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 ($0\nu\beta\beta$) decay by operating high-purity germanium detectors enriched in $<sup>{76}$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 $0\nu\beta\beta$ decay search based on 61 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 $0.5<sup>{+0.3}_{-0.2}$ cts/(keV ton yr) in the $0\nu\beta\beta$ 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 $2.8 \times 10<sup>{26}$ yr on the half-life of $0\nu\beta\beta$ decay, reveals no evidence for a signal and sets a new observed lower limit at $T^{0\nu}_{1/2} &gt; 1.9 \times 10<sup>{26}$ 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 $m_{\beta\beta} &lt; 75-200$ meV, depending on the adopted nuclear matrix element.