Double-Weak Decays of $^{124}$Xe and $^{136}$Xe in the XENON1T and XENONnT Experiments

TL;DR

This paper reports the first observation of double-electron capture in $^{124}$Xe with a significant signal, sets new limits on neutrinoless double-beta decay in $^{136}$Xe, and projects future sensitivities for the XENONnT experiment.

Contribution

It provides the first measurement of $^{124}$Xe double-electron capture and improves limits on $^{136}$Xe neutrinoless double-beta decay using the XENON1T detector, with projections for XENONnT.

Findings

Measured $^{124}$Xe double-electron capture half-life as $(1.1 extpm0.2_ ext{stat} extpm0.1_ ext{sys}) imes 10^{22}$ years.

Set a lower limit on $^{136}$Xe neutrinoless double-beta decay half-life at $>1.2 imes 10^{24}$ years.

Projected XENONnT sensitivity to $^{136}$Xe decay half-life as $>2.1 imes 10^{25}$ years.

Abstract

We present results on the search for double-electron capture ($2\nu\text{ECEC}$) of $^{124}$Xe and neutrinoless double-$\beta$ decay ($0\nu\beta\beta$) of $^{136}$Xe in XENON1T. We consider captures from the K- up to the N-shell in the $2\nu\text{ECEC}$ signal model and measure a total half-life of $T_{1/2}^{2\nu\text{ECEC}}=(1.1\pm0.2_\text{stat}\pm0.1_\text{sys})\times 10^{22}\;\text{yr}$ with a $0.87\;\text{kg}\times\text{yr}$ isotope exposure. The statistical significance of the signal is $7.0\,\sigma$. We use XENON1T data with $36.16\;\text{kg}\times\text{yr}$ of $^{136}$Xe exposure to search for $0\nu\beta\beta$. We find no evidence of a signal and set a lower limit on the half-life of $T_{1/2}^{0\nu\beta\beta} > 1.2 \times 10^{24}\;\text{yr}\; \text{at}\; 90\,\%\;\text{CL}$. This is the best result from a dark matter detector without an enriched target to date. We also report projections on the sensitivity of XENONnT to $0\nu\beta\beta$. Assuming a $275\;\text{kg}\times\text{yr}$ $^{136}$Xe exposure, the expected sensitivity is $T_{1/2}^{0\nu\beta\beta} > 2.1 \times 10^{25}\;\text{yr}\; \text{at}\; 90\,\%\;\text{CL}$, corresponding to an effective Majorana mass range of $\langle m_{\beta\beta} \rangle < (0.19 - 0.59)\;\text{eV/c}^2$.