We have modified the standard code for primordial nucleosynthesis to include the effect of the slight heating of neutrinos by $e^\pm$ annihilations. There is a small, systematic change in the $^4$He yield, $\Delta Y \simeq +1.5\times 10^{ -4}$, which is insensitive to the value of the baryon-to-photon ratio $\eta$ for $10^{ -10}\la \eta \la 10^{ -9}$. We also find that the baryon-to-photon ratio decreases by about 0.5\% less than the canonical factor of 4/11 because some of the entropy in $e^\pm$ pairs is transferred to neutrinos. These results are in accord with recent analytical estimates.
Categories: astro-ph hep-ph