Double-weak decays of ¹²⁴Xe and ¹³⁶Xe in the XENON1T and XENONnT experiments

We present results on the search for two-neutrino double-electron capture (2νECEC) of ¹²⁴Xe and neutrinoless double-β decay (0νββ) of ¹³⁶Xe in XENON1T. We consider captures from the K shell up to the N shell in the 2νECEC signal model and measure a total half-life of T^2νECEC_1/2=(1.1±0.2_stat±0.1_sys)×10²² 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 ¹³⁶Xe 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>1.2×10²⁴ 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 ¹³⁶Xe exposure, the expected sensitivity is T^0νββ_1/2>2.1×10²⁵ yr at 90% CL, corresponding to an effective Majorana mass range of ⟨m_ββ⟩<(0.19–0.59) eV/c².