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

Abstract

We conducted an improved search for the simultaneous capture of two K-shell electrons on the Xe-124 and Xe-126 nuclei with emission of two neutrinos using 800.0 days of data from the XMASS-I detector. A novel method to discriminate gamma-ray/X-ray or double electron capture signals from beta-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 2.1x10(22) and 1.9 x 10(22) years for Xe-124 and Xe-126, respectively, with 90% confidence level. These limits improve upon previously reported values by a factor of 4.5. © The Author(s) 2018. Published by Oxford University Press on behalf of the Physical Society of Japan.