Pulse shape analysis in GERDA Phase II

TL;DR

This paper discusses pulse shape analysis techniques used in GERDA Phase II to discriminate between signal and background events, significantly reducing background noise while maintaining high signal efficiency in the search for neutrinoless double-beta decay.

Contribution

It introduces and evaluates pulse shape discrimination methods applied to GERDA Phase II data, enhancing background suppression in neutrinoless double-beta decay searches.

Findings

Background reduced by a factor of about 5 near Qββ

81±3% of signal preserved after discrimination

Methods applied to 103.7 kg·yr of data

Abstract

The GERmanium Detector Array (GERDA) collaboration searched for neutrinoless double-$\beta$ decay in $^{76}$Ge 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 half-life 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 $^{228}$Th 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$\cdot$yr. 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$\pm$3)% of the signal. In addition, an exhaustive list of parameters is provided which were used in the final data analysis.