Primordial nucleosynthesis, majorons and heavy tau neutrinos

We determine the restrictions imposed by primordial nucleosynthesis upon a heavy tau neutrino, in the presence of ντ 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 mντ and the ντ-ντ-majoron coupling g. We show that for theoretically plausible g values >or∼10-4 present nucleosynthesis observations can not rule out ντ masses in the MeV range. Moreover, these models give Neq≤3 in the ντ mass region 1-10 MeV, for very reasonable values of g≥3 x 10-4. The evasion of the cosmological limits brings new interest to the improvement of the present laboratory limit on the ντ mass which can be achieved at a tau-charm factory. (orig.)