Double Chooz: Latest results

Abstract

The latest results from the Double Chooz experiment on the neutrino mixing angle $\theta_{13}$ are presented. A detector located at an average distance of 1050 m from the two reactor cores of the Chooz nuclear power plant has accumulated a live time of 467.90 days, corresponding to an exposure of 66.5 GW-ton-year (reactor power $\times$ detector mass $\times$ live time). A revised analysis has boosted the signal efficiency and reduced the backgrounds and systematic uncertainties compared to previous publications, paving the way for the two detector phase. The measured $\sin^2 2\theta_{13} = 0.090^{+0.032}_{-0.029}$ is extracted from a fit to the energy spectrum. A deviation from the prediction above a visible energy of 4 MeV is found, being consistent with an unaccounted reactor flux effect, which does not affect the $\theta_{13}$ result. A consistent value of $\theta_{13}$ is measured in a rate-only fit to the number of observed candidates as a function of the reactor power, confirming the robustness of the result.