Inelastic Dark Photon Dark Matter for the LUX-ZEPLIN High-Recoil Event and the Galactic Halo Gamma-Ray Excess

Abstract

We show that a single dark-photon dark matter framework can simultaneously account for two recently reported anomalies: the halo-like Galactic gamma-ray excess identified by Totani in 15 years of Fermi-LAT data, and the $E_{\rm nr}=248\pm23\,({\rm stat})\pm23\,({\rm sys})$ keV nuclear-recoil event of interest reported by the LUX-ZEPLIN Collaboration. The dark matter is a $U(1)_X$ dark photon $X$ with mass $m_X=420$ GeV, stabilized by a dark parity that forbids kinetic mixing with the Standard Model photon. A light scalar mediator $ϕ$, already required to explain the Totani excess via Sommerfeld enhancement, is extended to couple a nearly degenerate heavier vector partner $V$ to $X$. The resulting inelastic transition $X+N \to V+N$, with mass splitting $δ\simeq316$~keV close to the kinematic endpoint for $m_X=420$ GeV, naturally reproduces the localized LZ event once the detector energy resolution is included, while leaving the relic abundance and the halo phenomenology of the original Totani-motivated benchmark essentially unchanged.