Unstable massive tau-neutrinos and primordial nucleosynthesis

Abstract

The impact of unstable Majorana ν τ on primordial nucleosynthesis is considered. The mass and lifetime of ν τ are taken in the intervals 0.1–20 MeV and 0.001–400 s respectively. The studied decay modes are ν τ → ν μ + φ and ν τ → ν e + φ, where φ is a massless (or light) scalar. Integro-differential kinetic equations are solved numerically without any simplifying assumptions. Our results deviate rather strongly from earlier calculations. Depending on mass, lifetime, and decay channels of the ν τ , the number of effective neutrino species (found from 4He), in addition to the 3 standard ones, varies from −2 to +2.5. The abundances of 2H and 7Li are also calculated.