Improved measurements of the neutrino mixing angle O13 with the double chooz detector

The Double Chooz experiment presents improved measurements of the neutrino mixing angle O13 using the data collected in 467.90 live days from a detector positioned at an average distance of 1050m from two reactor cores at the Chooz nuclear power plant. Several novel techniques have been developed to achieve significant reductions of the backgrounds and systematic uncertainties with respect to previous publications, whereas the efficiency of the (Formula Presented.) signal has increased. The value of O13 is measured to be sin22O13 = 0.090-0.029+0.032 from a fit to the observed energy spectrum. Deviations from the reactor (Formula Presented.) prediction observed above a prompt signal energy of 4MeV and possible explanations are also reported. A consistent value of O13 is obtained from a fit to the observed rate as a function of the reactor power independently of the spectrum shape and background estimation, demonstrating the robustness of the O13 measurement despite the observed distortion.