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

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$\hat{I}½$2EC), utilizing a $1.39\mathrm{kg}\times \mathrm{yr}$ isotopic exposure from the first LZ science run. A half-life of $T_{1/2}^{2\nu 2\mathrm{EC}}=(1.09\pm 0.14_{\text{stat}}\pm 0.05_{\text{sys}})\times 10^{22}\mathrm{yr}$ is observed with a statistical significance of $8.3\ddot{I}ƒ$, 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 $1.4\ddot{I}ƒ$ level.

15 pages, 3 figures