Search for two-neutrino double electron capture on $^{124}$Xe with the XMASS-I detector

Double electron capture is a rare nuclear decay process in which two orbital electrons are captured simultaneously in the same nucleus. Measurement of its two-neutrino mode would provide a new reference for the calculation of nuclear matrix elements whereas observation of its neutrinoless mode would demonstrate lepton number violation. A search for two-neutrino double electron capture on Xe-124 is performed using 165.9 days of data collected with the XMASS-I liquid xenon detector. No significant excess above background was observed and we set a lower limit on the half-life as 4.7 x 10(21) years at 90% confidence level. The obtained limit has ruled out parts of some theoretical expectations. We obtain a lower limit on the Xe-126 two-neutrino double electron capture half-life of 4.3 x 10(21) years at 90% confidence level as well. (C) 2016 The Author(s). Published by Elsevier B.V.