The GERDA experiment searches for the neutrinoless double beta (0𝜈𝛽𝛽) decay of Ge-76. The observation of this decay would prove the Majorana character of the neutrino, i.e. that it is its own antiparticle. This would clarify the question which neutrino mass ordering is realized in nature and give a hint of the effective Majorana neutrino mass. Furthermore, the existence of the 0𝜈𝛽𝛽 decay would imply the violation of lepton number conservation which is a key feature in some theories explaining the asymmetry of matter and antimatter in the universe. The effective Majorana neutrino mass is connected with the half life of the 0𝜈𝛽𝛽 decay via a nuclear matrix element (NME), which is predicted by various theoretical models that are afflicted by large uncertainties. The accuracy of the different NMEs and their internal model assumptions can be increased by considering experimental investigations. While the NMEs for the 0𝜈𝛽𝛽 decay and the neutrino accompanied double beta (2𝜈𝛽𝛽) decay are numerically different, they rely on similar model assumptions. Thus, experimental constraints can be given by the 2𝜈𝛽𝛽 decay into the ground state, which has been already measured by GERDA with unprecedented precision for Ge-76, but also by the investigation of the 2𝜈𝛽𝛽 decay into excited states, which has not yet been observed for Ge-76. GERDA operates enriched germanium detectors in liquid argon (LAr) which serves as an additional background veto using the scintillation light that is created when energy is deposited in LAr. The signal signature of the decay into excited states can be enhanced with the application of the LAr veto, however, for that the efficiency of the LAr veto needs to be determined. One of the key parameters of the LAr efficiency is the attenuation of the scintillation light in LAr, which is dependent on the impurity composition and concentration in LAr. Therefore, the attenuation length of the scintillation light in LAr has been measured in GERDA with a dedicated setup in the course of this work. The analysis of the acquired data required intense computer simulations in order to describe the background for the measurement sufficiently. This also involved the measurement of the steel reflectivity in the visible and the UV region, where LAr scintillates. Therewith, the search for excited states has been performed in this work for the data accumulated in GERDA Phase I, Phase II and Phase II+ including the LAr veto for the latter two data sets. New limits have been set on the investigated excited states decay modes and some of the corresponding theoretical half life predictions could be disfavored, i.e. the underlying NMEs models can be constrained. The successor experiment LEGEND will continue searching for the 0𝜈𝛽𝛽 decay of Ge-76 using more germanium detectors together with an improved LAr veto. The investigation of the decay of Ge-76 into excited states will also be further pursued in LEGEND.

Le projet DUNE est un projet d’expérience d’oscillations de neutrinos sur faisceau longue-distance, dédié en particulier à la détermination de la hiérarchie de masse des neutrinos et à la mesure de la phase de violation CP intervenant dans le mécanisme d’oscillations. Ce projet consiste en un faisceau intense de neutrinos de 1300 km et un détecteur massif contenant plus de 40 kilotonnes d’argon liquide, basé sur la technologie des chambres à dérive à argon liquide (LArTPC). Deux approches de cette technologie sont actuellement en développement, menant à l'installation au CERN de deux prototypes dont la construction devrait s'achever fin 2018. Le travail présenté dans cette thèse s’intègre dans le cadre du projet ProtoDUNE-DP, qui vise à prouver la faisabilité de la technologie dite « double-phase », c’est-à-dire utilisant de l'argon liquide et gazeux, pour les LArTPC de cette envergure. Deux signaux principaux sont attendus dans un tel détecteur, un signal de charges et un signal lumineux de scintillation. Le signal lumineux peut être utilisé dans le système de déclenchement d'acquisition des données, dans l’identification et éventuellement la réjection du signal dû aux muons cosmiques, et pour des mesures calorimétriques de précision. Des simulations préalables de ce signal sont donc nécessaires afin d'en comprendre les particularités et de développer des algorithmes d'identification. Cette thèse porte en particulier sur le développement de cette simulation et sur l’étude de la propagation des photons de scintillation au sein du détecteur. Les différents mécanismes de production de la lumière de scintillation, la simulation développée au cours de cette thèse et les études réalisées sur la propagation de la lumière de scintillation dans ProtoDUNE-DP seront présentés. Ces simulations ont également pu être comparées aux données recueillies avec un pré-prototype double-phase installé au CERN en 2017, afin de tester la validité de la simulation et d’en améliorer les différents paramètres. / DUNE is a future long-baseline neutrino experiment designed to determine, among others, the neutrino mass hierarchy and to measure the CP violation phase that enters the neutrino oscillation process. This project is based on a 1300 km long high intensity neutrino beam and a massive detector containing more than 40 kilotons of liquid argon using the liquid argon time projection chamber technology (LArTPC). Two approaches of this technology are currently under development, leading to the construction of two prototypes to be in place at the end of 2018 at CERN. The work of this thesis is part of the ProtoDUNE-DP project, which aims at probing the capabilities of the so-called “dual-phase” technology, that uses both gaseous and liquid argon, for a large-scale detector. Two kind of signals, a charge signal and a scintillation light signal, are expected in a LArTPC. The light signal can be used as a trigger, for the identification and rejection of the cosmic background, and for precise calorimetric measurements. Prior simulations of this signal are needed in order to improve our understanding of the scintillation light signal and to develop the identification algorithms. This work addresses the development of this simulation and the study of the scintillation photon behavior in the liquid argon detector. The different scintillation light production mechanisms, the developed simulation and the different studies on the light propagation in ProtoDUNE-DP are presented. These simulations have also been compared with light data taken at CERN in 2017 with a first demonstrator, in order to validate and tune the simulation.