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

TL;DR

This paper reports an improved search for two-neutrino double electron capture in xenon isotopes using the XMASS-I detector, employing a novel particle identification method to discriminate signals from background, resulting in the most stringent half-life limits to date.

Contribution

The study introduces a new scintillation time profile technique for better background discrimination in double electron capture searches.

Findings

No significant signal detected.

Established new lower half-life limits for $^{124}$Xe and $^{126}$Xe.

Limits are 4.5 times more stringent than previous results.

Abstract

We conducted an improved search for the simultaneous capture of two $K$-shell electrons on the $^{124}$Xe and $^{126}$Xe 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 $2.1 \times 10^{22}$ and $1.9 \times 10^{22}$ years for $^{124}$Xe and $^{126}$Xe, respectively, with 90% confidence level. These limits improve upon previously reported values by a factor of 4.5.