Exothermic Dark Matter at LZ

Abstract

We investigate whether the high-energy nuclear recoil reported by the LUX-ZEPLIN (LZ) experiment can be explained by exothermic dark matter. The LZ event can occur in a preferred parameter region with dark matter mass around $m_χ\sim 30-200\mathrm{GeV}$, and mass splitting $δ\sim 0.5-1\mathrm{MeV}$, where a single event can be accommodated without additional activity at lower energies. We realize this framework in a minimal inelastic dark photon model and show that both freeze-out and low-reheating-temperature freeze-in cosmologies can reproduce the observed rate, with GeV-scale mediator masses and small kinetic mixing. The scenario is largely insensitive to halo uncertainties and has robust predictions for different targets. In particular, argon and germanium detectors are expected to observe events at energies above the current xenon window.