Impact of massive tau-neutrinos on primordial nucleosynthesis. Exact calculations Original Research Article

The influence of a massive Majorana vτ on primordial nucleosynthesis is rigorously calculated. The system of three integro-differential kinetic equations is solved numerically for mvτ in the interval from 0 to 20 MeV It is found that the usual assumption of kinetic equilibrium is strongly violated and non-equilibrium corrections considerably amplify the effect. Even a very weak restriction from nucleosynthesis, allowing for one extra massless neutrino species, permits to conclude that mvτ < 1 MeV For a stricter bound, e.g. for ΔNv < 0.3, the limit is tmvτ < 0.35MeV.