Nucleon-light dark matter annihilation through baryon number violation

Dark matter that participates in baryon-number violating interactions can annihilate with baryons if the dark matter particle is not protected under discrete symmetries. In this paper we investigate the dark matter-baryon annihilation in color-triplet extensions of the standard model, in which a fermionic dark matter can become kinematically stable within a small mass range near the proton mass. We demonstrate that the DM's annihilation with nucleons can be probed to stringent limits at large-volume water Cherenkov detectors like the Super-Kamiokande experiment, with the mediator scale m Phi constrained up to 10(7) GeV. In case of a Majorana light dark matter, this constraint is weaker than, yet close in magnitude to that from neutron-antineutron oscillation. In the Dirac DM case, the dark matter-nucleon annihilation gives stronger bounds than that from the uncertainties of the neutron decay lifetime. In a limited range of the DM mass above m(p) + m(e), the DM-nucleon annihilation bound can be higher than the requirement from the DM's stability in the Universe. Given the strong limits from Super-Kamiokande, we find it below the current experimental capabilities to detect indirectly the dark matter-nucleon annihilation signal in diffuse Galactic gamma rays or from neutron star heating.