Doctor of Philosophy / Department of Physics / Glenn Horton-Smith / Double Chooz is a reactor neutrino experiment which has shown evidence of electron anti-neutrino disappearance at 1 km distance. It has been able to exclude the no-oscillation hypothesis at 99.8% CL (2.9ς) with only one detector. From a rate plus spectral shape analysis, the value of sin²2θ₁₃ was found to be 0.109±0.030(stat) ± 0.025(syst). Correlated events mimicking an anti-neutrino event are one of the most important backgrounds for a reactor neutrino experiment like Double Chooz which measured the neutrino mixing angle θ₁₃. Cosmic muons passing through the rock surrounding the detector produce fast neutrons which give rise to correlated events through proton recoil followed by a neutron capture. Muons stopping around the chimney region subsequently decay into Michel electrons also contributing to the correlated background. Measurement of the shape and rate of this background is very important for the precise measurement of θ₁₃. Experimental techniques to estimate of the shape and rate of this background in the Double Chooz far detector are presented in this thesis.

For about 20 years now, observations of 6Li in several old metal-poor stars inside the halo of our galaxy have been reported, which are largely independent of the stars’ metallicity, and which point to a possible primordial origin. The observations exceed the predictions of the Standard Big-Bang Nucleosynthesis model by a factor of 500. In the relevant energy range, no directly measured S-factors were available yet for the main production reaction 2H(α,γ)6Li, while different theoretical estimations have an uncertainty of up to two orders of magnitude. The very small cross section in the picobarn range has been measured with a deuterium gas target at the LUNA acceler- ator (Laboratory for Underground Nuclear Astrophysics), located deep underground inside Laboratori Nazionali del Gran Sasso in Italy. A beam-induced, neutron-caused background in the γ-detector occurred which had to be analyzed carefully and sub- tracted in an appropriate way, to finally infer the weak signal of the reaction. For this purpose, a method to parameterize the Compton background has been developed. The results are a contribution to the discussion about the accuracy of the recent 6Li observations, and to the question if it is necessary to include new physics into the Standard Big-Bang Nucleosynthesis model.

For about 20 years now, observations of 6Li in several old metal-poor stars inside the halo of our galaxy have been reported, which are largely independent of the stars’ metallicity, and which point to a possible primordial origin. The observations exceed the predictions of the Standard Big-Bang Nucleosynthesis model by a factor of 500. In the relevant energy range, no directly measured S-factors were available yet for the main production reaction 2H(α,γ)6Li, while different theoretical estimations have an uncertainty of up to two orders of magnitude. The very small cross section in the picobarn range has been measured with a deuterium gas target at the LUNA acceler- ator (Laboratory for Underground Nuclear Astrophysics), located deep underground inside Laboratori Nazionali del Gran Sasso in Italy. A beam-induced, neutron-caused background in the γ-detector occurred which had to be analyzed carefully and sub- tracted in an appropriate way, to finally infer the weak signal of the reaction. For this purpose, a method to parameterize the Compton background has been developed. The results are a contribution to the discussion about the accuracy of the recent 6Li observations, and to the question if it is necessary to include new physics into the Standard Big-Bang Nucleosynthesis model.

La matière sombre compte pour 85% de la matière composant l’univers et nous ne savons toujours pas ce qu’elle est. Depuis plusieurs années, l’expérience Picasso, maintenant devenue l’expérience PICO, tente d’élucider ce mystère. Les fréons de la famille des CXFY sont utilisés comme cibles de choix dans les détecteurs à liquides surchauffés de l’expérience PICO. Situés à SNOLab, en Ontario, ces détecteurs font parties des plus performant de la recherche de la matière sombre. Lors d’interactions de particules avec le liquide en surchauffe, un changement de phase est induit par le dépôt d’énergie engendré par l’interaction. Les bulles créées par l’évènement sont alors détectées par différents capteurs afin de déterminer le type d’interaction qui a eu lieu. Dans ce travail seront présentés les détecteurs à liquides surchauffés dans le cadre de la recherche de la matière sombre. Principalement, nous y verrons trois types de détecteurs utilisés par les expériences PICO et Picasso. Le principe de fonctionnement de chacun des détecteurs sera exposé en premier lieu ainsi que leur fabrication, puis leur mode d’opération et l’analyse des données. Les méthodes de calibration seront par la suite expliquées pour terminer avec une description des résultats obtenus démontrant la performance de ce type de détection. / Dark matter makes up 85% of the matter content of the universe and we still don’t know what it is made of. The Picasso experiment, now named PICO, has been searching for it for several years with the use of superheated liquid detectors. Following the interaction of a particle with a superheated liquid freon of the CXFY family, a bubble is formed through a phase change and is detected with several types of sensors, telling us about the nature of the event. Located at SNOLab, in Ontario, these detectors produce some of the best results in the field. The present work will go through three types of superheated liquid detectors. A full description of the working principles will be presented for each of them. In addition, the fabrication, the operation mode and the data analysis will be shown. Detector calibration techniques will then be presented with different particle sources. Finally, the most recent results will be discussed, demonstrating the performance of the superheated liquid detector technique.