GERDA and LEGEND: Probing the Neutrino Nature and Mass at 100 meV and beyond

Abstract

The Gerda (GERmanium Detector Array) project, located at Laboratori Nazionali del Gran Sasso (LNGS), was started in 2005, a few years after the claim of evidence for the neutrinoless double beta decay (0νββ) of 76Ge to the ground state of 76Se: it is an ultra-rare process whose detection would directly establish the Majorana nature of the neutrino and provide a measurement of its mass and mass hierarchy. The aim of Gerda was to confirm or disprove the claim by an increased sensitivity experiment. After establishing the new technology of Ge detectors operated bare in liquid Argon and since 2011, Gerda efficiently collected data searching for 0νββ of 76Ge, first deploying the 76Ge-enriched detectors from two former experiments and later new detectors with enhanced signal-to-background rejection, produced from freshly 76Ge-enriched material. Since then, the Gerda setup has been upgraded twice, first in 2013–2015 and later in 2018. The period before 2013 is Phase I and that after 2015 is Phase II. Both the Gerda setup and the analysis tools evolved along the project lifetime, allowing to achieve the remarkable average energy resolution of ∼3.6 and ∼2.6 keV for Coaxial Germanium (Coax) detectors and for Broad Energy Germanium (BEGe), respectively, and the background index of 5.2−1.3+1.6·10−4 cts/(keV·kg·yr) in a 230 keV net range centered at Qββ. No evidence of the 0νββ decay at Qββ = 2039.1 keV has been found, hence the limit of 1.8·1026 yr on the half-life (T1/20ν) at 90% C.L. was set with the exposure of 127.2 kg·yr. The corresponding limit range for the effective Majorana neutrino mass mee has been set to 79–180 meV. The Gerda performances in terms of background index, energy resolution and exposure are the best achieved so far by 76Ge double beta decay experiments. In Phase II, Gerda succeeded in operating in a background free regime and set a world record. In 2017, the Legend Collaboration was born from the merging of the Gerda and Majorana Collaborations and resources with the aim to further improve the Gerda sensitivity. First, the Legend200 project, with a mass of up to 200 kg of 76Ge-enriched detectors, aims to further improve the background index down to < 0.6 ·10−3 cts/(keV·kg·yr) to explore the Inverted Hierarchy region of the neutrino mass ordering, then the Legend1000 (1 ton of 76Ge-enriched) will probe the Normal Hierarchy. In this paper, we describe the Gerda experiment, its evolution, the data analysis flow, a selection of its results and technological achievements, and finally the design, features and challenges of Legend, the Gerda prosecutor.

A multi–channel study of the 20Ne + 130Te system within the NUMEN project

“…Neutrinoless double beta decay (0νββ) is one of the "hot" subject in physics research nowadays [1,2]. The process, forbidden in the Standard Model framework, has not been experimentally observed yet and only experimental lower limits on the 0νββ half-time are available for some isotopes [3,4,5,6,7]. In this extremely rare yet elusive process, a nucleus with mass and atomic numbers (A,Z) is transformed into a new one with mass and atomic numbers (A,Z+2) emitting also two electrons or to (A,Z-2) emitting two positrons.…”

A Coupled Approach to Compute the Dislocation Density Development during Czochralski Growth and Its Application to the Growth of High-Purity Germanium (HPGe)

“…Such dislocation engineering would be very helpful in conjunction with the Legend project [5], with the aim of detecting the neutrinoless double-β decay. High-purity germanium (HPGe) crystals are enriched in the isotope 76 Ge, which is predicted to undergo a neutrinoless double-β decay.…”

“…Such experiments have been performed, e.g., in the GERDA project [6]. Further investigations will be performed within the Legend project [5], applying 200 kg of Ge detectors in the first step and 1000 kg in the second.…”