Search of Neutrinoless Double Beta Decay with the GERDA Experiment

Abstract The GERDA (GERmanium Detector Array) is an experiment for the search of neutrinoless double beta decay ( 0 ν β β ) in 76 Ge, 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 ⋅ 10 25  yr on the 0 ν β β 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 ⋅ 10 25 yr (90% C.L.) was established. Correspondingly, the limit on the effective Majorana neutrino mass is m e e 0.2 – 0.4  eV , depending on the considered nuclear matrix element. The previous claim for evidence of a 0 ν β β decay signal is strongly disfavored, and the field of research is open again.