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 ($0\nu \beta \beta$) 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 $2.4\cdot {10}^{25}\text{ yr}$ on the $0\nu \beta \beta$ 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 $m_{ee}<0.2\text{–}0.4\text{ eV}$, depending on the considered nuclear matrix element. The previous claim for evidence of a $0\nu \beta \beta$ decay signal is strongly disfavored, and the field of research is open again.