Double-weak decays of and in the XENON1T and XENONnT experiments

We present results on the search for two-neutrino double-electron capture ( 2 ν ECEC ) of 124 Xe and neutrinoless double- β decay ( 0 ν β β ) of 136 Xe in XENON1T. We consider captures from the K shell up to the N shell in the 2 ν ECEC signal model and measure a total half-life of T 2 ν ECEC 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 7.0 σ . We use XENON1T data with 36.16 kg yr of 136 Xe exposure to search for 0 ν β β . We find no evidence of a signal and set a lower limit on the half-life of T 0 ν β β 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 0 ν β β . Assuming a 275 kg yr 136 Xe exposure, the expected sensitivity is T 0 ν β β 1 / 2 > 2.1 × 10 25 yr at 90 % CL , corresponding to an effective Majorana mass range of ⟨ m β β ⟩ < ( 0.19 – 0.59 ) eV / c 2 .