First Results on the Search for Lepton Number Violating Neutrinoless Double Beta Decay with the LEGEND-200 Experiment

Abstract

The LEGEND collaboration is searching for neutrinoless double beta ($0\hat{I}½{\hat{I}}^2{\hat{I}}^2$) decay by operating high-purity germanium detectors enriched in ${}^{76}$Ge in a low-background liquid argon environment. Building on key technological innovations from GERDA and the MAJORANA DEMONSTRATOR, LEGEND-200 has performed a first $0\hat{I}½{\hat{I}}^2{\hat{I}}^2$ decay search based on 61.0 kg yr of data. Over half of this exposure comes from our highest performing detectors, including newly developed inverted-coaxial detectors, and is characterized by an estimated background level of $0.5_{-0.2}^{+0.3}$ cts/(keV kg yr) in the $0\hat{I}½{\hat{I}}^2{\hat{I}}^2$ decay signal region. A combined analysis of data from GERDA, the MAJORANA DEMONSTRATOR, and LEGEND-200, characterized by a 90% confidence level exclusion sensitivity of $2.8\times 10^{26}$ yr on the half-life of $0\hat{I}½{\hat{I}}^2{\hat{I}}^2$ decay, reveals no evidence for a signal and sets a new observed lower limit at $T_{1/2}^{0\hat{I}½}>1.9\times 10^{26}$ yr (90% confidence level). Assuming the decay is mediated by Majorana neutrinos, this corresponds to an upper limit on the effective Majorana mass in the range $m_{{\hat{I}}^2{\hat{I}}^2}<75-200$ meV, depending on the adopted nuclear matrix element.