Primordial nucleosynthesis, majorons and heavy tau neutrinos

We determine the restrictions imposed by primordial nucleosynthesis upon a heavy tau neutrino, in the presence of ~, ~ annihilations into majorons, as expected in a wide class of particle physics models of neutrino mass. We determine the equivalent umber of light neutrino species Neq as a function of m,, and the v~-vr-majoron coupling. We show that for theoretically plausible g values ~> 10-4 present nucleosynthesis observations can not rule out v ~ masses in the MeV range. Moreover, these models give Neq ~< 3 in the v ~ 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 ~, ~ mass which can be achieved at a tau-charm factory