Nucleosynthesis and the mass of the tau neutrino

The primordial abundance of long-lived heavy Majorana neutrinos is calculated from the full Boltzmann equation. Inclusion of scattering reactions drastically change the predicted abundance of a heavy neutrino species. This loosens the well known mass constraint on MeV neutrinos from Big Bang nucleosynthesis, and allows for the existence of a Majorana tau neutrino with mass m > 11 MeV. Further experimental efforts are therefore needed to investigate the range 11 MeV < m < 24 MeV. Some interesting cosmological consequences of an MeV nu_tau are also pointed out.