LEGEND and the Quest for Majorana Neutrinos
Prof. Dr. Stefan Schoenert (TU Munich)

Since neutrinos have no electric charges, they may be their own antiparticles, referred to as Majorana neutrinos, and thus violate lepton number conservation. Neutrinoless double beta decay would be a direct consequence, and the search for this decay mode is the most sensitive method to unravel the Majorana nature of neutrinos. By operating bare germanium diodes, enriched in Ge-76, in an active liquid argon shield, GERDA achieved an unprecedently low background index of 5.2∙10−4 counts/keV kg yr in the signal region and collected an exposure of 127 kg yr in a background-free regime. No signal was observed, and a limit on the half-life of 0νββ decay in Ge-76 is set at T1/2 > 1.8∙1026 yr (90 % C.L.) [1] and Majorana neutrino masses are constrained to mββ< 79–180meV (90\% C.L.). The LEGEND Collaboration builds on the success of GERDA and MJD, and develops a phased, Ge-76-based double-beta decay experimental program with a T1/2 - discovery potential beyond 1028 years. Its first stage, LEGEND-200, started data-taking in early 2023, and LEGEND-1000 is under preparation. The first results from LEGEND-200, based on 48.3 kg·yrs of data, were presented in June at the Neutrino 2024 conference in Milan and will be discussed.