Results from Reno

Abstract

The Reactor Experiment for Neutrino Oscillation (RENO) has been taking data near the Hanbit nuclear power plant in South Korea, using two identical detectors since August 2011. The experiment made a definitive measurement of the smallest neutrino mixing angle $\theta_{13}$ in 2012, based on the disappearance of reactor electron antineutrinos. The RENO experiment has obtained more precise values of the mixing angle and the neutrino squared-mass-difference $|\Delta m_{ee}^2|$ from an energy and baseline dependent reactor neutrino disappearance using $\sim$1500 live days of data. Based on the ratio of inverse-beta-decay (IBD) prompt spectra measured in two identical far and near detectors, we obtain $\sin^2(2\theta_{13}) = 0.086 \pm 0.006(stat.) \pm 0.005(syst.)$ and $|\Delta m_{ee}^2| = [2.61^{+0.15}_{-0.16}(stat.) ^{+0.09}_{-0.09}(syst.)] \times 10^{-3}~eV^2$. An excess of reactor antineutrinos near 5 MeV is observed in the measured prompt spectrum with respect to the most commonly used models. The excess is found to be consistent with coming from reactors and show a weak correlation with the $^{235}$U fuel fraction. A precise value of $\theta_{13}$ would provide important information on determination of the leptonic CP phase if combined with a result of an accelerator neutrino beam experiment.