GAUGE COUPLING UNIFICATION IN REALISTIC FREE-FERMIONIC STRING MODELS

We discuss the unification of gauge couplings within the framework of a wide class of realistic free-fermionic string models which have appe ared in the literature, including the flipped SU(5), SO(6) x SO(4), an d various SU(3) x SU(2) x U(1) models. If the matter spectrum below th e string scale is that of the Minimal Supersymmetric Standard Model (M SSM), then string unification is in disagreement with experiment. We t herefore examine several effects that may modify the minimal string pr edictions. First, we develop a systematic procedure for evaluating the one-loop heavy string threshold corrections in free-fermionic string models, and we explicitly evaluate these corrections for each of the r ealistic models. We find that these string threshold corrections are s mall, and we provide general arguments explaining why such threshold c orrections are suppressed in string theory. Thus heavy thresholds cann ot resolve the disagreement with experiment. We also study the effect of non-standard hypercharge normalizations, light SUSY thresholds, and intermediate-scale gauge structure, and similarly conclude that these effects cannot resolve the disagreement with low-energy data. Finally , we examine the effects of additional color triplets and electroweak doublets beyond the MSSM. Although not required in ordinary grand unif ication scenarios, such states generically appear within the context o f certain realistic free-fermionic string models. We show that if thes e states exist at the appropriate thresholds, then the gauge couplings will indeed unify at the string scale. Thus, within these string mode ls, string unification can be in agreement with low-energy data.