Reverse Direct Detection: Cosmic Ray Tests of Light Dark Matter Elastic Scattering

Many dark matter studies have considered indirect detection ($\chi\chi\rightarrow f f$), direct detection ($\chi f\rightarrow \chi f$), and collider searches ($f f\rightarrow \chi \chi$). We propose a new strategy in searching for dark matter elastic cross section by considering cosmic-ray propagation in the galactic dark matter halo. We find that cosmic rays can lose significant fraction of their energy through scattering with dark matter ($f \chi \rightarrow f \chi$). Using existing cosmic-ray data and a simple cosmic-ray propagation model, we study the qualitative effects of dark matter scattering on cosmic-ray propagation and obtain new constraints of dark matter elastic cross sections on light dark matter (keV--GeV), a regime that is difficult for traditional direct detection experiments to probe.