Improved search for two-neutrino double electron capture on 124Xe and 126Xe using particle identification in XMASS-I

We conducted an improved search for the simultaneous capture of two K-shell electrons on the 124Xe and 126Xe nuclei with emission of two neutrinos using 800.0 days of data from the XMASS-I detector. A novel method to discriminate γ-ray/X-ray or double electron capture signals from β-ray background using scintillation time profiles was developed for this search. No significant signal was found when fitting the observed energy spectra with the expected signal and background. Therefore, we set the most stringent lower limits on the half-lives at 2.1×1022 and 1.9×1022 years for 124Xe and 126Xe, respectively, with 90% confidence level. These limits improve upon previously reported values by a factor of 4.5.

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