Implications of anomalous U(1) symmetry in unified models: the flipped SU(5) x U(1) paradigm

A generic feature of string-derived models is the appearance of an anomalou s abelian U(1)(A) symmetry which, among other properties, constrains the Yu kawa couplings and distinguishes the three families from each other. In thi s paper, we discuss in a model-independent way the general constraints impo sed by such a U(1)(A) symmetry on fermion masses, R-violating couplings and proton-decay operators in a generic flpped SU(5) x U(1)' model. We constru ct all possible viable fermion mass textures and give various examples of e ffective low-energy models which are distinguished from each other by their different predictions for B-, L- and R-violating effects. We pay particula r attention to predictions for neutrino masses, in the light of the recent Super-Kamiokande data.