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); (iii) the decay of a nearby long-lived state surviving from the big bang, chi(0)(2) + 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. (c) 2007 Elsevier B.V. All rights reserved.