In a class of models with global SU(n) family symmetry, the possibility naturally exists that n-1 generations have light neutrinos and the remaining generation has a medium-heavy neutrino, of the order of the charged-lepton mass or the quark mass, as a consequence of an incomplete action of the seesaw mechanism. We discuss as applications two possibilities: (i) The heavy neutrino is identified with the neutral particle of mass 17 keV suggested by Simpson from β-decay experiments, and (ii) there may exist a fourth generation and the associated neutrino has a heavy mass, probably of the order of a hundred GeV.

• Received 18 January 1991

DOI:https://doi.org/10.1103/PhysRevLett.66.2705