An improved limit on the neutrinoless double-electron capture of $$^{36}$$Ar with GERDA

The GERmanium Detector Array (Gerda) experiment operated enriched high-purity germanium detectors in a liquid argon cryostat, which contains 0.33% of 36Ar, a candidate isotope for the two-neutrino double-electron capture (2νECEC) and therefore for the neutrinoless double-electron capture (0νECEC). If detected, this process would give evidence of lepton number violation and the Majorana nature of neutrinos. In the radiative 0νECEC of 36Ar, a monochromatic photon is emitted with an energy of 429.88 keV, which may be detected by the Gerda germanium detectors. We searched for the 36Ar 0νECEC with Gerda data, with a total live time of 4.34 year (3.08 year accumulated during Gerda Phase II and 1.26 year during Gerda Phase I). No signal was found and a 90% CL lower limit on the half-life of this process was established T1/2>1.5·1022 year. Supplementary information: The online version contains supplementary material available at 10.1140/epjc/s10052-023-12280-6.