Improved measurements of the neutrino mixing angle θ[subscript 13] with the Double Chooz detector

Abstract

The Double Chooz experiment presents improved measurements of the neutrino mixing angle θ[subscript 13] using the data collected in 467.90 live days from a detector positioned at an average distance of 1050 m 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 [bar over ν][subscript e] signal has increased. The value of θ[subscript 13] is measured to be sin[superscript 2]2θ[subscript 13] = 0.090[+ 0.032 over − 0.029] from a fit to the observed energy spectrum. Deviations from the reactor [bar over ν][subscript e] prediction observed above a prompt signal energy of 4 MeV and possible explanations are also reported. A consistent value of θ[subscript 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 θ[subscript 13] measurement despite the observed distortion.