Primordial nucleosynthesis, majorons and heavy tau neutrinos

We determine the restrictions imposed by primordial nucleosynthesis upon a heavy tau neutrino, in the presence of nu-tau annihilations into Majorons, as expected in a wide class of particle physics models of neutrino mass. We determine the equivalent number of light neutrino species Neq as a function of nu-tau mass and the nu-tau-Majoron coupling g. We show that for theoretically plausible g values $\gsim 10^{-4}$ present nucleosynthesis observations can not rule out nu-tau masses in the MeV range. Moreover, these models give Neq≤3 in the nu-tau mass region 1-10 MeV, for very reasonable values of g≥3×10−4. The evasion of the cosmological limits brings new interest to the improvement of the present laboratory limit on the nu-tau mass which can be achieved at a tau-charm factory