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Search for the neutrinoless double ß-decay in Gerda Phase I using a Pulse Shape Discrimination technique


Kirsch,  Andrea
Division Prof. Dr. Werner Hofmann, MPI for Nuclear Physics, Max Planck Society;

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Kirsch, A. (2014). Search for the neutrinoless double ß-decay in Gerda Phase I using a Pulse Shape Discrimination technique. PhD Thesis, Ruprecht-Karls-Universität, Heidelberg.

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The Germanium Detector Array (Gerda) experiment, located underground at the INFN Laboratori Nazionali del Gran Sasso (LNGS) in Italy, deploys high-purity germanium detectors to search for the neutrinoless double β-decay (0vββ) of 76Ge. An observation of this lepton number violating process, which is expected by many extensions of the Standard Model, would not only generate a fundamental shift in our understanding of particle physics, but also unambiguously prove the neutrino to have a non-vanishing Majorana mass component. A ifrst phase of data recording lasted from November 2011 to May 2013 - resulting in a total exposure (defined as the product of detector mass and measurement time) of 21.6 kg . yr. Within this thesis a thorough study of this data with special emphasis on the development and scrutiny of an active background suppression technique by means of a signal shape analysis has been performed. Among several investigated multivariate approaches, particularly a selection algorithm based on an artificial neural network is found to yield the best performance; i.a. the background index close to the Q-value of the 0vββ - decay could be suppressed by 45% to 1. 102 cts=(keV. kg.yr), while still retaining a considerably high signal survival fraction of (83 ± 3)% leading to a significant improvement of the experimental sensitivity. The efficiency is derived by a simulation and further validated by substantiated consistency checks availing themselves of measurements taken with different calibration sources and physics data. No signal is observed and a new lower limit of T0v½ (90% C.L.) > 2.2. 1025 yr for the half-life of neutrinoless double β-decay of 76Ge is established.