Abstract
The Gerda experiment located at the Laboratori Nazionali del Gran Sasso of INFN searches for neutrinoless double beta (0νββ) decay of 76Ge using germanium diodes as source and detector. In Phase I of the experiment eight semi-coaxial and five BEGe type detectors have been deployed. The latter type is used in this field of research for the first time. All detectors are made from material with enriched 76Ge fraction. The experimental sensitivity can be improved by analyzing the pulse shape of the detector signals with the aim to reject background events. This paper documents the algorithms developed before the data of Phase I were unblinded. The double escape peak (DEP) and Compton edge events of 2.615 MeV γ rays from 208Tl decays as well as two-neutrino double beta (2νββ) decays of 76Ge are used as proxies for 0νββ decay. For BEGe detectors the chosen selection is based on a single pulse shape parameter. It accepts 0.92±0.02 of signal-like events while about 80 % of the background events at Q ββ =2039 keV are rejected. For semi-coaxial detectors three analyses are developed. The one based on an artificial neural network is used for the search of 0νββ decay. It retains 90 % of DEP events and rejects about half of the events around Q ββ . The 2νββ events have an efficiency of 0.85±0.02 and the one for 0νββ decays is estimated to be $0.90^{+0.05}_{-0.09}$ . A second analysis uses a likelihood approach trained on Compton edge events. The third approach uses two pulse shape parameters. The latter two methods confirm the classification of the neural network since about 90 % of the data events rejected by the neural network are also removed by both of them. In general, the selection efficiency extracted from DEP events agrees well with those determined from Compton edge events or from 2νββ decays.
Highlights
The GERDA (GERmanium Detector Array) experiment searches for neutrinoless double beta decay (0νββ decay) of 76Ge
The GERDA experiment located at the Laboratori Nazionali del Gran Sasso of Istituto Nazionale di Fisica Nucleare (INFN) searches for neutrinoless double beta (0νββ) decay of 76Ge using germanium diodes as source and detector
The effect of the PSD selection on the physics data is typically always compared in the energy interval 1930– 2190 keV which is used for the 0νββ analysis [1]
Summary
The GERDA (GERmanium Detector Array) experiment searches for neutrinoless double beta decay (0νββ decay) of 76Ge. Large efforts went into the selection of radio pure materials surrounding the detectors. The latter are mounted in low mass holders made from screened copper and PTFE and are operated in liquid argon which serves as cooling medium and as a shield against external backgrounds. The effect of the PSD selection on the physics data is typically always compared in the energy interval 1930– 2190 keV which is used for the 0νββ analysis [1]. Events with an energy deposition in the window Qββ ± 5 keV (Qββ ± 4 keV) were hidden for the semi-coaxial (BEGe) detectors and were analyzed after all selections and calibrations had been finalized. This article presents the pulse shape analysis for GERDA Phase I developed in advance of the data unblinding
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