Do Neutrino Rest Masses Affect Cosmological Helium Production?

It is shown that the possibility of nonzero neutrino rest masses $m_{\nu }\approx 10$ ${\mathrm{e}\mathrm{V}/\mathit{c}}^2$ does not alter previous predictions of the standard big-bang model for the primordial ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ abundance. Although both left- and right-handed neutrinos could then be present, reactions mediated by the standard Weinberg-Salam-Glashow model extended to include Dirac neutrinos with nonzero rest mass cannot maintain ${\nu }_R$ in thermal equilibrium.