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 life-time is longer than O(100 sec), the constraint Neff < 3.6 rules out ντ masses in the range 0.5 (MeV ) < mντ < 35 (MeV ) for Majorana neutrinos and 0.74 (MeV ) < mντ < 35 (MeV ) for Dirac neutrinos. Given that the present laboratory bound is 35 MeV, our results lower the present bound to 0.5 and 0.74 for Majorana and Dirac neutrinos respectively. Permanent address: ITEP 113259, Moscow, Russia. 1 Despite a considerable experimental effort it is still unknown whether or not neutrinos have a nonzero mass. The following upper bounds are known [1]: