Effect of neutrino heating on primordial nucleosynthesis.

We modify the standard code for primordial nucleosynthesis to include the effect of the slight heating of neutrinos by [ital e][sup [plus minus]] annihilations. There is a small, systematic change in the [sup 4]He yield, [Delta][ital Y][congruent]+1.5[times]10[sup [minus]4], which is insensitive to the value of the baryon-to-photon ratio [eta] for 10[sup [minus]10][approx lt][eta][approx lt]10[sup [minus]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 [ital e][sup [plus minus]] pairs is transferred to neutrinos. These results are in accord with recent analytical estimates.