Dark forces and light dark matter

We consider a simple class of models in which the dark matter, X, is coupled to a new gauge boson, n•, with a relatively low mass (m _n•∼100MeV-3GeV). Neither the dark matter nor the new gauge boson have tree-level couplings to the Standard Model. The dark matter in this model annihilates to pairs, and for a coupling of g _X0. 06×(m _X/10GeV) ¹2 yields a thermal relic abundance consistent with the cosmological density of dark matter. The n•'s produced in such annihilations decay through a small degree of kinetic mixing with the photon to combinations of Standard Model leptons and mesons. For dark matter with a mass of ∼10GeV, the shape of the resulting gamma-ray spectrum provides a good fit to that observed from the Galactic center, and can also provide the very hard electron spectrum required to account for the observed synchrotron emission from the Milky Way's radio filaments. For kinetic mixing near the level naively expected from loop-suppressed operators (∼10 ^-4), the dark matter is predicted to scatter elastically with protons with a cross section consistent with that required to accommodate the signals reported by DAMA/LIBRA, CoGeNT and CRESST-II.