Measurement of the Reactor Antineutrino Flux and Spectrum at Daya Bay

This Letter reports a measurement of the flux and energy spectrum of electron antineutrinos from six 2.9 ${\mathrm{GW}}_{\mathrm{t}\mathrm{h}}$ nuclear reactors with six detectors deployed in two near (effective baselines 512 and 561 m) and one far (1579 m) underground experimental halls in the Daya Bay experiment. Using 217 days of data, 296 721 and 41 589 inverse $\beta$ decay (IBD) candidates were detected in the near and far halls, respectively. The measured IBD yield is ($1.55\pm 0.04$) $\times {10}^{-18}{\mathrm{cm}}^2{\mathrm{GW}}^{-1}{\text{day}}^{-1}$ or ($5.92\pm 0.14$) $\times {10}^{-43}{\mathrm{cm}}^2{\text{fission}}^{-1}$. This flux measurement is consistent with previous short-baseline reactor antineutrino experiments and is $0.946\pm 0.022$ ($0.991\pm 0.023$) relative to the flux predicted with the $\mathrm{Huber}\text{−}\mathrm{Mueller}$ ($\mathrm{ILL}\text{-Vogel}$) fissile antineutrino model. The measured IBD positron energy spectrum deviates from both spectral predictions by more than $2\sigma$ over the full energy range with a local significance of up to $\sim 4\sigma$ between 4–6 MeV. A reactor antineutrino spectrum of IBD reactions is extracted from the measured positron energy spectrum for model-independent predictions.

Figure 1

Top: Rate of reactor antineutrino candidate events in the six ADs with corrections for three-flavor oscillations (closed circles), and additionally for the variation of flux-weighted fission fractions at the different sites (open squares). The average of the three near detectors is shown as a gray line (and extended through the three far detectors as a dotted gray line) with its $1\sigma$ systematic uncertainty (gray band). The rate predicted with the Huber-Mueller (ILL-Vogel) model and its uncertainty are shown in blue (orange). Bottom: The measured reactor ${\overline{\nu }}_e$ rate as a function of the distance from the reactor, normalized to the theoretical prediction with the Huber-Mueller model. The rate is corrected for three-flavor neutrino oscillations at each baseline. The blue shaded region represents the global average and its $1\sigma$ uncertainty. The 2.7% model uncertainty is shown as a band around unity. Measurements at the same baseline are combined for clarity. The Daya Bay measurement is shown at the flux-weighted baseline (573 m) of the two near halls.

Figure 2

Top panel: Predicted and measured prompt-energy spectra. The prediction is based on the Huber-Mueller model and normalized to the number of measured events. The highest energy bin contains all events above 7 MeV. The gray hatched and red filled bands represent the square root of diagonal elements of the shape-only covariance matrix ($\sqrt{V_{ii}}$) for the reactor related and the full (reactor, detector, and background) systematic uncertainties, respectively. The error bars on the data represent the statistical uncertainty. Middle panel: Ratio of the measured prompt-energy spectrum to the predicted spectrum (Huber-Mueller model). The blue curve shows the ratio of the prediction based on the ILL-Vogel model to that based on the Huber-Mueller model. Bottom panel: The defined ${\chi }^2$ distribution ($\widetilde{{\chi }_i}$) of each bin (black dashed curve) and local $p$ values for 1-MeV energy windows (magenta solid curve). See the text for the definitions of these quantities.

Figure 3

Top panel: The extracted reactor antineutrino spectrum and its correlation matrix. Bottom panel: Ratio of the extracted reactor antineutrino spectrum to the Huber-Mueller prediction. The error bars of the data points are the square roots of the diagonal elements of the data covariance matrix, which included statistical and systematic uncertainties, as well as bias and the AD-correlated uncertainty from Table 1. The solid red band represents the square roots of the diagonal elements of the prediction covariance matrix, which included reactor and Huber-Mueller model uncertainties. The horizontal dashed line represents the normalization factor $R=0.946$. The vertical dashed line denotes that above 8 MeV, the Huber-Mueller model was extrapolated.