# Spectral shapes of forbidden argon beta decays as background component for rare-event searches

## TL;DR

This paper calculates the spectral shapes of forbidden beta decays of argon isotopes Ar-39 and Ar-42 to improve background modeling in dark matter searches and explore fundamental weak interaction parameters.

## Contribution

It presents the first calculation of the spectral shapes of these forbidden decays to next-to-leading order, aiding background characterization and fundamental physics tests.

## Key findings

- Spectral shape of Ar-39 decay characterized for the first time.
- Potential to compare experimental spectra with theoretical predictions over a wide energy range.
- Implications for understanding the ratio of weak vector and axial-vector constants.

## Abstract

The spectral shape of the electrons from the two first-forbidden unique beta- decays of Ar-39 and Ar-42 were calculated for the first time to the next-to-leading order. Especially the spectral shape of the Ar-39 decay can be used to characterise this background component for dark matter searches based on argon. Alternatively, due to the low thresholds of these experiments, the spectral shape can be investigated over a wide energy range with high statistics and thus allow a sensitive comparison with the theoretical predictions. This might lead to interesting results for the ratio of the weak vector and axial-vector constants in nuclei.