MeV-mass dark matter and primordial nucleosynthesis

The annihilation of new dark matter candidates with masses mX 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 2H, 4He, and 7Li. If the new particles are in thermal equilibrium with neutrinos during the nucleosynthesis epoch they increase the helium mass fraction for mX < or approx. 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 mX=4-10 MeV they can even improve the overall agreement between the predicted and observed 2H and 4He abundances