Search for Two-Neutrino Double Electron Capture of $^{124}$Xe with XENON100

TL;DR

This paper reports a search for the rare two-neutrino double electron capture in $^{124}$Xe using the XENON100 detector, setting new lower limits on its half-life and evaluating future experiment sensitivities.

Contribution

It provides the first experimental lower limit on the half-life of two-neutrino double electron capture in $^{124}$Xe and assesses the potential of the XENON1T experiment for future detection.

Findings

No significant excess observed, setting a lower half-life limit of >6.5×10^{20} years.

Projected sensitivity of XENON1T is >6.1×10^{22} years after 2 ton-year exposure.

Demonstrates the feasibility of using dark matter detectors for rare nuclear decay searches.

Abstract

Two-neutrino double electron capture is a rare nuclear decay where two electrons are simultaneously captured from the atomic shell. For $^{124}$Xe this process has not yet been observed and its detection would provide a new reference for nuclear matrix element calculations. We have conducted a search for two-neutrino double electron capture from the K-shell of $^{124}$Xe using 7636 kg$\cdot$d of data from the XENON100 dark matter detector. Using a Bayesian analysis we observed no significant excess above background, leading to a lower 90 % credibility limit on the half-life $T_{1/2}>6.5\times10^{20}$ yr. We also evaluated the sensitivity of the XENON1T experiment, which is currently being commissioned, and find a sensitivity of $T_{1/2}>6.1\times10^{22}$ yr after an exposure of 2 t$\cdot$yr.