Pulse shape discrimination for Gerda Phase I data.

The Gerda experiment located at the Laboratori Nazionali del Gran Sasso of INFN searches for neutrinoless double beta (0 νββ) 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 (2 νββ) decays of ⁷⁶Ge are used as proxies for 0 νββ 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 0 νββ decay. It retains 90 % of DEP events and rejects about half of the events around Q_ββ. The 2 νββ events have an efficiency of 0.85±0.02 and the one for 0 νββ 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 2 νββ decays. [ABSTRACT FROM AUTHOR]