Luminous signals of inelastic dark matter in large detectors

Abstract

We study luminous dark matter signals in models with inelastic scattering. Dark matter χ1 that scatters inelastically off elements in the Earth is kicked into an excited state χ2 that can subsequently decay into a monoenergetic photon inside a detector. The photon signal exhibits large sidereal-daily modulation due to the daily rotation of the Earth and anisotropies in the problem: the dark matter wind comes from the direction of Cygnus due to the Sun's motion relative to the galaxy, and the rock overburden is anisotropic, as is the dark matter scattering angle. This allows outstanding separation of signal from backgrounds. We investigate the sensitivity of two classes of large underground detectors to this modulating photon line signal: large liquid scintillator neutrino experiments, including Borexino and JUNO, and the proposed large gaseous scintillator directional detection experiment CYGNUS. Borexino's (JUNO's) sensitivity exceeds the bounds from xenon experiments on inelastic nuclear recoil for mass splittings delta underset{sim }{>} 240 (180) keV, and is the only probe of inelastic dark matter for 350 keV underset{sim }{<}delta underset{sim }{<} 600 keV. CYGNUS's sensitivity is at least comparable to xenon experiments with ~ 10m3 volume detector for delta underset{sim }{<} 150 keV, and could be substantially better with larger volumes and improved background rejection. Such improvements lead to the unusual situation that the inelastic signal becomes the superior way to search for dark matter even if the elastic and inelastic scattering cross sections are comparable.