Exciting dark matter and the INTEGRAL/SPI 511 keV signal

Abstract

We propose a dark matter candidate with an ``excited state'' 1--2 MeV above the ground state, which may be collisionally excited and deexcites by ${e}^{+}{e}^{\ensuremath{-}}$ pair emission. By converting its kinetic energy into pairs, such a particle could produce a substantial fraction of the 511 keV line observed by the International Gamma-Ray Astrophysics Laboratory/SPI in the inner Milky Way. Only a small fraction of the dark matter candidates have sufficient energy to excite, and that fraction drops sharply with galactocentric radius, naturally yielding a radial cutoff, as observed. Even if the scattering probability in the inner kpc is $\ensuremath{\ll}1%$ per Hubble time, enough power is available to produce the $\ensuremath{\sim}3\ifmmode\times\else\texttimes\fi{}{10}^{42}$ pairs per second observed in the galactic bulge. We specify the parameters of a pseudo-Dirac fermion designed to explain the positron signal, and find that it annihilates chiefly to ${e}^{+}{e}^{\ensuremath{-}}$ and freezes out with the correct relic density. We discuss possible observational consequences of this model.