Cosmic Imprints of XENON1T Axions

TL;DR

This paper investigates how solar axions, potentially explaining the XENON1T electron recoil excess, could leave detectable imprints on the cosmic microwave background through an increased effective neutrino number, testable by future CMB surveys.

Contribution

It proposes minimal models where solar axions produce a measurable $ ext{N}_ ext{eff}$ signal, linking particle physics explanations to cosmological observations.

Findings

Predicted $ ext{N}_ ext{eff}$ exceeds 0.03-0.04 in all models.

Future CMB-S4 experiments could detect these axion-induced signals.

Cosmological tests can potentially confirm the solar axion interpretation of XENON1T excess.

Abstract

The recent electron recoil excess observed by XENON1T has a possible interpretation in terms of solar axions coupled to electrons. If such axions are still relativistic at recombination they would also leave a cosmic imprint in the form of an additional radiation component, parameterized by an effective neutrino number $\Delta N_\text{eff}$. We explore minimal scenarios with a detectable signal in future CMB surveys: axions coupled democratically to all fermions, axion-electron coupling generated radiatively, the DFSZ framework for the QCD axion. The predicted $\Delta N_\text{eff}$ is larger than $0.03-0.04$ for all cases, close to the $2\sigma$ forecasted sensitivity of CMB-S4 experiments. This opens the possibility of testing with cosmological observations the solar axion interpretation of the XENON1T excess.