New upper limits on the Tau-neutrino mass from primordial he'lium considerations

We reconsider recently derived bounds on MeV tau neutrinos, taking into account previously unaccounted for effects. We find that, assuming that the neutrino lifetime is longer than about 100 sec, the constraint N(eff) less than 3.6 rules out nu(tau) masses in the range 0.5-35 MeV for Majorana and 0.3-35 MeV for Dirac neutrinos. Given that the present laboratory upper bound is 31 MeV, our results imply an upper bound of 0.5 MeV for Majorana neutrinos and of 0.3 MeV for Dirac neutrinos. (Author (revised))