The mass-hierarchy puzzle and the 17-keV neutrino in the context of a universal seesaw model

In the light of renewed evidence for the existence of a 17 keV neutrino, the possible mass patterns for the charged and the neutral leptons are studied. The investigation is performed in the context of a generalized seesaw type of model, which implements a horizontal U(1)(sub A) Peccei-Quinn symmetry. Under some general assumptions concerning the structure of the mass matrix, it is found 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. An additional assumption is made; namely, that the Majorana and Dirac type coupling constants, generally assumed to be independent to each other, are proportional. In this model, the spectrum contains an intermediate mass Dirac tau neutrino and two very light Majorana states, which are essentially the electron and the muon neutrinos. Identifying the 17 keV neutrino with the Dirac state, and using the bounds on the Peccei-Quinn scale, it is found that the mass of the two light states turns out to be in the right range, required by the Mikheyev-Smirnov-Wolfenstein solution of the solar neutrino problem. A cosmologically consistent lifetime for the Simpson neutrino is obtained through its decay into an axion and the electron neutrino.