Implications of anomalous U(1) symmetry in unified models: the flipped SU(5)xU(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)xU(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-, L- and R-violating effects. We pay particular attention to predictions for neutrino masses, in the light of the recent Super-Kamiokande data. (author)