String Unification, Higher-Level Gauge Symmetries, and Exotic Hypercharge Normalizations

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We explore the extent to which string theories with higher-level gauge symmetries and non-standard hypercharge normalizations can reconcile the discrepancy between the string unification scale and the GUT scale extrapolated from the Minimal Supersymmetric Standard Model (MSSM). We determine the phenomenologically allowed regions of (kY,k₂,k₃) parameter space, and investigate the proposal that there might exist string models with exotic hypercharge normalizations kY which are less than their usual value kY=5/3. For a broad class of heterotic string models (encompassing most realistic string models which have been constructed), we prove that kY >= 5/3. Beyond this class, however, we show that there exist consistent MSSM embeddings which lead to kY < 5/3. We also consider the constraints imposed on kY by demanding charge integrality of all unconfined string states, and show that only a limited set of hypercolor confining groups and corresponding values of kY are possible.