German Title: Pulsformanalyse im GERDA-Experiment zur Bestimmung einer neuen Untergrenze der Halbwertszeit des neutrinolosen Doppelbetazerfalls von 76 Ge

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Abstract

The GERDA experiment searches for neutrinoless double beta (0nbb) decay of Ge-76 using high purity germanium (HPGe) detectors operated in liquid argon (LAr). The aim is to explore half-lives of the order of 10^26 yr. Therefore, GERDA relies on improved active background reduction techniques such as pulse shape discrimination (PSD) in which the time structure of the germanium signals is analyzed to discriminate signal- from background-like events. Two types of HPGe detectors are operated: semi-coaxial detectors previously used in the Heidelberg-Moscow and IGEX experiments and new Broad Energy Germanium (BEGe) detectors which feature an improved energy resolution and enhanced PSD. In Phase I of the experiment, five enriched BEGe detectors were used for the first time in the search for 0nbb decay. A PSD based on a single parameter, the ratio of the maximum current amplitude over the energy A/E is applied. 83% of the background events in a 232 keV region around Qbb are rejected with a high signal efficiency of (92.1 +/- 1.9) %. The achieved background index (BI) is (5.4 +4.1 -3.4) * 10^-3 counts/(keV*kg*yr). This is an improvement by a factor of 10 compared to previous germanium based 0nbb experiments. Phase II of the experiment includes a major upgrade: for further background rejection, the LAr cryostat is instrumented to detect argon scintillation light. Additional 25 BEGe detectors are installed. After PSD and LAr veto a BI of (0.7 +1.3 -0.5) * 10^-3 counts/(keV*kg*yr) is achieved. This is the best BI achieved in 0nbb experiments so far. A frequentist statistical analysis is performed on the combined data collected in GERDA Phase I and the first Phase II release. A new limit on the half-life of 0nbb decay of Ge-76 is set to T_1/2 > 5.3 * 10^25 yr at 90% C.L., with a median sensitivity of T_1/2 > 4.0 * 10^25 yr at 90%C.L.