Primordial Nucleosynthesis, Majorons and Heavy Tau Neutrinos

Abstract

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 nu-tau mass and the nu-tau-Majoron coupling $g$. We show that for theoretically plausible $g$ values $\text{gsim}10^{-4}$ present nucleosynthesis observations can not rule out nu-tau masses in the MeV range. Moreover, these models give $N_{eq}\le 3$ in the nu-tau mass region 1-10 MeV, for very reasonable values of $g\ge 3\times 10^{-4}$. 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.

Comments: 20 pages, LaTex + 6 ps figures. Discussion of the majoron couplings is clarified in new appendix. To be published in NPB