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

Abstract

We present results on the search for double-electron capture ($2\hat{I}½\text{ECEC}$) of ${}^{124}$Xe and neutrinoless double-${\hat{I}}^2$ decay ($0\hat{I}½{\hat{I}}^2{\hat{I}}^2$) of ${}^{136}$Xe in XENON1T. We consider captures from the K- up to the N-shell in the $2\hat{I}½\text{ECEC}$ signal model and measure a total half-life of $T_{1/2}^{2\hat{I}½\text{ECEC}}=(1.1\pm 0.2_{\text{stat}}\pm 0.1_{\text{sys}})\times 10^{22}\text{yr}$ with a $0.87\text{kg}\times \text{yr}$ isotope exposure. The statistical significance of the signal is $7.0\ddot{I}ƒ$. We use XENON1T data with $36.16\text{kg}\times \text{yr}$ of ${}^{136}$Xe exposure to search for $0\hat{I}½{\hat{I}}^2{\hat{I}}^2$. We find no evidence of a signal and set a lower limit on the half-life of $T_{1/2}^{0\hat{I}½{\hat{I}}^2{\hat{I}}^2}>1.2\times 10^{24}\text{yr}\text{at}90\%\text{CL}$. 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 $0\hat{I}½{\hat{I}}^2{\hat{I}}^2$. Assuming a $275\text{kg}\times \text{yr}$ ${}^{136}$Xe exposure, the expected sensitivity is $T_{1/2}^{0\hat{I}½{\hat{I}}^2{\hat{I}}^2}>2.1\times 10^{25}\text{yr}\text{at}90\%\text{CL}$, corresponding to an effective Majorana mass range of $⟨m_{{\hat{I}}^2{\hat{I}}^2}⟩<(0.19-0.59){\text{eV/c}}^2$.

23 pages, 14 figures, 9 tables, version accepted for publication in Phys. Rev. C