Two-neutrino double electron capture is a rare nuclear decay where two electrons are simultaneously captured from the atomic shell. For ${}^{124}\mathrm{Xe}$ 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 ${}^{124}\mathrm{Xe}$ using 7636 $\mathrm{kg}\mathrm{d}$ 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 $T_{1/2}>6.5\times {10}^{20}$ yr. We have also evaluated the sensitivity of the XENON1T experiment, which is currently being commissioned, and found a sensitivity of $T_{1/2}>6.1\times {10}^{22}$ yr after an exposure of 2 $\mathrm{t}\mathrm{yr}$.

• Received 12 September 2016

DOI:https://doi.org/10.1103/PhysRevC.95.024605