Exploring New Physics with O(keV) Electron Recoils in Direct Detection Experiments

TL;DR

This paper investigates various new physics models, including light boson absorption and dark matter-electron interactions, to explain the XENON1T electron recoil excess, proposing novel mechanisms and analyzing their viability against experimental and astrophysical constraints.

Contribution

The paper introduces a comprehensive analysis of multiple new physics scenarios, including a Chameleon-like axion model and exothermic dark matter, providing new insights into potential explanations for the XENON1T excess.

Findings

Sun-produced keV bosons fit data but face stellar cooling constraints

Dark photon or axion absorption can explain the excess under certain conditions

Exothermic dark matter with keV splittings can fit the data

Abstract

Motivated by the recent XENON1T results, we explore various new physics models that can be discovered through searches for electron recoils in O(keV)-threshold direct-detection experiments. First, we consider the absorption of light bosons, either as dark matter relics or being produced directly in the Sun. In the latter case, we find that keV mass bosons produced in the Sun provide an adequate fit to the data but are excluded by stellar cooling constraints. We address this tension by introducing a novel Chameleon-like axion model, which can explain the excess while evading the stellar bounds. We find that absorption of bosonic dark matter provides a viable explanation for the excess only if the dark matter is a dark photon or an axion. In the latter case, photophobic axion couplings are necessary to avoid X-ray constraints. Second, we analyze models of dark matter-electron scattering to determine which models might explain the excess. Standard scattering of dark matter with electrons is generically in conflict with data from lower-threshold experiments. Momentum-dependent interactions with a heavy mediator can fit the data with dark matter mass heavier than a GeV but are generically in tension with collider constraints. Next, we consider dark matter consisting of two (or more) states that have a small mass splitting. The exothermic (down)scattering of the heavier state to the lighter state can fit the data for keV mass splittings. Finally, we consider a subcomponent of dark matter that is accelerated by scattering off cosmic rays, finding that dark matter interacting though an O(100 keV)-mass mediator can fit the data. The cross sections required in this scenario are, however, typically challenged by complementary probes of the light mediator. Throughout our study, we implement an unbinned Monte Carlo analysis and use an improved energy reconstruction of the XENON1T events.