Interpretation of XENON1T excess with MeV boosted dark matter

TL;DR

This paper proposes a model with two scalar MeV dark matter particles, where boosted $S_B$ particles explain the XENON1T keV excess through electron scattering mediated by a vector boson, consistent with various astrophysical and experimental constraints.

Contribution

It introduces a novel two-scalar MeV dark matter model with resonant annihilation and boosted particles to interpret the XENON1T excess, addressing existing experimental bounds.

Findings

Boosted $S_B$ particles can account for the XENON1T excess.

The model remains consistent with BBN, CMB, and low-energy experimental constraints.

A small relic fraction of $S_B$ is compatible with detection bounds.

Abstract

The XENON1T excess of keV electron recoil events may be induced by the scattering of electrons and long-lived particles with MeV mass and high-speed. We consider a tangible model composed of two scalar MeV dark matter (DM) particles $S_A$ and $S_B$ to interpret the XENON1T keV excess via boosted $S_B$. A small mass splitting $m_{S_A}-m_{S_B}>0$ is introduced and the boosted $S_B$ can be produced by the dark annihilation process of $S_A S_A^\dagger \to \phi \to S_B S_B^\dagger$ via a resonant scalar $\phi$. The $S_B-$electron scattering is intermediated by a vector boson $X$. Although the constraints from BBN, CMB and low-energy experiments set the $X-$mediated $S_B-$electron scattering cross section to be $\lesssim 10^{-35} \mathrm{cm}^2$, the MeV scale DM with a resonance enhanced dark annihilation today can still provide enough boosted $S_B$ and induce the XENON1T keV excess. The relic density of $S_B$ is significantly reduced by the $s$-wave process of $S_B S_B^\dagger \to X X$ which is allowed by the constraints from CMB and 21-cm absorption. A very small relic fraction of $S_B$ is compatible with the stringent bounds on un-boosted $S_B$-electron scattering in DM direct detection and the $S_A$-electron scattering is also allowed.