Sensitivity of the NEXT experiment to Xe-124 double electron capture

TL;DR

This paper evaluates the sensitivity of the NEXT-White detector to the rare double electron capture in Xe-124, aiming to establish benchmarks for detecting neutrinoless decay modes and guiding future experimental efforts.

Contribution

The study presents the first sensitivity analysis of the NEXT-White detector to Xe-124 double electron capture and projects potential improvements with NEXT-100 for future searches.

Findings

Estimated sensitivity to $2 u ECEC$ half-life of $6 imes 10^{22}$ years for NEXT-100.

Optimal event selection method enhances detection sensitivity.

Projected 5-year run could significantly advance detection limits.

Abstract

Double electron capture by proton-rich nuclei is a second-order nuclear process analogous to double beta decay. Despite their similarities, the decay signature is quite different, potentially providing a new channel to measure the hypothesized neutrinoless mode of these decays. The Standard-Model-allowed two-neutrino double electron capture ($2\nu ECEC$) has been predicted for a number of isotopes, but only observed in $^{78}$Kr, $^{130}$Ba and, recently, $^{124}$Xe. The sensitivity to this decay establishes a benchmark for the ultimate experimental goal, namely the potential to discover also the lepton-number-violating neutrinoless version of this process, $0\nu ECEC$. Here we report on the current sensitivity of the NEXT-White detector to $^{124}$Xe $2\nu ECEC$ and on the extrapolation to NEXT-100. Using simulated data for the $2\nu ECEC$ signal and real data from NEXT-White operated with $^{124}$Xe-depleted gas as background, we define an optimal event selection that maximizes the NEXT-White sensitivity. We estimate that, for NEXT-100 operated with xenon gas isotopically enriched with 1 kg of $^{124}$Xe and for a 5-year run, a sensitivity to the $2\nu ECEC$ half-life of $6 \times 10^{22}$ y (at 90% confidence level) or better can be reached.