Speaker
Description
The search for neutrinoless double beta ($0\nu\beta\beta$) decay with $^{76}$Ge detectors tests whether neutrinos are Majorana particles and requires ultra-low background conditions, making background identification and mitigation crucial for next-generation experiments.
The LEGEND (Large Enriched Germanium Experiment for Neutrinoless $\beta\beta$ Decay) collaboration aims to improve the 3$\sigma$ half-life discovery sensitivity of $^{76}$Ge-based $0\nu\beta\beta$ experiments beyond $10^{28}$ yr through a staged approach.
The first phase, LEGEND-200, started operations at the Laboratori Nazionali del Gran Sasso in Italy in spring 2023.
During its first physics run, it operated 142 kg of high-purity germanium detectors enriched in $^{76}$Ge, immersed in instrumented liquid argon for active background suppression. LEGEND-200 achieved the lowest background index ever reached in a $^{76}$Ge $0\nu\beta\beta$ experiment, with an estimated background level of $0.5^{+0.3}_{-0.2}$ cts/(keV ton yr) in the signal region. By combining an exposure of 61.0 kg yr with data from previous experiments GERDA and MAJORANA DEMONSTRATOR, the highest half-life sensitivity to date for the search for $0\nu\beta\beta$ decay has been achieved.
In this talk, I will present the current status and performance of LEGEND-200 following an extensive cleaning, re-mounting, and background investigation and mitigation campaign aimed at reducing residual background contributions observed during the first physics run. The outcome of these studies will guide the next ton-scale phase of the experiment, LEGEND-1000, which is currently in the technical design phase and requires further improvements in low-background materials and radiation shielding.
This work is supported by the U.S. DOE and the NSF; the LANL, ORNL, and LBNL LDRD programs; the European ERC and Horizon programs; the German DFG, BMBF, and MPG; the Italian INFN; the Polish NCN and MNiSW; the Czech MEYS; the Slovak SRDA; the Swiss SNF; the UK STFC; the Canadian NSERC and CFI; the LNGS and SURF facilities.