Measurement of Reactor Antineutrino Oscillation Amplitude and Frequency at RENO

The RENO experiment reports more precisely measured values of ${\theta }_{13}$ and $|\mathrm{\Delta }{m}_{ee}^2|$ using $\sim 2200$ live days of data. The amplitude and frequency of reactor electron antineutrino (${\overline{\nu }}_e$) oscillation are measured by comparing the prompt signal spectra obtained from two identical near and far detectors. In the period between August 2011 and February 2018, the far (near) detector observed 103 212 (850 666) ${\overline{\nu }}_e$ candidate events with a background fraction of 4.8% (2.0%). A clear energy and baseline dependent disappearance of reactor ${\overline{\nu }}_e$ is observed in the deficit of the measured number of ${\overline{\nu }}_e$. Based on the measured far-to-near ratio of prompt spectra, we obtain ${\sin }^{2}2{\theta }_{13}=0.0896\pm 0.0048(\mathrm{stat})\pm 0.0047(\mathrm{syst})$ and $|\mathrm{\Delta }{m}_{ee}^2|=[2.68\pm 0.12(\mathrm{stat})\pm 0.07(\mathrm{syst})]\times {10}^{-3}{\mathrm{eV}}^2$.

Figure 1

Spectral shape comparison of observed and expected IBD prompt events in the near and far detectors [13]. The observed spectra are obtained from subtracting the remaining background spectra as shown in the insets. The expected distributions are obtained from the best-fit oscillation results that are applied to the no-oscillation MC spectra. The deviation from the expectation near 5 MeV is larger than the uncertainty of an expected spectrum (shaded band) from the reactor antineutrino model [18, 19].

Figure 2

(Top panel) Comparison of the observed IBD prompt spectrum in the far detector (dots) with the no-oscillation prediction (blue shaded histogram) obtained from the measurement in the near detector. The prediction from the best-fit oscilation parameters is also shown (yellow shaded histogram). Both blue and yellow bands represent uncertainties. (Bottom panel) Ratio of IBD events measured in the far detector to the no-oscillation prediction (dots), and the ratio from the MC simulation with best-fit results folded in (shaded band). Errors include the statistical and background subtraction uncertainties.

Figure 3

Allowed regions of 68.3, 95.5, and 99.7% C.L. in the $|\mathrm{\Delta }{m}_{ee}^2|$ vs ${\sin }^{2}2{\theta }_{13}$ plane. The best-fit values are given by the black dot. The $\mathrm{\Delta }{\chi }^2$ distributions for (top panel) ${\sin }^{2}2{\theta }_{13}$ and (right panel) $|\mathrm{\Delta }{m}_{ee}^2|$ are also shown with a $1\sigma$ band. The rate-only result for ${\sin }^{2}2{\theta }_{13}$ is shown by the cross.

Figure 4

Measured reactor ${\overline{\nu }}_e$ survival probability in the far detector as a function of $L_{\mathrm{eff}}/E_{\nu }$ [13]. The curve is a predicted survival probability, obtained from the observed probability in the near detector, for the best-fit values of $|\mathrm{\Delta }{m}_{ee}^2|$ and ${\sin }^{2}2{\theta }_{13}$. The $L_{\mathrm{eff}}/E_{\nu }$ value of each data point is given by the average of the counts in each bin.

Figure 5

Comparison of experimental results on ${\sin }^{2}2{\theta }_{13}$ and $|\mathrm{\Delta }{m}_{32}^2|$ for normal (NH) and inverted (IH) mass hierarchies. The world average values and the experimental results of Daya Bay [22], Double Chooz [23], T2K [24], MINOS [25], and $\mathrm{NO}\nu \mathrm{A}$ [26] are taken from Ref. [21].