Final Results of GERDA on the Search for Neutrinoless Double Beta Decay

The GERmanium Detector Array (GERDA) experiment searched for the lepton-number-violating double beta (0νββ) decay of ⁷⁶Ge, whose discovery would have far-reaching implications in cosmology and particle physics. Using High Purity Germanium (HPGe) detectors enriched in the isotope ⁷⁶Ge, which were directly immersed into liquid argon (LAr), it exploited the combination of excellent energy resolution of germanium detectors and scintillating properties of LAr. The GERDA experiment achieved an unprecedentedly low background index of 5.2·10^-4 counts/(keV·kg·yr) in the signal region and met the design goal to collect 100 kg·yr of exposure in a quasi-background-free regime. In November 2019, after fulfilling and exceeding the design goals of the experiment, data taking was stopped. No signal has been observed, hence a lower limit on the half-life of 0νββ decay in ⁷⁶Ge has been set at T_1/2^0ν > 1.8·10²⁶ years (90% C.L.) corresponding to an effective Majorana neutrino mass of m_ββ < 79-180 meV (90% C.L.). The limit coincides with the sensitivity, defined as the median expectation under the no signal hypothesis.

An experiment covering the full inverted mass ordering region, with effective Majorana neutrino masses of 15-50 meV, will require a ton-scale target mass foreseen by the Large Enriched Germanium Experiment for Neutrinoless double beta Decay (LEGEND) with extremely low backgrounds and excellent energy resolution. The GERDA infrastructure is therefore a suitable facility for the first phase of LEGEND deploying 200 kg of ⁷⁶Ge-enriched HPGe detectors in a liquid-argon cryostat. This first stage, LEGEND-200, aims for a tenfold improvement in the half-life sensitivity with a further reduction factor of 2.5 in the background. 

This talk will present the basic design concept of the GERDA experiment and discuss its final results, and how it paves the way for LEGEND-200.