Observation of Electron-Antineutrino Disappearance at Daya Bay

The Daya Bay Reactor Neutrino Experiment has measured a nonzero value for the neutrino mixing angle ${\theta }_{13}$ with a significance of 5.2 standard deviations. Antineutrinos from six 2.9 $\mathrm{G}{\mathrm{W}}_{\mathrm{th}}$ reactors were detected in six antineutrino detectors deployed in two near (flux-weighted baseline 470 m and 576 m) and one far (1648 m) underground experimental halls. With a $43000\mathrm{ton}–\mathrm{G}{\mathrm{W}}_{\mathrm{th}}–\mathrm{day}$ live-time exposure in 55 days, 10 416 (80 376) electron-antineutrino candidates were detected at the far hall (near halls). The ratio of the observed to expected number of antineutrinos at the far hall is $R=0.940\pm 0.011(\mathrm{stat}.)\pm 0.004(\mathrm{syst}.)$. A rate-only analysis finds ${sin}^{2}2{\theta }_{13}=0.092\pm 0.016(\mathrm{stat}.)\pm 0.005(\mathrm{syst}.)$ in a three-neutrino framework.

Figure 1

Layout of the Daya Bay experiment. The dots represent reactors, labeled as D1, D2, L1, L2, L3, and L4. Six ADs, AD1–AD6, are installed in three EHs.

Figure 2

Schematic diagram of the Daya Bay detectors.

Figure 3

Daily average measured IBD rates per AD in the three experimental halls as a function of time. Data between the two vertical dashed lines were used in this analysis. The solid curves represent no-oscillation predictions based on reactor flux analyses and detector simulation for comparison. The predictions have been corrected with the best-fit normalization parameter in determining ${sin}^{2}2{\theta }_{13}$.

Figure 4

Ratio of measured versus expected signal in each detector, assuming no oscillation. The error bar is the uncorrelated uncertainty of each AD, including statistical, detector-related, and background-related uncertainties. The expected signal is corrected with the best-fit normalization parameter. Reactor and survey data were used to compute the flux-weighted average baselines. The oscillation survival probability at the best-fit value is given by the smooth curve. The AD4 and AD6 data points are displaced by $-30$ and $+30\mathrm{m}$ for visual clarity. The ${\chi }^2$ versus ${sin}^{2}2{\theta }_{13}$ is shown in the inset.

Figure 5

Top: Measured prompt-energy spectrum of the far hall (sum of three ADs) compared with the no-oscillation prediction from the measurements of the two near halls. Spectra were background subtracted. Uncertainties are statistical only. Bottom: The ratio of measured and predicted no-oscillation spectra. The solid curve is the best-fit solution with ${sin}^{2}2{\theta }_{13}=0.092$ obtained from the rate-only analysis. The dashed line is the no-oscillation prediction.