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

Abstract

The GERmanium Detector Array (GERDA) experiment operated enriched high-purity germanium detectors in a liquid argon cryostat, which contains 0.33% of ${}^{36}$Ar, a candidate isotope for the two-neutrino double-electron capture (2$\hat{I}½$ECEC) and therefore for the neutrinoless double-electron capture (0$\hat{I}½$ECEC). If detected, this process would give evidence of lepton number violation and the Majorana nature of neutrinos. In the radiative 0$\hat{I}½$ECEC of ${}^{36}$Ar, 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 ${}^{36}$Ar 0$\hat{I}½$ECEC with GERDA data, with a total live time of 4.34 yr (3.08 yr accumulated during GERDA Phase II and 1.26 yr during GERDA Phase I). No signal was found and a 90% C.L. lower limit on the half-life of this process was established T${}_{1/2}$ > 1.5x10${}^{22}$ yr

10 pages, 5 figures, 1 table