Inelastic dark matter, small scale problems, and the XENON1T excess

We study a generic model in which the dark sector is composed of a Majorana dark matter χ1, its excited state χ2, both at the electroweak scale, and a light dark photon Z′ with mz′ ∼ 10−4 eV. The light Z′ enhances the self-scattering elastic cross section χ1χ1 → χ1χ1 enough to solve the small scale problems in the N-body simulations with the cold dark matter. The dark matter communicates with the SM via kinetic mixing parameterized by ϵ. The inelastic scattering process χ1χ1 → χ2χ2 followed by the prompt decay χ2 → χ1Z′ generates energetic Z′. By setting δ ≡ mχ2− mχ1 ≃ 2.8 keV and ϵ ∼ 10−10 the excess in the electron-recoil data at the XENON1T experiment can be explained by the dark-photoelectric effect. The relic abundance of the dark matter can also be accommodated by the thermal freeze-out mechanism via the annihilation χ1χ1(χ2χ2) → Z′Z′ with the dark gauge coupling constant αX ∼ 10−3.