The galactic 511 keV line from electroweak scale WIMPs

We consider possible mechanisms via which electroweak scale WIMPs \chi^0 could provide the source of the INTEGRAL/SPI 511 keV photon flux from the galactic centre. We consider scenarios where the WIMP spectrum contains near-degeneracies, with MeV-scale splitting, and focus on three possible production mechanisms for galactic positrons: (i) collisional excitation of the WIMP to a nearby charged state, \chi^0 + \chi^0 -> \chi^+ + \chi^-, with the subsequent decay producing positrons; (ii) capture of the WIMP by nuclei in the galactic interstellar medium, \chi^0 + N -> e^+ + (\chi^- N); and (iii) the decay of a nearby long-lived state surviving from the Big Bang, \chi^0_2 -> \chi_1^0 + e^+ + e^-. We find that process (i) requires a cross-section which is significantly larger than the unitarity bound, process (ii) is allowed by unitarity, but is impractical due to terrestrial bounds on the \chi-N cross-section, while process (iii) is viable and we construct a simple model realization with singlet dark matter fields interacting with the Standard Model via the Higgs sector.