Cosmic Ray-Dark Matter Scattering: a New Signature of (Asymmetric) Dark
  Matter in the Gamma Ray Sky
  We consider the process of scattering of Galactic cosmic-ray electrons and
protons off of dark matter with the radiation of a final-state photon. This
process provides a novel way to search for Galactic dark matter with gamma
rays. We argue that for a generic weakly interacting massive particle, barring
effects such as co-annihilation or a velocity-dependent cross section, the
gamma-ray emission from cosmic-ray scattering off of dark matter is typically
smaller than that from dark matter pair-annihilation. However, if dark matter
particles cannot pair-annihilate, as is the case for example in asymmetric dark
matter scenarios, cosmic-ray scattering with final state photon emission
provides a unique window to detect a signal from dark matter with gamma rays.
We estimate the expected flux level and its spectral features for a generic
supersymmetric setup, and we also discuss dipolar and luminous dark matter. We
show that in some cases the gamma-ray emission might be large enough to be
detectable with the Fermi Large Area Telescope.
