Updated Nucleosynthesis Constraints on Unstable Relic Particles

TL;DR

This work refines cosmological bounds on unstable massive relics by modeling electromagnetic cascades from decays and updating photonuclear cross sections, while anchoring η with CMB data. It computes coupled networks of photodissociation and secondary production for D, He-3, He-4, Li-6, and Li-7, deriving analytic insights and performing full numerical treatments. The results show Li-6 and He-4 place the strongest, robust exclusions of parameter space beyond D alone, with the Li-6 constraint dominating for typical lifetimes, and they illustrate the implications for gravitino cosmology by constraining the reheating temperature after inflation. The study highlights the potential to reconcile or further challenge standard BBN and inflationary scenarios, and calls for extending the analysis to hadronic showers and improved cross sections.

Abstract

We revisit the upper limits on the abundance of unstable massive relic particles provided by the success of Big-Bang Nucleosynthesis calculations. We use the cosmic microwave background data to constrain the baryon-to-photon ratio, and incorporate an extensively updated compilation of cross sections into a new calculation of the network of reactions induced by electromagnetic showers that create and destroy the light elements deuterium, he3, he4, li6 and li7. We derive analytic approximations that complement and check the full numerical calculations. Considerations of the abundances of he4 and li6 exclude exceptional regions of parameter space that would otherwise have been permitted by deuterium alone. We illustrate our results by applying them to massive gravitinos. If they weigh ~100 GeV, their primordial abundance should have been below about 10^{-13} of the total entropy. This would imply an upper limit on the reheating temperature of a few times 10^7 GeV, which could be a potential difficulty for some models of inflation. We discuss possible ways of evading this problem.

Figures (8)

• Figure 1: Contours of the ${}^{4}$ He mass fraction $Y_p$ (a) in the $(\zeta_X, \tau_X)$ plane, for $\eta_{10} = 6$, and (b) in the $(\zeta_X, \eta)$ plane, for $\tau_X = 10^8$ sec. See the text for discussion.
• Figure 2: Deuterium abundance contours plotted as in Fig. \ref{['fig:He4tau']}.
• Figure 3: Contours of ${}^{7}$ Li/H, plotted as in Fig.~\ref{['fig:He4tau']}.
• Figure 4: Contours of ${}^{6}$ Li/H, plotted as in Fig.~\ref{['fig:He4tau']}.
• Figure 5: Contours of ${}^{6}$ Li/${}^{7}$ Li, plotted as in Fig.~\ref{['fig:He4tau']}.