New Upper Limits on the Tau Neutrino Mass from Primordial Helium Considerations

Abstract

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 $O(100sec)$, the constraint $N_{eff}<3.6$ rules out ${\nu }_{\tau }$ masses in the range $0.5(MeV)<m_{{\nu }_{\tau }}<35(MeV)$ for Majorana neutrinos and $0.74(MeV)<m_{{\nu }_{\tau }}<35(MeV)$ 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.

Comments: 9 pages (2 figures available upon request), UM-AC-93-02