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

TL;DR

This paper reports a search for two-neutrino double electron capture on $^{124}$Xe using the XMASS-I detector, setting new lower limits on the half-life and constraining theoretical models.

Contribution

First experimental search for two-neutrino double electron capture on $^{124}$Xe with new half-life limits using the XMASS-I detector.

Findings

No significant excess observed above background.

Lower limit on $^{124}$Xe half-life set at $4.7 imes 10^{21}$ years.

Lower limit on $^{126}$Xe half-life set at $4.3 imes 10^{21}$ years.

Abstract

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 $^{124}$Xe 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 $4.7 \times 10^{21}$ years at 90% confidence level. The obtained limit has ruled out parts of some theoretical expectations. We obtain a lower limit on the $^{126}$Xe two-neutrino double electron capture half-life of $4.3 \times 10^{21}$ years at 90% confidence level as well.