Three-neutrino decay of a 17-keV Dirac neutrino

We construct a particle physics scenario 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 (i.e., δNν≤0.3) is saturated in this case. Identifying the recently reported 17-keV neutrino with ντ, we predict its lifetime to be 108≤τντ≤1013 sec. A hierarchy in the Dirac neutrino masses where mντ congruent 17 keV and mνe much-lt mνμ congruent 10-3 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