The GERDA (GERmanium Detector Array) is an experiment for the search of neutrinoless double beta decay (0νββ) in 76Ge, located at Laboratori Nazionali del Gran Sasso of INFN (Italy). In the first phase of the experiment, a 90% confidence level (C.L.) sensitivity of 2.4⋅1025 yr on the 0νββ decay half-life was achieved with a 21.6 kg⋅yr exposure and an unprecedented background index in the region of interest of 10−2 counts/(keV⋅kg⋅yr). No excess of signal events was found, and an experimental lower limit on the half-life of 2.1 ⋅ 1025 yr (90% C.L.) was established. Correspondingly, the limit on the effective Majorana neutrino mass is mee<0.2–0.4 eV, depending on the considered nuclear matrix element. The previous claim for evidence of a 0νββ decay signal is strongly disfavored, and the field of research is open again.
GERDA Collaboration, Agostini, M., Allardt, M., Bakalyarov, A. M., Balata, M., Barabanov, I., … Zuzel, G. (2016). Search of Neutrinoless Double Beta Decay with the GERDA Experiment. Nuclear and Particle Physics Proceedings, 273, 1876–1882. https://doi.org/10.1016/j.nuclphysbps.2015.09.303