Primordial nucleosynthesis, majorons and heavy tau neutrinos Original Research Article

We determine the restrictions imposed by primordial nucleosynthesis upon a heavy tau neutrino, in the presence of vτ 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 mvτ, and the vτ−vτ-majoron coupling g. We show that for theoretically plausible g values ≳ 10-4 present nucleosynthesis observations can not rule out vτ masses in the MeV range. Moreover, these models give Neq ⩽ 3 in the vτ 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 vτ mass which can be achieved at a tau-charm factory.