Probing inelastic sub-GeV dark matter at the DUNE near detector

Abstract

We study inelastic dark matter (iDM) in a minimal and ultraviolet-complete framework in which the dark photon mass arises from a dark Higgs mechanism. The spontaneous breaking of a $U(1{)}_D$ symmetry splits the Dirac fermion into two Majorana states, thus opening new annihilation channels mediated by the dark Higgs. Focusing on sub-GeV dark matter, we assess the sensitivity of DUNE's liquid argon cube at the near detector (ND-LAr) to this scenario. We find that the ND-LAr can probe regions of parameter space consistent with the observed relic abundance due to these new annihilation channels, particularly for large dark photon-to-DM mass ratios where decay-based searches lose sensitivity. Our results highlight the complementarity between cosmological constraints and fixed-target experiments and demonstrate the potential of DUNE's ND-LAr to explore iDM scenarios with extended dark sectors.

Comments: 20 pages, 5 figures; v2: references updated, minor changes, version accepted for publication in PRD