Lithium-6: A Probe of the Early Universe

TL;DR

The synthesized 6 Li/H ratio may be compared to 6Li/H in metal-poor stars which, in the absence of stellar depletion of 6Li, yields significantly stronger constraints on relic particle densities than the usual consideration of overproduction of 3He.

Abstract

I consider the synthesis of 6Li due to the decay of relic particles, such as gravitinos or moduli, after the epoch of Big Bang Nucleosynthesis. The synthesized 6Li/H ratio may be compared to 6Li/H in metal-poor stars which, in the absence of stellar depletion of 6Li, yields significantly stronger constraints on relic particle densities than the usual consideration of overproduction of 3He. Production of 6Li during such an era of non-thermal nucleosynthesis may also be regarded as a possible explanation for the relatively high 6Li/H ratios observed in metal-poor halo stars.

Figures (2)

• Figure 1: Total number of $^6$Li nuclei produced per MeV of energy in energetic electro-magnetically interacting particles injected at an epoch with redshift $z$.
• Figure 2: Limits on the abundance of a relic, decaying particle with mass $m_X$, and density $n_X$, relative to photon density $n_{\gamma}$, as a function of its lifetime $\tau_X$. The limits based on $^3$He and $^2$H destruction/production are taken from Mass.