Search for two-neutrino double electron capture in ³⁶Ar with the DarkSide-50 detector

Two-neutrino double electron capture is a rare nuclear decay where two electrons are simultaneously captured from the atomic shells and two neutrinos are emitted. We report on the first search for two-neutrino double electron capture in the KK- and KL-shells of ³⁶Ar using the low-radioactivity liquid argon target from underground sources in the DarkSide-50 experiment. No statistically significant excess was observed with approximately 12 ton-day exposure of underground argon (UAr) and, taking into account the ³⁶Ar isotopic abundance in UAr (0.007%), we set a limit on the half-life of the two-electron capture process in ³⁶Ar of T1/2 > 9.2 × 10¹⁹ yr at 90% C.L. The sensitivity of the DarkSide-20k experiment, which will become operational in the next few years, was also evaluated and is expected to increase by a factor ~100 with 10 years of expected operation and assuming the same ³⁶Ar abundance as in the DarkSide-50 underground argon target.