Abstract:
Direct searches for TeV-scale Dark Matter (DM) are extremely powerful. However, existing constraints become increasingly weak when the dominant scatteringis inelastic, and they 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 $\lesssim \delta \lesssim 600$ keV. For smaller mass splittings $\delta \lesssim 150$ 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.