Effect of neutrino heating on primordial nucleosynthesis.

We have modified the standard code for primordial nucleosynthesis to include the effect of the slight heating of neutrinos by e annihilations. There is a small, systematic change in the He yield, ∆Y ≃ +1.5 × 10, which is insensitive to the value of the baryon-to-photon ratio η for 10 < ∼ η <∼ 10 . 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 pairs is transferred to neutrinos. These results are in accord with recent analytical estimates.