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

Abstract

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 $\text{KK}$- and $\text{KL}$-shells of ${}^{36}$Ar 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 ${}^{36}$Ar isotopic abundance in UAr (0.007%), we set a limit on the half-life of the two-electron capture process in ${}^{36}$Ar of $T_{1/2}>9.2\times 10^{19}$ 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 $\sim$100 with 10 years of expected operation and assuming the same ${}^{36}$Ar abundance as in the DarkSide-50 underground argon target.

12 pages, 5 figures; corrected author list (added author A. Derbin, D. Semenov, E. Unzhakov, V. Muratova; updated author order)

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