Results of the LEGEND-200 experiment in the search for neutrinoless double beta decay

Abstract

The LEGEND experiment is looking for the extremely rare neutrinoless double beta ($0\nu\beta\beta$) decay of $^{76}$Ge using isotopically-enriched high-purity germanium (HPGe) detectors. The detection of this process would imply that the neutrino is a Majorana particle and the total lepton number would not be conserved, which could be related to the cosmological asymmetry between matter and antimatter through leptogenesis. The long-term goal of the collaboration is LEGEND-1000: a 1-ton detector array planned to run for 10 years, with a projected half-life sensitivity exceeding $10^{28}$ years, fully covering the inverted neutrino mass hierarchy. A first search for the $0\nu\beta\beta$ decay has been carried out by LEGEND-200 building on the experience gained from GERDA and the MAJORANA DEMONSTRATOR. The experiment has been collecting physics data for a year at the Gran Sasso National Laboratory in Italy with 140 kg of HPGe detectors. With a total exposure of 61 kg yr, LEGEND-200 has achieved a background index of $5^{+3}_{-2}\times10^{-4}$ counts/(keV kg yr) in the $0\nu\beta\beta$ decay signal region from the highest performing detectors. After combining the results from GERDA, the MAJORANA Demonstrator and LEGEND-200, an exclusion sensitivity $ > 2.8\times10^{26}$ yr has been obtained at 90\% confidence level for the $0\nu\beta\beta$ decay half-life, with no evidence for a signal. A new observed lower limit of $T^{0\nu}_{1/2} > 1.9\times10^{26}$ yr at 90\% confidence level has been established.