Three-neutrino decay of a 17-keV dirac neutrino

A particle physics scenario is constructed which avoids the cosmological mass-density constraint on a Dirac-type neutrino in the mass range 100 eV-1 MeV by letting it decay primarily to three light Dirac neutrino final states. The nucleosynthesis constraints on the extra number of neutrinos is saturated in this case. Identifying the recently reported 17-keV neutrino with nu(tau), its lifetime is predicted to be 10 to the 8th to 10 to the 13th sec. A hierarchy in the Dirac neutrino masses where m nu(tau) = 17 keV and m nu(e) much less than m nu(mu) = 0.001 eV (so that the solar neutrino puzzle is resolved by the Mikheyev-Smirnov-Wolfenstein mechanism) can naturally be realized in an extension of the standard model. (Author)