NEUTRINO MASSES IN FLIPPED SU(5)

It is demonstrated that the, recently proposed, SU(5) × U(1) unification scheme is one of only a small number of the current candidates that allows, in its parameter space, the possibility of heavy neutrinos. This is due to the fact that the usual GIM suppression mechanism does not operate, leading to fast decays of heavy tau neutrinos of the form ν→νγ, with an estimated lifetime of O(1 yr) for a tau neutrino mass of 1 MeV. Using well known cosmological arguments, based on the observed 3 K background radiation, the mass of the electron neutrino is constrained to be either greater than O(1 eV), or less than usual limit of O(10-2 eV). © 1990.