An improved cosmological bound on the tau-neutrino mass

Abstract

We consider the influence of non-equilibrium electronic neutrinos (and anti-neutrinos) on the neutron-to-proton ratio. These neutrinos would come from massive $ν_τ$ annihilations $\bar ν_τν_τ\rightarrow \bar ν_e ν_e$. For sufficiently large $ν_τ$ masses this new effect would strongly enhance the (n/p)-ratio, leading to a very stringent bound on the $ν_τ$ mass, even adopting a rather weak upper bound on the effective number on neutrino species during nucleosynthesis.