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 νe 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 νe around 5 MeV relative to the prediction from a most commonly used model. A clear energy and baseline dependent disappearance of reactor νe is observed in the deficit of the observed number of νe. Based on the measured far-to-near ratio of prompt spectra, we obtain sin 2 2θ13 = 0. The reactor ν e disappearance has been firmly observed to determine the smallest neutrino mixing angle θ 13 [1-3]. All of the three mixing angles in the Pontecorvo-MakiNakagawa-Sakata matrix [4,5] have been measured to provide a comprehensive picture of neutrino transformation. The successful measurement of a rather large θ 13 value opens the possibility of searching for CP violation in the leptonic sector and determining the neutrino mass ordering. Appearance of ν e from an accelerator ν µ beam is also observed by the T2K [6] and NOνA [7] experiments.Using the ν e survival probability P [8], reactor experiments with a baseline distance of ∼1 km can determine the mixing angle θ 13 and an effective squared-massdifference ∆m where ∆ ij ≡ 1.267∆m 2 ij L/E, E is the ν e energy in MeV, and L is the distance between the reactor and detector in meters.The first measurement of θ 13 by RENO was based on the rate-only analysis of deficit found in ∼220 live days of data [1]. The oscillation frequency |∆m 2 ee | in the measurement was approximated by the measured value |∆m 2 31 | assuming the normal ordering in the ν µ disappearance [10]. In this Letter, we present a more precisely measured value of θ 13 and our first determination of |∆m 2 ee |, based on the rate, spectral and baseline information (rate+spectrum analysis) of reactor ν e disappearance using ∼500 live days of data. The Daya Bay collaboration has also reported spectral measurements [11].The RENO uses identical near and far ν e detectors located at 294 m and 1383 m, respectively, from the center of six reactor cores of the Hanbit (known as Yonggwang) Nulcear Power Plant. The far (near) detector is under a 450 m (120 m) of water equivalent overburden. Six pressurized water reactors, each with maximum thermal output of 2.8 GW th , are situated in a linear array spanning 1.3 km with equal spacings. The reactor flux-weighted baseline is 410.6 m for the near detector and 1445.7 m for the far detector.The reactor ν e is detected through the inverse beta decay (IBD) interaction, ν e + p → e + + n, with free protons in hydrocarbon liquid scintillator (LS) with 0.1% Gadolinium (Gd) as a target. The coincidence of a prompt positron signal and a mean time of ∼28 µs delayed signal from neutron capture by Gd (n-Gd) provides the distinctive IBD signatur...

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