Loop-induced dark matter direct detection signals from gamma-ray lines
  Improved limits as well as tentative claims for dark matter annihilation into
gamma-ray lines have been presented recently. We study the direct detection
cross section induced from dark matter annihilation into two photons in a
model-independent fashion, assuming no additional couplings between dark matter
and nuclei. We find a striking non-standard recoil spectrum due to different
destructively interfering contributions to the dark matter nucleus scattering
cross section. While in the case of s-wave annihilation the current sensitivity
of direct detection experiments is insufficient to compete with indirect
detection searches, for p-wave annihilation the constraints from direct
searches are comparable. This will allow to test dark matter scenarios with
p-wave annihilation that predict a large di-photon annihilation cross section
in the next generation of experiments.
