Inelastic Dark Matter in Large Volume Detectors

Direct searches for TeV-scale Dark Matter (DM) are extremely powerful. However, direct searches become increasingly weak when the dominant scattering process is inelastic, and the constraints disappear completely when the mass splitting delta between nearby DM states exceeds 350 keV. In this work, we show how this ‘inelastic frontier’ can be probed when the DM upscatters against heavy elements in the Earth. The heavier state subsequently decays to a photon, giving rise to a monoenergetic signal which is highly directional, and can be searched for in large volume neutrino detectors like Borexino. The resulting sidereal-daily modulation allows outstanding separation of signal from backgrounds, and this scenario represents at present the only probe of inelastic DM for 350 keV <~ delta <~ 600 keV. For smaller mass splittings delta <~ 150 keV, this scenario also suggests that directional DM detectors (coming online in the near future) can probe inelastic DM to smaller cross sections than even the most sensitive existing direct detection experiments.