GERDA results and the future perspectives for the neutrinoless double beta decay search using 76Ge

The GERmanium Detector Array (GERDA) is a low background experiment at the Laboratori Nazionali del Gran Sasso (LNGS) of INFN designed to search for the rare neutrinoless double beta decay (0νββ) of 76Ge. In the first phase (Phase I) of the experiment, high purity germanium diodes were operated in a “bare” mode and immersed in liquid argon. The overall background level of 10−2cts/(keV⋅kg⋅yr) was a factor of ten better than those of its predecessors. No signal was found and a lower limit was set on the half-life for the 0νββ decay of 76Ge T0ν1/2>2.1×1025 yr (90% CL), while the corresponding median sensitivity was 2.4×1025 yr (90% CL). A second phase (Phase II) started at the end of 2015 after a major upgrade. Thanks to the increased detector mass and performance of the enriched germanium diodes and due to the introduction of liquid argon instrumentation techniques, it was possible to reduce the background down to 10−3cts/(keV⋅kg⋅yr). After analyzing 23.2 kg⋅yr of these new data no signal was seen. Combining these with the data from Phase I a stronger half-life limit of the 76Ge 0νββ decay was obtained: T0ν1/2>8.0×1025 yr (90% CL), reaching a sensitivity of 5.8×1025 yr (90% CL). Phase II will continue for the collection of an exposure of 100 kg⋅yr. If no signal is found by then the GERDA sensitivity will have reached 1.4×1026 yr for setting a 90% CL. limit. After the end of GERDA Phase II, the flagship experiment for the search of 0νββ decay of 76Ge will be LEGEND. LEGEND experiment is foreseen to deploy up to 1-ton of 76Ge. After ten years of data taking, it will reach a sensitivity beyond 1028 yr, and hence fully cover the inverted hierarchy region.