Two-neutrino double electron capture of $^{124}$Xe in the first LUX-ZEPLIN exposure

TL;DR

This paper reports a precise measurement of the rare two-neutrino double electron capture decay of $^{124}$Xe using LUX-ZEPLIN data, providing the first empirical KK capture fraction and confirming theoretical models.

Contribution

It presents the first empirical measurement of the $^{124}$Xe 2$ u$2EC decay half-life and KK capture fraction, expanding understanding of rare nuclear decay processes.

Findings

Measured half-life: (1.09 ± 0.14 (stat) ± 0.05 (sys)) × 10^{22} years

Observed decay with 8.3 sigma significance

First empirical KK capture fraction consistent with models

Abstract

The broad physics reach of the LUX-ZEPLIN (LZ) experiment covers rare phenomena beyond the direct detection of dark matter. We report precise measurements of the extremely rare decay of $^{124}$Xe through the process of two-neutrino double electron capture (2$\nu$2EC), utilizing a $1.39\,\mathrm{kg} \times \mathrm{yr}$ isotopic exposure from the first LZ science run. A half-life of $T_{1/2}^{2\nu2\mathrm{EC}} = (1.09 \pm 0.14_{\text{stat}} \pm 0.05_{\text{sys}}) \times 10^{22}\,\mathrm{yr}$ is observed with a statistical significance of $8.3\,\sigma$, in agreement with literature. First empirical measurements of the KK capture fraction relative to other K-shell modes were conducted, and demonstrate consistency with respect to recent signal models at the $1.4\,\sigma$ level.