Solving the cosmic lithium problems with primordial late-decaying particles

TL;DR

This work addresses the lithium problems in standard Big-Bang Nucleosynthesis by exploring how exotic late-decaying particles with lifetime $\tau_X$ and hadronic branching $B_h$ modify primordial light-element abundances via non-thermal cascades parameterized by $Y_X = n_X/s$. Using a model-independent framework (KKM), the authors identify regions in the generalized parameter space, notably $B_h E_{ m vis} Y_X$ and $\tau_X$, that can reduce ${^7}$Li and boost ${^6}$Li without spoiling D, ${^3}$He, or ${^4}$He, and they predict a robustly high ${^6}$Li/${^7}$Li ratio as a signature. They examine the impact of $\alpha$–$\alpha$ collisions and employ conservative estimates for ${^4}$He energies to bound non-thermal ${^7}$Li/${^7}$Be production, ultimately finding a viable region at 2σ that persists under various observational constraints. The results provide a concrete particle-physics motivated route to resolve the lithium problems, with implications for non-thermal dark-matter production and stringent bounds on long-lived, massive particles in beyond-Standard-Model theories.

Abstract

We investigate the modifications to predictions for the abundances of light elements from standard Big-Bang nucleosynthesis when exotic late-decaying particles with lifetimes exceeding ~1 sec are prominent in the early Universe. Utilising a model-independent analysis of the properties of these long-lived particles, we identify the parameter space associated with models that are consistent with all observational data and hence resolve the much discussed discrepancies between observations and theoretical predictions for the abundances of Li^7 and Li^6.

Figures (2)

• Figure 1: Bounds on $B_h E_{\rm vis}Y_X$ as a function of $\tau_X$. In the panel (a), $\alpha$--$\alpha$ collisions are excluded and in (b), $\alpha$--$\alpha$ collisions are included. In each panel, the lower (upper) shaded region indicates the parameter space which is consistent with all observations when we adopt Low (D/H) and $^{7}$Li/H (MR) (High (D/H) and $^{7}$Li/H (Bonifacio et al.) ). The element's name is written beside each contour.
• Figure 2: Results assuming stellar depletion. Here we have adopted the depletion factor $D_{7}$ = 0, 0.1, and 0.2 (thin line, moderate line, and thick line). We see that the most severe bounds on $^{7}$Li agrees with the theory with $D_{7}$ = 0.2. The star symbol means the allowed region. Here we have included $\alpha+\alpha \to$ Li and Be processes.