The mass-hierarchy puzzle and the 17 keV neutrino in the context of a Universal Seesaw Model

Abstract

In the light of renewed evidence for the existence of a 17 keV neutrino, we study the possible mass patterns for the charged and the neutral leptons, in the context of a generalized “seesaw”-type of model, which implements a horizontal U(1) A Peccei-Quinn symmetry. Under some general assumptions concerning the structure of the mass matrix we find that the mass hierarchy between the first two generations of charged leptons and the third one is explained in terms of the natural scales of the model. At the same time, with the additional assumption of the proportionality of Majorana- and Dirac-type couplings, the spectrum of the neutral leptons contains two very light Majorana neutrinos, such as required by the MSW interpretation of the solar neutrino deficit, and the 17 keV “Simpson” neutrino. A cosmologically consistent decay mode of this neutrino is into a ν e and the axion.