XENON1T excess in local Z(2) DM models with light dark sector

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 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 similar to O (100) MeV for scalar DM and similar to O (1 - 10) MeV for fermion DM cases. And even lighter dark Higgs phi plays an important role in the DM relic density calculation: XX dagger -> Z'phi for scalar DM (X) and chi(chi) over bar -> 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. (C) 2020 The Author(s). Published by Elsevier B.V.