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

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.