New cosmological limit on neutrino mass

We show that considerations of the equilibration of right- and left-handed neutrino seas in the early Universe and the well-known ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ constraint on the number of relativistic degrees of freedom extant at the nucleosynthesis epoch lead to a new limit on the Dirac neutrino mass of ${\mathit{m}}_{\nu }$≪300 keV. This constraint would apply to any neutrino so long as it had a purely Dirac mass and a lifetime exceeding the nucleosynthesis time scale.