Precision Measurement of Reactor Antineutrino Oscillation at Kilometer-Scale Baselines by Daya Bay

We present a new determination of the smallest neutrino mixing angle ${\theta }_{13}$ and the mass-squared difference $\mathrm{\Delta }{m}_{32}^2$ using a final sample of $5.55\times {10}^6$ inverse beta-decay (IBD) candidates with the final-state neutron captured on gadolinium. This sample is selected from the complete dataset obtained by the Daya Bay reactor neutrino experiment in 3158 days of operation. Compared to the previous Daya Bay results, selection of IBD candidates has been optimized, energy calibration refined, and treatment of backgrounds further improved. The resulting oscillation parameters are ${\sin }^{2}2{\theta }_{13}=0.0851\pm 0.0024$, $\mathrm{\Delta }{m}_{32}^2=(2.466\pm 0.060)\times {10}^{-3}{\mathrm{eV}}^2$ for the normal mass ordering or $\mathrm{\Delta }{m}_{32}^2=-(2.571\pm 0.060)\times {10}^{-3}{\mathrm{eV}}^2$ for the inverted mass ordering.

Figure 1

Error ellipses in the $\mathrm{\Delta }{m}_{ee}^2\text{−}{\sin }^{2}2{\theta }_{13}$ space with the best-fit point indicated. The error bars display the one-dimensional 1-standard-deviation confidence intervals. The colored contours correspond to 1, 2, and 3 standard deviations. The $\mathrm{\Delta }{\chi }^2$ distributions are also shown. These one-dimensional distributions were obtained by determining the smallest $\mathrm{\Delta }{\chi }^2$ value after scanning through $\mathrm{\Delta }{m}_{ee}^2$ (${\sin }^{2}2{\theta }_{13}$) for a given ${\sin }^{2}2{\theta }_{13}$ ($\mathrm{\Delta }{m}_{ee}^2$).

Figure 2

The measured prompt-energy spectra of EH1, EH2, and EH3 with the best-fit and no-oscillation curves superimposed in the upper panels. The shape of the backgrounds are apparent in the spectra with a logarithmic ordinate shown in the insets. The backgrounds shown in the legend are in descending order according to their contribution. The lower panels shows the ratio of the observed spectrum to the predicted no-oscillation distribution. The error bars are statistical.

Figure 3

Measured disappearance probability as a function of the ratio of the effective baseline $L_{\mathrm{eff}}$ to the mean antineutrino energy $⟨E_{{\overline{\nu }}_e}⟩$.