Speaker
Description
The LEGEND collaboration aims to achieve an unambiguous discovery of neutrinoless double-beta decay (0νββ) using high-purity germanium (HPGe) detectors enriched in $^{76}$Ge (Q$_{ββ}$ = 2039 keV). These detectors are operated in liquid argon, which serves as both a coolant and an active shield, enabling a quasi-background-free search for 0νββ. The first phase, LEGEND-200, uses up to 200 kg of enriched HPGe detectors and is currently running in Hall A of the Laboratori Nazionali del Gran Sasso (LNGS), Italy. The next phase, LEGEND-1000, will scale the detector mass to 1000 kg, and preparation for its installation in Hall C at LNGS will begin in 2026.
To reach a 3σ discovery sensitivity for 0νββ with a half-life of 10$^{28}$ years in LEGEND-1000, the background index at Q$_{ββ}$ must be below 10$^{-5}$ cts/(keV·kg·yr). This strict limit makes previously sub-dominant backgrounds in LEGEND-200 become relevant, especially delayed decays of muon-induced isotopes. Notably, $^{77(m)}$Ge from neutron capture on $^{76}$Ge in HPGe detectors alone would exceed the background goal.
This talk presents new active suppression techniques that use delayed-coincidence tagging to identify isotope production during muon showers and delayed de-excitations from metastable daughter isotopes. These methods reduce the $^{77(m)}$Ge background to a sub-dominant 10% of the total budget, with minimal impact on the 0νββ detection efficiency (97%), supporting LNGS as a suitable site for LEGEND-1000.
This work is supported by the US DOE and NSF; the LANL, ORNL and LBNL LDRD programs; the European ERC and Horizon programs; the German DFG, BMFTR, and MPG; the Italian INFN; the Polish NCN and MNiSW; the Czech MEYS; the Slovak APVV; the Swiss SNF; the UK STFC; the Canadian NSERC; the Chinese (Taiwan) MOE and NSTC; and the LNGS and SURF facilities.