A measurement of electron antineutrino oscillation by the Daya Bay Reactor Neutrino Experiment is described in detail. Six 2.9-GWth nuclear power reactors of the Daya Bay and Ling Ao nuclear power facilities served as intense sources of ν e 's. Comparison of theν e rate and energy spectrum measured by antineutrino detectors far from the nuclear reactors (∼1500-1950 m) relative to detectors near the reactors (∼350-600 m) allowed a precise measurement ofν e disappearance. More than 2.5 millionν e inverse beta-decay interactions were observed, based on the combination of 217 days of operation of six antineutrino detectors (December, 2011-July, 2012) with a subsequent 1013 days using the complete configuration of eight detectors (October, 2012-July, 2015. Theν e rate observed at the far detectors relative to the near detectors showed a significant deficit, R ¼ 0.949 AE 0.002ðstatÞAE 0.002ðsystÞ. The energy dependence ofν e disappearance showed the distinct variation predicted by neutrino oscillation. Analysis using an approximation for the three-flavor oscillation probability yielded the flavor-mixing angle sin 2 2θ 13 ¼ 0.0841 AE 0.0027ðstatÞ AE 0.0019ðsystÞ and the effective neutrino mass-squared difference of jΔm 2 ee j ¼ ð2.50 AE 0.06ðstatÞ AE 0.06ðsystÞÞ × 10 −3 eV 2 . Analysis using the exact three-flavor probability found Δm

Reactor neutrino experiments play a crucial role in advancing our knowledge of neutrinos. A precise measurement of reactor electron antineutrino flux and spectrum evolution can be key inputs in improving the knowledge of neutrino mass and mixing as well as reactor nuclear physics and searching for physics beyond the standard model. In this work, the evolution of the flux and spectrum as a function of the reactor isotopic content is reported in terms of the inverse-beta-decay yield at Daya Bay with 1958 days of data and improved systematic uncertainties. These measurements are compared with two signature model predictions: the Huber-Mueller model based on the conversion method and the SM2018 model based on the summation method. The measured average flux and spectrum, as well as their evolution with the 239 Pu isotopic fraction, are inconsistent with the predictions of the Huber-Mueller model. In contrast, the SM2018 model is shown to agree with the average flux and its evolution but fails to describe the energy spectrum. Altering the predicted IBD spectrum from 239 Pu fission does not improve the agreement with the measurement for either model. The models can be brought into better agreement with the measurements if either the predicted spectrum due to 235 U fission is changed or the predicted 235 U, 238 U, 239 Pu, and 241 Pu spectra are changed in equal measure.

This Letter reports the first measurement of the oscillation amplitude and frequency of reactor antineutrinos at Daya Bay via neutron capture on hydrogen using 1958 days of data. With over 3.6 million signal candidates, an optimized candidate selection, improved treatment of backgrounds and efficiencies, refined energy calibration, and an energy response model for the capture-on-hydrogen sensitive region, the relative νe rates and energy spectra variation among the near and far detectors gives sin 2 2θ13 = 0.0759 +0.0050 −0.0049 and ∆m 2 32 = (2.72 +0.14 −0.15 )× 10 −3 eV 2 assuming the normal neutrino mass ordering, and ∆m 2 32 = (−2.83 +0.15 −0.14 )×10 −3 eV 2 for the inverted neutrino mass ordering. This estimate of sin 2 2θ13 is consistent with and essentially independent from the one obtained using the capture-on-gadolinium sample at Daya Bay. The combination of these two results yields sin 2 2θ13 = 0.0833 ± 0.0022, which represents an 8% relative improvement in precision regarding the Daya Bay full 3158-day capture-on-gadolinium result.

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