New nuclear physics for big bang nucleosynthesis

We discuss nuclear reactions which could play a role in big bang nucleosynthesis. Most of these reactions involve lithium and beryllium isotopes and the rates for some of these have not previously been included in BBN calculations. Few of these reactions are well studied in the laboratory. We also discuss novel effects in these reactions, including thermal population of nuclear target states, resonant enhancement, and nonthermal neutron reaction products. We perform sensitivity studies which show that even given considerable nuclear physics uncertainties, most of these nuclear reactions have minimal leverage on the standard BBN abundance yields of ${}^6\mathrm{Li}$ and ${}^7\mathrm{Li}$. Although a few have the potential to alter the yields significantly, we argue that this is unlikely.

Figure 1

Reaction network for BBN, as modified from 39. The nuclei indicated in dotted boxes are completely unstable, whereas those in dashed boxes have some stable states and some others that are relevant to BBN that undergo particle decay to some other nuclei. Lighter (red) lines indicate reactions newly added to the BBN code, and darker (blue) lines indicate reactions studied in the context of nonthermal neutrons. Double thickness lines indicate more than one possible reaction.

Figure 2

Nuclear abundance as a function of temperature $T_9$, where $T_9=T/{10}^9\mathrm{K}$. Abundances are given as mass fraction for ${}^4\mathrm{He}$ and number abundance relative to hydrogen for all others.

Figure 3

${}^6\mathrm{Li}$ destruction rates as a function of temperature $T_9$.

Figure 4

Neutron energy distribution.

Figure 5

The fraction of neutrons with energies above $E_m=2.5\mathrm{MeV}$ is shown by the solid curve. The fraction predicted by Maxwell-Boltzmann distribution is shown by the dashed curve.

Figure 6

The cross section for the $n+d\to 2n+p$ reaction as a function of incident neutron energy from Ref. [63].