Improved measurements of the neutrino mixing angle θ13 with the Double Chooz detector

The Double Chooz experiment presents improved measurements of the neutrino mixing angle theta(13) 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 (nu) over bar (e) signal has increased. The value of theta(13) is measured to be sin(2)2 theta(13) = 0.090(-0.029)(+0.032) from a fit to the observed energy spectrum. Deviations from the reactor (nu) over bar (e) prediction observed above a prompt signal energy of 4 MeV and possible explanations are also reported. A consistent value of theta(13) 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 theta(13) measurement despite the observed distortion.