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

Abstract

The Reactor Experiment for Neutrino Oscillation (RENO) began gathering data in August, 2011. The RENO is designed to measure the neutrino mixing angle Theta 13 and mass squared di fference

Delta m^2_ee

which is the electron neutrino weighted average mass square di fference

Delta m^2_31

and

Delta m^2_32

. The experimental site of RENO is the Hanbit nuclear power plant, which is 280 km away from the capital city of Koera, Seoul. There are 6 reactors in the Hanbit nuclear power plant, equally spaced in a single line. The Experiment used two identical detectors. The near detector (far detector) is located at 294 m (1384 m) from the cen- ter of this six reactor array. and the flux-weighted average distance between the reactors and the far (near) detector is 408.6 m (1444.0 m). The experimental has analyzed about 500 live days of data in the period between August 2011 and January 2013. In this period, the far (near) detector observed 31541 (290775) electron antineu- trino candidate events with a background fraction of 4.9% (2.8%). The measured prompt spectra has an excess of reactor e of approx- imately 5 MeV relative to the prediction from the most commonly used model. The excess is proportional to reactor thermal output power, therefore the excess is the neutrino signal coming from reac- tor cores and not a background. To aviod this problem, the data are analysed based on the measured far-to-near ratio of prompt spectra. A clear energy and baseline dependent disappearance of reactor e is observed in the de cit of the observed amount of e. From this de cit, we can determine sin^2 2theta_13 = 0.082 +- 0.009(stat.) +- 0.006(syst.) and

Delta m^2_ee

= 2.62+0.21-0.23(stat.) +0.12 - 0.13(syst.) (X10^-3eV^2) based on a rate and spectra analysis. The precise measurement of Theta_ 13 would be a important milestone in determination of the leptonic CP phase if combined with a result of an accelerator neutrino beam experiment.