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

Understand

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.