First results from LEGEND-200 on the search for neutrinoless double beta decay

Abstract

Neutrinoless double beta decay ($0\nu\beta\beta$) is a rare process which could take place if neutrinos are Majorana fermions: the observation of this decay would provide unambiguous evidence for the existence of new Physics beyond the Standard Model, as it entails a two-units lepton number violation. The LEGEND experiment (Large Enriched Germanium Experiment for Neutrinoless $\beta\beta$ Decay) searches for the $0\nu\beta\beta$ of $^{76}$Ge using High Purity Germanium detectors enriched in $^{76}$Ge beyond the 86%. The LEGEND project foresees two phases, LEGEND-200, aiming at a final 3$\sigma$ discovery sensitivity beyond 10$^{27}$ yr, and LEGEND-1000, aiming at a final sensitivity beyond 10$^{28}$ yr. LEGEND-200 started taking data in 2023 at Laboratori Nazionali del Gran Sasso (LNGS): in about one year it collected a total physics exposure of 61 kg yr, with a background index of $0.5^{+0.3}_{-0.2} \cdot 10^{-3}$ counts/(keV kg yr) in the so-called "golden dataset" and of $1.3^{+0.8}_{-0.5} \cdot 10^{-3}$ counts/(keV kg yr) in the so-called "silver dataset". The performed statistical analysis finds no evidence for a $0\nu\beta\beta$ signal and sets a lower limit on its half life to $T_{1/2}^{0\nu} > 0.5 \cdot 10^{26}$ yr at 90% CL. A combined analysis of the three germanium-based $0\nu\beta\beta$ experiments GERDA, MAJORANA Demonstrator and LEGEND-200 provides a lower limit of $T_{1/2}^{0\nu} > 1.9 \cdot 10^{26}$ yr at 90% CL.