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., {δ}N_ν{≤}0.3) is saturated in this case. Identifying the recently reported 17-keV neutrino with ν_τ, we predict its lifetime to be 10⁸{≤}τ_ντ{≤}10¹³ sec. A hierarchy in the Dirac neutrino masses where m_ντ{}17 keV and m_νe{}m_νμ{}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.