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 (2⁢𝜈⁢ECEC) of 124Xe and neutrinoless double-𝛽 decay (0⁢𝜈⁢𝛽⁢𝛽) of 136Xe 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 T₁⸝₂(2⁢𝜈⁢ECEC) = (1.1±0.2stat±0.1sys)×10²² 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 136Xe 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 T₁⸝₂(0⁢𝜈⁢𝛽⁢𝛽) > 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 0⁢𝜈⁢𝛽⁢𝛽. Assuming a 275 kg yr 136Xe exposure, the expected sensitivity is T₁⸝₂(0⁢𝜈⁢𝛽⁢𝛽) > 2.1×10²⁵ yr at 90% CL, corresponding to an effective Majorana mass range of ⟨𝑚𝛽⁢𝛽⟩ < (0.19–0.59)⁢ eV/c².