XENON1T excess in local $Z_2$ DM models with light dark sector

TL;DR

This paper proposes that the XENON1T electron recoil excess can be explained by exothermic scattering of excited dark matter within a local dark $U(1)$ gauge symmetry model, with light dark sector particles and specific annihilation channels.

Contribution

It introduces a dark $U(1)$ gauge symmetry model with light dark sector particles that explains the XENON1T excess and satisfies relic density constraints.

Findings

Exothermic scattering of excited dark matter explains the XENON1T excess.

Light dark sector particles (~100 MeV for scalars, 1-10 MeV for fermions) are required.

p-wave annihilation channels evade CMB bounds.

Abstract

Recently XENON1T Collaboration announced that they observed some excess in the electron recoil energy around a 2-3 keV. We show that this excess can be interpreted as exothermic scattering of excited dark matter (XDM), $XDM + e_{atomic} \rightarrow DM + e_{free}$ on atomic electron through dark photon exchange. We consider DM models with local dark $U(1)$ gauge symmetry that is spontaneously broken into its $Z_2$ subgroup by Krauss-Wilczek mechanism. In order to explain the XENON1T excess with the correct DM thermal relic density within freeze-out scenario, all the particles in the dark sector should be light enough, namely $\sim O(100)$ MeV for scalar DM and $\sim O(1-10)$ MeV for fermion DM cases. And even lighter dark Higgs $\phi$ plays an important role in the DM relic density calculation: $X X^\dagger \rightarrow Z' \phi$ for scalar DM ($X$) and $\chi \bar{\chi} \rightarrow \phi \phi$for fermion DM ($\chi$) assuming $m_{Z'} > m_\chi$. Both of them are in the $p$-wave annihilation, and one can easily evade stringent bounds from Planck data on CMB on the $s$-wave annihilations, assuming other dangerous $s$-wave annihilations are kinematically forbidden.