Background Studies for ν-bar_e Oscillations Measurement at the Double Chooz Experiment

Double Chooz is a reactor anti-neutrino experiment which measures the θ₁₃ mixing parameter in the neutrino oscillation phenomenon thanks to two identical underground detectors located at different distances from the two reactors of the Chooz nuclear power plant in the French Ardennes. The Far Detector started data taking in april 2011 and the Near Detector began operations in January 2015. This thesis presents an analysis of both the single- and two- detector phases. Neutrinos interact in Double Chooz through inverse β decay on protons, where a positron and a neutron are emitted, creating two coincident signals. Analyses were performed for each detector on independent samples using the neutron capture on Gadolinium or on Hydrogen. A combined analysis, using both neutron captures is also presented. Backgrounds to the neutrino sample originate from the ambient radioactivity and the abundance of atmospheric muons (cosmogenic (β-n) emitters, fast neutron showers, and muons stopping and decaying in-side the detector). This thesis focuses on developing techniques for measuring and reducing the Stopping Muon background. With colleagues from APC and MIT, a 60 litre neutron TPC was installed in both underground laboratories at Chooz. This low pressure He/CF₄-based time projection chamber, called DCTPC, measures the flux, energy spectrum and direction of fast neutrons in both locations, with different overburdens (150 and 300 meters water equivalent for the Near and Far Laboratories respectively). (author)