GERDA. GERmanium Detector Array GERDA for the detection of neutrinoless double beta decay in 76Ge. Subproject 2. Final report

GERDA is the first experiment in the world to achieve a sensitivity for the half-life of neutrinoless double beta decay of >10${}^{26}$ years. The original objective of the experiment was significantly exceeded and a background index (pre-unblinding) of 1.2${}^{{+1.8}_{-0.8}}$ x 10${}^{-4}$ cts/(keV kg yr) was achieved. Averaged over Phase II, a background index of 3.6${}^{{+1.6}_{-1.2}}$ x 10${}^{-4}$ cts/(keV kg yr) was achieved, a factor of 3 lower than specified in the GERDA proposal. Thus, the target for the background of the follow-up experiment LEGEND-200 of <2 x 10${}^{-4}$ cts/(keV kg yr) has already been achieved. The final results (GERDA Legacy data) were analyzed in 2020 and presented to the scientific public at the 2020 International Neutrino Conference and subsequently published in Physical Review Letters 125, 252502 (2020). After the successful upgrade in 2018, the search for neutrinoless double beta decay with GERDA was continued as planned in July 2018. The measurement campaign was carried out with an extremely high duty cycle and was successfully completed after 481 days on November 11, 2019. On average, a daily exposure of 0.091 kg yr per day was achieved during this time, so that GERDA Phase II has already achieved a total of 103.7 kg yr exposure after less than 4 years. In detail, the enriched BEGe detectors with 53.3 kg yr, the enriched coaxial detectors with 41.8 kg yr and the new enriched inverted coaxial detectors with 8.5 kg yr contributed to this exposure. The excellent performance of the inverted coaxial detectors is of utmost importance, especially for the LEGEND-200 follow-up experiment.