Measurement of neutrino mixing angle θ 13 and mass difference Δ m ee 2 from reactor antineutrino disappearance in the RENO experiment

摘要

RENO (Reactor Experiment for Neutrino Oscillation) made a definitive measurement of the smallest neutrino mixing angle θ 13 in 2012, based on the disappearance of reactor electron antineutrinos. The experiment has obtained a more precise value of the mixing angle and the first result on neutrino mass difference Δ m ee 2 from an energy and baseline dependent reactor neutrino disappearance using ～500 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 θ 13 ) = 0.082 ± 0.009 ( stat . ) ± 0.006 ( syst . ) and | Δ m ee 2 | = [ 2.62 ? 0.23 + 0.21 ( stat . ) ? 0.13 + 0.12 ( syst . ) ] × 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. A successful measurement of θ 13 is also made in an IBD event sample with a delayed signal of neutron capture on hydrogen. A precise value of θ 13 would provide important information on determination of the leptonic CP phase if combined with a result of an accelerator neutrino beam experiment.