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), greater than or similar to 300 sec the mass must be 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) greater than or similar to 2.5 x 10(-12) sec, 15 keV (Dirac) provided tau(nu) greater than or similar to 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%) He-4 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.