Pulse shape analysis in GERDA Phase II

The GERmanium Detector Array (GERDA) collaboration searched for neutrinoless double-beta decay in Ge-76 using isotopically enriched high purity germanium detectors at the Laboratori Nazionali del Gran Sasso of INFN. After Phase I (2011-2013), the experiment benefited from several upgrades, including an additional active veto based on LAr instrumentation and a significant increase of mass by point-contact germanium detectors that improved the halflife sensitivity of Phase II (2015-2019) by an order of magnitude. At the core of the background mitigation strategy, the analysis of the time profile of individual pulses provides a powerful topological discrimination of signal-like and background-like events. Data from regular Th-228 calibrations and physics data were both considered in the evaluation of the pulse shape discrimination performance. In this work, we describe the various methods applied to the data collected in GERDA Phase II corresponding to an exposure of 103.7 kg year. These methods suppress the background by a factor of about 5 in the region of interest around Q(beta beta) = 2039 keV, while preserving (81 +/- 3)% of the signal. In addition, an exhaustive list of parameters is provided which were used in the final data analysis.

Automated summary

The GERDA collaboration conducted a search for neutrinoless double-beta decay using high purity germanium detectors. Through upgrades and analysis of pulse time profiles, they were able to significantly reduce background and improve the sensitivity of the experiment. This work presents the methods used for data analysis and provides a list of parameters.