Observation of Reactor Electron Antineutrinos Disappearance in the RENO Experiment

The RENO experiment has observed the disappearance of reactor electron antineutrinos, consistent with neutrino oscillations, with a significance of 4.9 standard deviations. Antineutrinos from six $2.8{\mathrm{GW}}_{\mathrm{th}}$ reactors at the Yonggwang Nuclear Power Plant in Korea, are detected by two identical detectors located at 294 and 1383 m, respectively, from the reactor array center. In the 229 d data-taking period between 11 August 2011 and 26 March 2012, the far (near) detector observed 17102 (154088) electron antineutrino candidate events with a background fraction of 5.5% (2.7%). The ratio of observed to expected numbers of antineutrinos in the far detector is $0.920\pm 0.009(\mathrm{stat})\pm 0.014(\mathrm{syst})$. From this deficit, we determine ${sin}^{2}2{\theta }_{13}=0.113\pm 0.013(\mathrm{stat})\pm 0.019(\mathrm{syst})$ based on a rate-only analysis.

Figure 1

A schematic view of a RENO detector. The near and far detectors are identical.

Figure 2

Measured daily-average rates of reactor neutrinos after background subtraction in the near and far detectors as a function of running time. The solid curves are the predicted rates for no oscillation.

Figure 3

The ${\chi }^2$ distribution as a function of ${sin}^{2}2{\theta }_{13}$. Bottom: Ratio of the measured reactor neutrino events relative to the expected with no oscillation. The curve represents the oscillation survival probability at the best-fit, as a function of the flux-weighted baselines.

Figure 4

Observed spectrum of the prompt signals in the far detector compared with the nonoscillation predictions from the measurements in the near detector. The backgrounds shown in the inset are subtracted for the far spectrum. The background fraction is 5.5% (2.7%) for far (near) detector. Errors are statistical uncertainties only. Bottom: The ratio of the measured spectrum of far detector to the nonoscillation prediction.