Impact of massive tau-neutrinos on primordial nucleosynthesis. Exact calculations
Abstract
The influence of a massive Majorana tau-neutrino on primordial nucleosynthesis is rigorously calculated. The system of three integro-differential kinetic equations is solved numerically for the tau-neutrino mass 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 m_τ < 1 MeV. For a stricter bound, e.g. for dN < 0.3, the limit is m_τ < 0.35 MeV.
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