Antimatter regions in the early universe and big bang nucleosynthesis

We have studied big bang nucleosynthesis in the presence of regions of antimatter. Depending on the distance scale of the antimatter region, and thus the epoch of their annihilation, the amount of antimatter in the early universe is constrained by the observed abundances. Small regions, which annihilate after weak freezeout but before nucleosynthesis, lead to a reduction in the ${}^4\mathrm{He}$ yield, because of neutron annihilation. Large regions, which annihilate after nucleosynthesis, lead to an increased ${}^3\mathrm{He}$ yield. Deuterium production is also affected but not as much. The three most important production mechanisms of ${}^3\mathrm{He}$ are (1) photodisintegration of ${}^4\mathrm{He}$ by the annihilation radiation, (2) $p^4\mathrm{He}$ annihilation, and (3) $n^4\mathrm{He}$ annihilation by “secondary” antineutrons produced in ${}^4\overline{\mathrm{He}}$ annihilation. Although $p^4\mathrm{He}$ annihilation produces more ${}^3\mathrm{He}$ than the secondary $n^4\mathrm{He}$ annihilation, the products of the latter survive later annihilation much better, since they are distributed further away from the annihilation zone. Our results are in qualitative agreement with similar work by Rehm and Jedamzik, but we get a larger ${}^3\mathrm{He}$ yield.