Atomic ionization by scalar dark matter and solar scalars

TL;DR

This paper calculates atomic ionization cross-sections by scalar particles, corrects previous overestimations, and uses these results to set new bounds on scalar dark matter interactions, explaining the Xenon1T excess with solar scalars.

Contribution

It provides accurate cross-section calculations correcting past errors and applies these to interpret experimental data, establishing new bounds on scalar dark matter and explaining the Xenon1T excess.

Findings

Corrected ionization cross-sections are several orders of magnitude lower than previous estimates.

Established first direct bounds on scalar-electron coupling from Xenon1T data.

Proposed scalar emission from the Sun as an explanation for the Xenon1T excess.

Abstract

We calculate the cross-sections of atomic ionization by absorption of scalar particles in the energy range from a few eV to 100 keV. We consider both nonrelativistic particles (dark matter candidates) and relativistic particles which may be produced inside Sun. We provide numerical results for atoms relevant for direct dark matter searches (O, Na, Ar, Ca, Ge, I, Xe, W and Tl). We identify a crucial flaw in previous calculations and show that they overestimated the ionization cross sections by several orders of magnitude due to violation of the orthogonality of the bound and continuum electron wave functions. Using our computed cross-sections, we interpret the recent data from the Xenon1T experiment, establishing the first direct bounds on coupling of scalars to electrons. We argue that the Xenon1T excess can be explained by the emission of scalars from the Sun. While our finding is in a similar tension with astrophysical bounds as the solar axion hypothesis, we establish direct limits on scalar DM for the $\sim 1-10\,\mathrm{keV}$ mass range. We also update axio-ionization cross-sections. Numerical data files are provided.