The essential role of string-derived symmetries in ensuring proton-stability and light neutrino masses

Abstract

The paper addresses the problem of suppressing naturally the unsafe d = 4 as well as the color-triplet mediated and/or gravity-linked d = 5 proton-decay operators, which generically arise in SUSY-unification. It also attempts to give light masses to the neutrinos, of the type suggested by current experiments. It is noted that neither the symmetries in SO(10), nor those in E 6, suffice for the purpose - especially in the matter of suppressing naturally the d = 5 proton-decay operators. By contrast, it is shown that a certain string-derived symmetry, which cannot arise within conventional grand unification, but which does arise within a class of three-generation string-solutions, suffices, in conjunction with B - L, to safeguard proton-stability from all potential dangers, including those which may arise through higher dimensional operators and the color-triplets in the infinite tower of states. At the same time, the symmetry in question permits neutrinos to acquire appropriate masses. This shows that string theory plays an essential role in ensuring natural consistency of SUSY-unification with two low-energy observations - proton-stability and light masses for the neutrinos. The correlation between the masses of the extra Z′ boson (or bosons), which arise in these models, and proton-decay rate is noted.