The Large Enriched Germanium Experiment for Neutrinoless Double Beta Decay (LEGEND)

The LEGEND collaboration has formed by merging the two leading 76Ge-based neutrinoless double beta decay (0νββ) groups, MAJORANA DEMONSTRATOR and GERDA. The observation of 0νββ, a lepton number conservation violating process beyond the Standard Model of particle physics, would reveal the nature of neutrinos as either Dirac or Majorana particles, and give hints on both neutrino absolute mass scale and ordering. By conclusively demonstrating the Majorana nature of neutrinos, i. e. the equivalence of the neutrino and the anti-neutrino, a detection could also lead to a better understanding of the matter-antimatter asymmetry in the early Universe. The expected experimental signature in the energy spectrum of the emitted electrons would be a peak-like signal at the Q-value, which is 2039 keV for the ββ-decay in 76Ge (Qββ). One of the main challenges for a detection is the unavoidable continuous 2νββ background, ranging up to Qββ. Thus, a sufficient energy resolution is needed to separate the signal from these background events. With germanium detectors an excellent resolution of the order of 0.1-0.2% at Qββ can be achieved, which makes them one of the most preferable isotopes to search for 0νββ. Currently, the best limit on the half-life of 0νββ in 76Ge is T1/2 > 1.8·1026 yr, determined by the GERDA collaboration. The LEGEND collaboration aims to further improve the sensitivity to 0νββ via a phased experimental program. The ultimate goal of LEGEND is to achieve a discovery potential to half-lives of T1/21028 yr, and thus to probe neutrino masses of the inverted ordering region.


Half-life sensitivities
FIg. 1: Projected half-life sensitivity of the several experimental phases of LEGEND to neutrinoless double-beta decay; Credits: J. Myslik

In the first phase of LEGEND, approximately 200 kg of enriched 76Ge detectors will be operated. LEGEND-200 is located at the Laboratori Nazionali del Gran Sasso (LNGS), and will largely reuse the existing GERDA infrastructure, including some of the germanium detectors, the outer water tank and the inner cryostat. The cryostat itself will be filled with liquid argon (LAr), which cools the detectors to around 87 K, shields them from external backgrounds, and acts as an active scintillation medium. To detect scintillation light, the volume is instrumented with wavelength shifting fibers, silicon photomultipliers, and wavelength-shifting reflectors. These can be used to veto background events that deposit part of their energy in the LAr. The installation of the veto, detectors and calibration systems is currently ongoing, with the first commissioning run expected to start in September 2021. The beginning of data taking is then scheduled for early 2022. After five years of operation, sensitivities of T1/21027 yr can be expected, as indicated in Fig. 1.


Baseline design
Fig. 2: Cryostat baseline design for LEGEND-1000

In the second stage the final sensitivity goal shall be achieved by increasing the mass of the germanium detector array to at least 1000 kg, with a total run time of around one decade. Due to the long time scale needed for the production of the detector material, a novel baseline cryostat concept has been designed, as sketched in Fig. 2. It is planned to immerse the detectors into the cryostat in individual batches. In this way data taking can continue with the detectors being already in operation when further batches will be installed. Discussions on the final location of the experiment in its subsequent stage are still ongoing, as in a different deep underground facility with a larger natural shielding the background induced by cosmic muons could be reduced. In any case, LEGEND-1000 would be among the world leading experiments in the search for 0νββ decay in any isotope.


University of Zurich contributions: calibration system

LNGS experience
Fig. 3: Sketch of the LEGEND-200 calibration system

Our group very actively contributes to the current installation of LEGEND-200, and future planning of LEGEND-1000. In particular, we are responsible for the LEGEND-200 calibration system. For the calibration of the germanium detectors an upgraded version of the existing GERDA source-insertion system will be used to deploy multiple thorium-228 sources per system into the cryostat, see Fig. 3.

Following this multiple-source approach, we can achieve an optimal energy resolution and a stable energy scale for each individual detector. Thus, our work focuses on the needed update and extension of the existing calibration system, as well as hardware tests under realistic experimental conditions, such as the required cryogenic temperatures. Furthermore, we contribute to the monitoring and software control of the system, to ensure both a flexible handling and a constant operability of the calibration system.

Contributions: Wavelenth-shifting reflectors

Fig. 4: Wavelength shifting reflectors of LEGEND-200

Additionally, we are involved in the design, installation and characterization of the wavelength shifting reflectors for the new liquid argon veto system. These reflectors were recently mounted inside the cryostat and in-situ coated with the wavelength shifter TPB. This wavelength shifter emits blue light when illuminated with VUV light,  as shown in Fig.4. The samples from these reflectors are then characterized using a liquid argon setup in our lab. Apart from hardware and material characterization, we further contribute to simulations for both experimental phases.