LEGEND

The Large Enriched Germanium Experiment for Neutrinoless Double Beta Decay

LEGEND is an international project aiming to probe a rare radioactive decay process, neutrinoless double beta (0νββ) decay, which, if observed, would prove the Majorana nature of neutrinos and manifest the first measured signal of the violation of total lepton number conservation.

The LEGEND collaboration consists of about 250 researchers from more than 50 international institutions, including members from the two preceding Ge-based 0νββ decay groups, GERDA and the MAJORANA DEMONSTRATOR. The experiment is hosted at the Gran Sasso National Laboratory (Laboratori Nazionali del Gran Sasso, LNGS) in Italy, located 1.4 km underground. It operates high-purity germanium (HPGe) detectors in liquid argon to search for 0νββ decay in 76Ge.

The main experimental signature of a 0νββ event is a characteristic peak at the energy distribution of the summed energy of the two emitted electrons, located at the decay Q-value of Qββ = 2039 keV for 76Ge. Experimental research requires a high isotopic abundance, an ultra-low background level, an excellent energy resolution, and a high efficiency for detecting the two emitted electrons.

Continuous experimental searches for 0νββ decay have been employed with a variety of techniques and isotopes. No evidence of 0νββ decay has been observed so far. The currently best limit on the 0νββ decay half-life of 76Ge is T1/2 > 1.8×1026 yr, determined by GERDA. The LEGEND collaboration aims to further improve the sensitivity of measuring 0νββ decay via a phased experimental program separated into two stages. The ultimate goal is to achieve a discovery sensitivity of T1/2 ≈ 1028 yr and thus to probe neutrino masses of the inverted ordering region.

Credit: APS/Alan Stonebraker

The LEGEND collaboration meeting (March 2025, at GSSI, L'Aquila, Italy)

Phase I --- LEGEND-200

In the first phase of the experiment, LEGEND-200 will operate with a target mass of approximately 200 kg of enriched Ge materials and an estimated operational live-time of five years, aiming to reach a discovery sensitivity of the 0νββ decay half-life of T1/2 > 1027 yr with one ton-year of exposure, given the background level of 2×10-4 counts/(keV·kg·yr) is reached.

LEGEND-200 reuses much of the existing infrastructure from GERDA, including the outer water tank and the inner cryostat. At the same time, multiple subsystems were upgraded, including a new lock system to allow longer HPGe detector strings to be installed into the LAr cryostat, four new detector-calibration source-insertion systems permitting multiple radioactive sources to be deployed per unit, and a wavelength-shifting reflector plus an optical-fiber shroud surrounding the detector array to detect the LAr scintillation light.

LEGEND-200 has started its physics data-taking since spring 2023 with an initial 142 kg of HPGe detectors and unblinded the analysis of the first year of data taken between spring 2023 and spring 2024, accumulated up to 61 kg·yr of exposure. The results were submitted for publication while a preprint can be found in arXiv:2505.10440 [hep-ex].

The LEGEND-200 apparatus, including the HPGe detector array surrounded by optical-fiber shrouds and a wavelength-shifting reflector, the LAr cryostat containing a lock system for material deployment, and a water tank instrumented with photomultiplier tubes (PMTs) to detect Cherenkov light produced by external radiation. Credit: P. Krause

The HPGe detector array of LEGEND-200 (with an initial 142 kg of Ge materials) during maintenance while the array was lifted from the cryostat and stored in a glove box. Credit: M. Willers

Phase II --- LEGEND-1000

Reference layout of the LEGEND-1000 apparatus. The three-layered volumes from inside to outside are the underground-sourced-LAr-filled main volume in which the HPGe detectors are immersed, the cryostat filled with atmospheric LAr instrumented with a PMMA neutron moderator, and the water tank equipped with PMTs. Credit: P. Krause

The ton-scale second-phase experiment aims to achieve a discovery sensitivity that is another order of magnitude better than the first phase to fully cover the inverted-neutrino-mass-ordering regime. This requires a background rate of < 10-4 counts/(keV·kg·yr) together with a ten-year operation period, implying less than one background count in a 4σ region-of-interest in 10 t·yr of exposure, assuming an energy resolution of σ/Qββ = 0.05%, as achieved in GERDA and MAJORANA.

LEGEND-1000 will use the water tank of the Borexino experiment with modifications. The LEGEND-1000 apparatus will follow the LEGEND-200 concept by deploying HPGe detectors in LAr with various upgrades. One main difference is the layered structure of the water tank and two separate LAr volumes. The detector array will be immersed in a thin underground-electroformed-copper cylinder filled with underground-sourced LAr to minimize the amount of 42Ar, which contributes largely to the background, in the atmospheric argon, and the copper cylinder will be placed within the main cryostat filled with atmospheric LAr. The separation of the inner and the outer LAr volumes also prevents radon from drifting through the liquid. Additionally, a polymethyl methacrylate (PMMA) structure will be installed in the outer LAr volume serving as a neutron moderator.

LEGEND-1000 is in an advanced planning stage. Plenty of R&D projects are underway to mitigate the background level, including updating the infrastructure and improving the analysis techniques. A modular approach is planned, with different payloads being instrumented over a few years, allowing continuous data-taking while the experiment scales up to its full size. A phased commission is foreseen to start in 2030.

Our Contributions

Our group contributes to all aspects of the LEGEND experiment including hardware R&D, data analyses, and physics simulations.

Liquid Argon Test Stand

In LEGEND, liquid argon (LAr) serves as both a cooling material to provide a cryogenic environment for the HPGe detectors and an active veto system to discriminate 0νββ signals from background events. Our group is constructing a multi-purpose LAr test stand at NTU to intensively contribute to LEGEND’s efforts in R&D to its LAr facility. The LAr test stand will be used to study next-generation wavelength-shifting materials and to design an energy calibration mechanism for LEGEND's LAr system.

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