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

Abstract

We study a generic model in which the dark sector is composed of a Majorana dark matter ${\chi }_1$, its excited state ${\chi }_2$, both at the electroweak scale, and a light dark photon $Z^{\prime }$ with $m_{Z^{\prime }}\sim 10^{-4}$ eV. The light $Z^{\prime }$ enhances the self-scattering elastic cross section ${\chi }_1{\chi }_1\to {\chi }_1{\chi }_1$ 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 $\epsilon$. The inelastic scattering process ${\chi }_1{\chi }_1\to {\chi }_2{\chi }_2$ followed by the prompt decay ${\chi }_2\to {\chi }_1{Z}^{\prime }$ generates energetic $Z^{\prime }$. By setting $\delta \equiv m_{{\chi }_2}-m_{{\chi }_1}\simeq 2.8$ keV and $\epsilon \sim 10^{-10}$ 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 ${\chi }_1{\chi }_1({\chi }_2{\chi }_2)\to Z^{\prime }{Z}^{\prime }$ with the dark gauge coupling constant ${\alpha }_X\sim 10^{-3}$.

13 pages, 1 figure

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