Ge-based ton-scale neutrinoless double-beta decay experiment*

The seesaw mechanism can explain the small but finite neutrino masses and if neutrinos are Majorana particles, i.e. they are their own antiparticles. The observation of neutrinoless double-beta decay (0νββ) would prove in a model independent way that neutrinos are Majorana particles and show that lepton number is violated. Under the assumption of a light neutrino exchange, an experiment approaching 0νββ half-life of 10²⁸ years will be able to cover the inverted hierarchy. Such an experiment requires, beside a ton-scale target, excellent energy resolution and an extremely low background level of ~0.1 count/(FWHM·t·yr) in the region of interest.

The current-generation experiments - GERDA, and the MAJORANA DEMONSTRATOR - utilize p-type point contact detectors that are highly-enriched in ⁷⁶Ge, and have achieved the best intrinsic energy resolution and the lowest background in the signal region. These achievements were realized by placing the experiments deep underground, careful selection of construction and shielding materials, minimizing the exposure of the enriched germanium from cosmic-ray activation, and the use of an active veto for in-situ background rejection.

Building on these successes, a new international collaboration - LEGEND (Large Enriched Germanium Experiment for Neutrinoless ββ Decay) - has been formed to pursue a ton-scale ⁷⁶Ge experiment [1]. It aims to develop a phased experimental program with the discovery potential reaching a half-life of 10²⁸ years or longer. In the first 200-kg phase, LEGEND will use existing resources as appropriate to expedite physics results.

In this talk, I will present the results from ongoing efforts, and give an overview on the planning, development and execution of the LEGEND experimental program.

[1] N. Abgrall et al, https://arxiv.org/pdf/1709.01980