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 extraction of individual antineutrino spectra from 235 U and 239 Pu fission and an improved measurement of the prompt energy spectrum of reactor antineutrinos at Daya Bay. The analysis uses 3.5 × 10 6 inverse beta-decay candidates in four near antineutrino detectors in 1958 days. The individual antineutrino spectra of the two dominant isotopes, 235 U and 239 Pu, are extracted using the evolution of the prompt spectrum as a function of the isotope fission fractions. In the energy window of 4-6 MeV, a 7% (9%) excess of events is observed for the 235 U (239 Pu) spectrum compared with the normalized Huber-Mueller model prediction. The significance of discrepancy is 4.0σ for 235 U spectral shape compared with the Huber-Mueller model prediction. The shape of the measured inverse beta-decay prompt energy spectrum disagrees with the prediction of the Huber-Mueller model at 5.3σ. In the energy range of 4-6 MeV, a maximal local discrepancy of 6.3σ is observed.

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Searches for a light sterile neutrino have been performed independently by the MINOS and the Daya Bay experiments using the muon (anti)neutrino and electron antineutrino disappearance channels, respectively. In this Letter, results from both experiments are combined with those from the Bugey-3 reactor neutrino experiment to constrain oscillations into light sterile neutrinos. The three experiments are sensitive to complementary regions of parameter space, enabling the combined analysis to probe regions allowed by the Liquid Scintillator Neutrino Detector (LSND) and MiniBooNE experiments in a minimally extended four-neutrino flavor framework. Stringent limits on sin 2 2θµe are set over 6 orders of magnitude in the sterile mass-squared splitting ∆m 2 41 . The sterile-neutrino mixing phase space allowed by the LSND and MiniBooNE experiments is excluded for ∆m 2 41 < 0.8 eV 2 at 95% CLs.