Systematics in the XENON1T data: the 15-keV anti-axion

TL;DR

The paper investigates the XENON1T excess and suggests that data mismodeling could reduce the significance of the solar-axion interpretation, highlighting the need for careful background modeling in rare event searches.

Contribution

It provides a novel side-band analysis revealing potential mismodeling effects that impact the interpretation of the XENON1T excess as a solar axion signal.

Findings

Mismodeling may reduce the axion signal significance to about 0.1 p-value.

Significance peaks when the spectral model is shifted by ~15 keV.

Data does not follow expected chi-square distribution, indicating modeling issues.

Abstract

The XENON1T collaboration has found an excess of electron recoil events in their Science Run 1 data below ~7 keV with a spectral shape consistent with that expected from a solar-axion-induced signal. The claimed statistical significance of the solar-axion model over the null hypothesis is 3.5$\sigma$. In this work we provide evidence for mismodeling in the electron recoil data that may decrease the local significance of the axion model to as low as $p \approx 0.1$. To reach this conclusion, we search for a signal with the spectral template of the solar axion model, but shifted to higher (unphysical) energies above ~7 keV. We find that the distribution of significances found from this side-band analysis does not follow the expected chi-square distribution, which allows us to quantify the extent to which mismodeling may be affecting the interpretation of the data at energies below ~7 keV. For example, we find that there is an even higher-significance fit to the data when the solar axion model is shifted upwards in energy by ~15 keV and allowed to have a negative normalization.