MeV Tau Neutrino: Astrophysical and Cosmological Constraints and   Mischief
  Terrestrial and ``Heavenly'' experiments severely constrain the mass and lifetime of an MeV tau neutrino. Irrespective of decay mode, for $\tau_\nu \ga 300\sec$ the mass of the tau neutrino must be either approximately $30\MeV$ or less than $0.4\MeV$ (Majorana), $15\keV$ (Dirac). If the dominant decay mode includes electromagnetic daughter products, the mass must be less than $0.4\MeV$ (Majorana or Dirac) provided $\tau_\nu \ga 2.5\times 10^{-12}\sec$, $15\keV$ (Dirac) provided $\tau_\nu \ga 10^{-6}\sec (m_\nu /\MeV )$. A tau neutrino of mass between $1\MeV$ and $30\MeV$ can have a host of interesting astrophysical and cosmological consequences: relaxing the big-bang nucleosynthesis bound to the baryon density and the number of neutrino species, allowing big-bang nucleosynthesis to accommodate a low (less than 22\%) $^4$He mass fraction or high (greater than $10^{-4}$) deuterium abundance, improving significantly the agreement between the cold dark matter theory of structure formation and observations, and helping to explain how type II supernovae explode. Exploring the MeV mass range not only probes fundamental particle physics, but also interesting astrophysical and cosmological scenarios. 
Geza Gyuk and Michael S. Turner