MeV-mass dark matter and primordial nucleosynthesis
Pasquale D. Serpico, Georg G. Raffelt
TL;DR
This work investigates whether MeV-mass dark matter candidates, treated as thermal relics, can influence Big-Bang Nucleosynthesis and thereby be constrained by light-element abundances. The authors modify a modern BBN calculation to include MeV-scale species in thermal equilibrium during nucleosynthesis, accounting for whether the particles couple to the neutrino sector or to the electromagnetic plasma, which alters both the expansion history and weak interaction rates. They find that neutrino-coupled X particles tend to increase the helium fraction $Y_{ m p}$ for $m_X$ in the range of a few MeV, disfavoring such models, whereas electromagnetically coupled particles can lower both $Y_{ m p}$ and D/H, with $m_X$ in the 4–10 MeV range potentially improving concordance with observations. The results place meaningful constraints on MeV-scale dark matter scenarios and have implications for models proposed to explain the galactic 511 keV line, while highlighting sensitivity to $ ext{N}_{ m eff}$ and the baryon density $ig(\Omega_{ m B} h^2ig)$ in BBN analyses.
Abstract
The annihilation of new dark matter candidates with masses $m_X$ in the MeV range may account for the galactic positrons that are required to explain the 511 keV $γ$-ray flux from the galactic bulge. We study the impact of MeV-mass thermal relic particles on the primordial synthesis of $^2$H, $^4$He, and $^7$Li. If the new particles are in thermal equilibrium with neutrinos during the nucleosynthesis epoch they increase the helium mass fraction for $m_X\alt 10$ MeV and are thus disfavored. If they couple primarily to the electromagnetic plasma they can have the opposite effect of lowering both helium and deuterium. For $m_X=4$--10 MeV they can even improve the overall agreement between the predicted and observed $^2$H and $^4$He abundances.