Two-neutrino double electron capture of 124Xe 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 ¹²⁴Xe through the process of two-neutrino double electron capture (2v2EC), utilizing a 1.39 kg X yr isotopic exposure from the first LZ science run. A half-life of T_1/2^2v2EC = (1.09 + 0.14_stat + 0.05_sys) X 10²² 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.