NEW CONSTRAINTS ON SO(10) MODEL-BUILDING FROM STRING THEORY

A recent analysis of the methods by which higher-level gauge symmetrie s are realized in string theory has made it possible to systematically obtain new constraints on string GUT model-building. In this paper, w e perform such a study for the case of SO(10) free-field string GUT mo dels, and find a number of significant new results. First, we show tha t within the conventional string models using the so-called ''diagonal '' embeddings, all representations larger than the 16 must always tran sform as singlets under all gauge symmetries beyond SO(10). This inclu des the 45 and 54 representations, and holds regardless of the affine level at which SO(10) is realized. Furthermore, we show that such cons tructions can never give rise to the 120, 126, or 144 representations of SO(10) - again regardless of the affine level. We also perform a si milar analysis for E(6), and find that the adjoint 78 representation m ust always transform as a singlet under all gauge symmetries beyond E( 6); moreover, all representations beyond the 78 representation are pro hibited. Taken together, these results thus severely limit the types o f phenomenologically realistic field-theoretic SO(10) and E(6) models that can be obtained using such string constructions. Finally, we also explore the possibility of using non-diagonal free-field embeddings, and show that in the SO(10) case such embeddings may have improved phe nomenological prospects.