Measurement of the neutrino mixing angle θ₁₃ with the Double Chooz detector

The θ₁₃ parameter of the PMNS mixing matrix remained unknown until first hints and estimates by both Double Chooz and beam experiments in 2011. The Double Chooz reactor antineutrino experiment aims for a precise measurement of this parameter. Located at the Chooz nuclear power plant in France, it relies on a two identical detector measurement, canceling most of systematic uncertainties related to neutrino flux emission and detection. The near detector, located at a few hundred meters from the two reactor cores, aims to monitor the flux ν_e from the cores. The far detector, located at a distance of about one kilometer from the reactor cores near the expected first maximum of the oscillation, measures an energy dependent deficit in the electron antineutrino spectrum. Different approaches are used to extract θ₁₃: A combined rate and spectral shape analysis as well as a background-model-independent analysis based on reactor power variations. A unique feature of the Double Chooz experiment is that it was able to take data during a time period with both reactors off. This provides access to the background only measurement, allowing to crosscheck the background models used in the oscillation analysis. A new analysis based on 467.90 live days with 66.5 GW-ton-years of exposure with far detector only, leads to a value of sin²2θ₁₃ = 0:090^+0:032_-0.029. Data taking in the near detector has started end of 2014, enabling a significant reduction of both reactor and detector related systematics uncertainties in a near future. (paper)