Implications of Anomalous U ( 1 ) Symmetry in Unified Models : the Flipped SU ( 5 ) × U ( 1 ) Paradigm

A generic feature of string-derived models is the appearance of an anomalous Abelian U (1) A symmetry which, among other properties, constrains the Yukawa couplings and distinguishes the three families from each other. In this paper, we discuss in a model-independent way the general constraints imposed by such a U (1) A symmetry on fermion masses, R-violating couplings and proton-decay operators in a generic flipped SU (5) × U (1) ′ model. We construct all possible viable fermion mass textures and give various examples of effective low-energy models which are distinguished from each other by their different predictions for B-, Land R-violating effects. We pay particular attention to predictions for neutrino masses, in the light of the recent Super-Kamiokande data.