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We investigate the non-radiative decay during nucleosynthesis of a massivetau neutrino with mass 0.1 - 1 MeV into an electron neutrino and a scalar orpseudoscalar particle, $\phi$. The full Boltzmann equation is used and shown togive markedly different results than the usual non-relativistic formalism forrelativistic or semi-relativistic neutrino decays. Indeed, the region weinvestigate is where the formalism that has previously been applied to solvingthis problem is expected to break down. We also compare the nucleosynthesispredictions from this scenario with results from the standard model and withsome of the available observational determinations of the primordialabundances. It is found that for relativistic or semi-relativistic decays thehelium abundance can be significantly lowered without changing other lightelement abundances. Since a problem with the standard model of Big Bangnucleosynthesis is that helium appears to be overproduced, a decay of the typewe discuss can be a possible solution.Comment: 7 pages RevTeX, 6 figures. To be published in Phys. Rev.

τneutrino decays and big bang nucleosynthesis