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

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 (0 nu beta beta) of Ge-76 to the ground state of Se-76: 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 0 nu beta beta of Ge-76, first deploying the Ge-76-enriched detectors from two former experiments and later new detectors with enhanced signal-to-background rejection, produced from freshly (7)6Ge-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 similar to 3.6 and similar to 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 Q(beta beta). No evidence of the 0 nu beta beta decay at Q(beta beta) = 2039.1 keV has been found, hence the limit of 1.8.10(26) yr on the half-life (T-1/2(0 nu)) 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 Ge-76 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 Ge-76-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 Ge-76-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.