Observation of Energy and Baseline Dependent Reactor Antineutrino Disappearance in the RENO Experiment

Abstract

The RENO experiment has analyzed about 500 live days of data to observe an energy dependent disappearance of reactor $\overline{\nu}_e$ by comparison of their prompt signal spectra measured in two identical near and far detectors. In the period between August 2011 and January 2013, the far (near) detector observed 31541 (290775) electron antineutrino candidate events with a background fraction of 4.9\% (2.8\%). The measured prompt spectra show an excess of reactor $\overline{\nu}_e$ around 5 MeV relative to the prediction from a most commonly used model. A clear energy and baseline dependent disappearance of reactor $\overline{\nu}_e$ is observed in the deficit of the observed number of $\overline{\nu}_e$. Based on the measured far-to-near ratio of prompt spectra, we obtain $\sin^2 2 \theta_{13} = 0.082 \pm 0.009({\rm stat.}) \pm 0.006({\rm syst.})$ and $|\Delta m_{ee}^2| =[2.62_{-0.23}^{+0.21}({\rm stat.}) _{-0.13}^{+0.12}({\rm syst.})]\times 10^{-3}$~eV$^2$.