String Inspired Z' Model With Stable Proton and Light Neutrino Masses

Grand unification, and its incarnation in the form of heterotic-string unification, are the only extensions of the Standard Model that are rooted in the structure of the Standard Model itself. In this context, it was proposed that the exclusiveness of proton stability and suppression of neutrino masses necessitates the existence of an additional U(1)_Z' symmetry, which is of non-GUT origin and remains unbroken down to intermediate, or low, energies. Realistic string models frequently give rise to non-GUT U(1) symmetries, which arise from the flavor symmetries in the models. In this paper we demonstrate in a string-inspired toy model that such a stringy Z' can indeed guarantee proton longevity and viable phenomenology in the neutrino sector, as well as in the quark and charged lepton sectors.