that of the minimal supersymmetric standard model. In the absence of string threshold corrections the predicted values sin2-theta-w = 0.218 and alpha-2 = 0.20 are in gross conflict with experiment, but these corrections are generically important. One can express the string threshold corrections to sin2-theta-w and alpha-s in terms of certain modular weights of quark, lepton and Higgs superfields as well as the moduli of the string model. We find that in order to get agreement with the experimental measurements within the context of this minimal scheme, certain constraints on the modular weights of the quark, lepton and Higgs superfields should be obeyed. Our analysis indicates that this minimal string unification scheme is a rather constrained scenario.