Abstract

The Double Chooz experiment has observed 8249 candidate electron antineutrino events in 227.93 live days with 33.71 GW-ton-years ($\mathrm{\text{reactor power}}\ifmmode\times\else\texttimes\fi{}\mathrm{\text{detector mass}}\ifmmode\times\else\texttimes\fi{}\mathrm{\text{live time}}$) exposure using a $10.3\text{ }\text{ }{\mathrm{m}}^{3}$ fiducial volume detector located at 1050 m from the reactor cores of the Chooz nuclear power plant in France. The expectation in case of ${\ensuremath{\theta}}_{13}=0$ is 8937 events. The deficit is interpreted as evidence of electron antineutrino disappearance. From a rate plus spectral shape analysis we find ${sin}^{2}2{\ensuremath{\theta}}_{13}=0.109\ifmmode\pm\else\textpm\fi{}0.030(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.025(\mathrm{syst})$. The data exclude the no-oscillation hypothesis at 99.8% CL ($2.9\ensuremath{\sigma}$).