Double-weak decays of $^{124}Xe$ and $^{136}Xe$ in the XENON1T and XENONnT experiments

Abstract

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