Temperature-dependent wave function renormalization and weak interaction rates prior to nucleosynthesis

We present a general prescription for the effects of medium-dependent wave function renormalization on rates of reactions involving a Dirac particle. The result is applied to the corrections of order ${\mathit{e}}^2$ to the weak interaction rates prior to nucleosynthesis, due to the thermal photon and ${\mathit{e}}^{+}$ ${\mathit{e}}^{\mathrm{-}}$ baths. The results coming from the standard ‘‘finite temperature’’ corrections in the literature, a decrease of the predicted He abundance by 0.16%, are found to be replaced by an increase of 0.02%. But there are indications that plasma corrections that are not included at the ${\mathit{e}}^2$ level may be more important than these corrections. © 1996 The American Physical Society.