Threshold corrections in orbifold models and superstring unification of gauge interactions

The string one loop renormalization of the gauge coupling constants is examined in Abelian orbifold models. The contributions to string threshold corrections independent of the compactification moduli fields are evaluated numerically for several representative examples of orbifold models. We consider cases with standard and nonstandard embeddings as well as cases with discrete Wilson line background fields which match reasonably well with low energy phenomenology. The gap separating the observed grand unification scale MGUT≅2x1016 GeV from the string unification scale MX≅5x1017 GeV is discussed on the basis of standardlike orbifold models. We examine one loop gauge coupling constant unification in a description incorporating the combined effects of moduli-dependent and -independent threshold corrections, an adjustable affine level for the hypercharge group factor, and a large mass threshold associated with an anomalous U(1) mechanism. copyright 1996 The American Physical Society