Exothermic Dark Matter for XENON1T Excess

TL;DR

This paper explores exothermic dark matter as an explanation for the XENON1T electron recoil excess, analyzing the necessary mass splitting, cross sections, and underlying models involving a $Z'$ mediator.

Contribution

It introduces a detailed model of exothermic dark matter with specific mass splitting and interactions to account for the XENON1T excess, including effective theory and microscopic realizations.

Findings

Mass difference around 2.5 keV explains the recoil peak.

Calculated scattering cross section matches the observed excess.

Proposed models include a massive $Z'$ mediator for dark matter-electron interactions.

Abstract

Motivated by the recent excess in the electron recoil from XENON1T experiment, we consider the possibility of exothermic dark matter, which is composed of two states with mass splitting. The heavier state down-scatters off the electron into the lighter state, making an appropriate recoil energy required for the Xenon excess even for the standard Maxwellian velocity distribution of dark matter. Accordingly, we determine the mass difference between two component states of dark matter to the peak electron recoil energy at about $2.5\,{\rm keV}$ up to the detector resolution, accounting for the recoil events over $E_R=2-3\,{\rm keV}$, which are most significant. We include the effects of the phase-space enhancement and the atomic excitation factor to calculate the required scattering cross section for the Xenon excess. We discuss the implications of dark matter interactions in the effective theory for exothermic dark matter and a massive $Z'$ mediator and provide microscopic models realizing the required dark matter and electron couplings to $Z'$.