MeV-mass dark matter and primordial nucleosynthesis

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 $\gamma$-ray flux from the galactic bulge. We study the impact of MeV-mass thermal relic particles on the primordial synthesis of ${}^2\mathrm{H},$ ${}^4\mathrm{He},$ and ${}^7\mathrm{Li}.$ If the new particles are in thermal equilibrium with neutrinos during the nucleosynthesis epoch they increase the helium mass fraction for $m_X\lesssim 10\mathrm{MeV}$ 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 $m_X=4\mathrm{}–10\mathrm{MeV}$ they can even improve the overall agreement between the predicted and observed ${}^2\mathrm{H}\hspace{0.5em}\mathrm{and}{\hspace{0.5em}}^4\mathrm{He}$ abundances.