Making ends meet: String unification and low-energy data

We examine the effects that may resolve the disagreement between string-scale unification and low-energy data. These include string threshold corrections, nonstandard hypercharge normalizations, light supersymmetric thresholds, intermediate gauge structure, and extra matter beyond the minimal supersymmetric standard model. We explicitly evaluate these effects within the flipped SU(5), SO(6)×SO(4), and various SU(3)×SU(2)×U(1) string models, and surprisingly find that the only way to reconcile string unification with low-energy data is through certain types of extra matter. Remarkably, many of the realistic string models contain precisely this required matter in their massless spectra.