PHENOMENOLOGY OF 3-FAMILY GRAND UNIFIED STRING MODELS

In the 3-family grand unified string models constructed so far, there is only one adjoint (and no higher dimensional representation) Higgs h eld in the grand unified gauge group, In this preliminary analysis, we address the proton-decay problem in the 3-family E-6 and related SO(1 0) string models. In particular, we analyze the doublet-triplet splitt ing (within certain assumptions about non-perturbative dynamics), It a ppears that generically some fine-tuning is necessary to arrange for a pair of Higgs doublets to be light, while having color Higgs triplets superheavy. We also discuss charge-2/3 quark mass matrix that generic ally also seems to require some fine-tuning to have rank 1. (C) 1997 E lsevier Science B.V.