Abstract
Among the several experiments and techniques conceived of to search for neutrinoless double β decay (0ν2β) in a handful of isotopes, presently the best lower limit on the half-life for this rare process, is provided by those using 76Ge, a rare isotope of germanium. Such a lower limit is of 1.8 × 1026 y. Building from such a successful achievement of the GERDA and Majorana Demonstrator experiments, the baton with 76Ge passes now to the LEGEND experiment. Using a two-stage approach with about 200 kg and then 1 t of germanium, LEGEND aims to attain a sensitivity of around 1028 y, which will enable it to probe the standard inverted-ordering neutrino mass scenario. We touch upon the past generation of experiments to illustrate their strong and weak points, review the general concept and design of LEGEND, and describe the LEGEND-200 detector and its preliminary performance. We also illustrate how the backgrounds can have a dramatic effect on the search and in which way the latter can be mitigated.
Highlights
In this Special Issue on “Neutrino-less Double Beta Decay” (0ν2β), several experiments are reviewed, which search for Lepton Flavor Violation (LFV) in the neutrino sector by looking for such a rare process
Four types of diodes have been developed that are being applied to the 0ν2β experiments: coaxial and Broad-Energy Germanium (BEGe, in Germanium Detector Array (GERDA)), the P-type Point Contact (PPC, in Majorana Demonstrator (MJD)), and the Inverted Coaxial Point Contact (ICPC), which is going to have the lion’s share in LEGEND
A nylon shroud was placed around each string and coated with Wavelength Shifter (WLS), in order to isolate it from 42K ions developed in the Liquid Argon (LAr) and attracted to the n+ dead layer, a background whose impact we describe in the following paragraph Section 4.1.2
Summary
In this Special Issue on “Neutrino-less Double Beta Decay” (0ν2β), several experiments are reviewed, which search for Lepton Flavor Violation (LFV) in the neutrino sector by looking for such a rare process. Its competitor based in the USA, the Majorana Demonstrator (MJD), has published a competitive figure, confirming the suitability and reliability of germanium diodes for the purpose at hand Based on this successful past, the “germanium community” has joined forces to build the next-generation experiment “LEGEND”, which will take the technique to the ton scale and approach the half-life range of 1028 y. Here, we talk only briefly about the relationship between the measured half-life and the Majorana effective mass mββ, the relation with the mass ordering, and the question of the Nuclear Matrix Element (NME) and its computation for the various isotopes according to different nuclear models. Depending on the NME value one picks, for example for 76Ge, a lower limit on T10/ν2 translates into an upper limit on mββ and, in turn, allows ruling out regions of probability for the inverse ordering This is illustrated, from [11].
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