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The Daya Bay experiment has observed correlations between reactor core fuelevolution and changes in the reactor antineutrino flux and energy spectrum.Four antineutrino detectors in two experimental halls were used to identify 2.2million inverse beta decays (IBDs) over 1230 days spanning multiple fuel cyclesfor each of six 2.9 GW$_{\textrm{th}}$ reactor cores at the Daya Bay and LingAo nuclear power plants. Using detector data spanning effective $^{239}$Pufission fractions, $F_{239}$, from 0.25 to 0.35, Daya Bay measures an averageIBD yield, $\bar{\sigma}_f$, of $(5.90 \pm 0.13) \times 10^{-43}$cm$^2$/fission and a fuel-dependent variation in the IBD yield,$d\sigma_f/dF_{239}$, of $(-1.86 \pm 0.18) \times 10^{-43}$ cm$^2$/fission.This observation rejects the hypothesis of a constant antineutrino flux as afunction of the $^{239}$Pu fission fraction at 10 standard deviations. Thevariation in IBD yield was found to be energy-dependent, rejecting thehypothesis of a constant antineutrino energy spectrum at 5.1 standarddeviations. While measurements of the evolution in the IBD spectrum showgeneral agreement with predictions from recent reactor models, the measuredevolution in total IBD yield disagrees with recent predictions at 3.1$\sigma$.This discrepancy indicates that an overall deficit in measured flux withrespect to predictions does not result from equal fractional deficits from theprimary fission isotopes $^{235}$U, $^{239}$Pu, $^{238}$U, and $^{241}$Pu.Based on measured IBD yield variations, yields of $(6.17 \pm 0.17)$ and $(4.27\pm 0.26) \times 10^{-43}$ cm$^2$/fission have been determined for the twodominant fission parent isotopes $^{235}$U and $^{239}$Pu. A 7.8% discrepancybetween the observed and predicted $^{235}$U yield suggests that this isotopemay be the primary contributor to the reactor antineutrino anomaly.

Evolution of the Reactor Antineutrino Flux and Spectrum at Daya Bay