Sensitive search for double electron capture on 124Xe in XMASS-I

Abstract

Abstract Double electron capture is a rare nuclear decay process in which two orbital electrons are captured simultaneously in the same nucleus. We have conducted an improved search for two-neutrino double electron capture on 124Xe and 126Xe using 800.0 days of the XMASS-I data. As a result of fitting the observed energy spectra with the expected signal and background, no significant signal is found. Therefore, we set the most stringent lower limits on their half-lives at 2.1 × 1022 years for 124Xe and 1.9 × 1022 years for 126Xe at 90% confidence level. The limits get improved by a factor of 4.5 compared to the previous result.