Indication of Reactor ν¯e Disappearance in the Double Chooz Experiment

The Double Chooz experiment presents an indication of reactor electron antineutrino disappearance consistent with neutrino oscillations. An observed-to-predicted ratio of events of 0.944±0.016(stat)±0.040(syst) was obtained in 101 days of running at the Chooz nuclear power plant in France, with two 4.25GW_th reactors. The results were obtained from a single 10m³ fiducial volume detector located 1050 m from the two reactor cores. The reactor antineutrino flux prediction used the Bugey4 flux measurement after correction for differences in core composition. The deficit can be interpreted as an indication of a nonzero value of the still unmeasured neutrino mixing parameter sin⁡²2θ₁₃. Analyzing both the rate of the prompt positrons and their energy spectrum, we find sin⁡²2θ₁₃=0.086±0.041(stat)±0.030(syst), or, at 90% C.L., 0.017<sin⁡²2θ₁₃<0.16.