New upper limits on the tau-neutrino mass from primordial helium considerations

In this paper 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 {~}100 sec, the constraint Neff3.6 rules out ν_τ masses in the range 0.5m_ντ35 MeV for Majorana and 0.3m_ντ35 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.