An improved cosmological bound on the tau-neutrino mass

We consider the influence of non-equilibrium electronic neutrinos (and anti-neutrinos) on the neutron-to-proton ratio. These neutrinos would come from massive ντ annihilations ντντ → νeνe. For sufficiently large ντ masses this new effect would strongly enhance the (np)-ratio, leading to a very stringent bound on the ντ mass, even adopting a rather weak upper bound on the effective number on neutrino species during nucleosynthesis.