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 N-eq as a function of m(nu tau) and the nu(tau)-nu(tau)-majoron coupling g. We show that for theoretically plausible g values greater than or similar to 10(-4) present nucleosynthesis observations can not rule out nu(tau) masses in the MeV range. Moreover, these models give N-eq less than or equal to 3 in the nu(tau) mass region 1-10 MeV, for very reasonable values of g greater than or equal to 3 x 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, (C) 1997 Elsevier Science B.V.