Improved Measurement of the Evolution of the Reactor Antineutrino Flux and Spectrum at Daya Bay

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 239Pu 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 239Pu 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 235U is changed or the predicted 235U, 238U, 239Pu, and 241Pu spectra are changed in equal measure.