Joint Determination of Reactor Antineutrino Spectra from ${}^{235}$U and ${}^{239}$Pu Fission by Daya Bay and PROSPECT

A joint determination of the reactor antineutrino spectra resulting from the fission of ${}^{235}$U and ${}^{239}$Pu has been carried out by the Daya Bay and PROSPECT collaborations. This Letter reports the level of consistency of ${}^{235}$U spectrum measurements from the two experiments and presents new results from a joint analysis of both data sets. The measurements are found to be consistent. The combined analysis reduces the degeneracy between the dominant ${}^{235}$U and ${}^{239}$Pu isotopes and improves the uncertainty of the ${}^{235}$U spectral shape to about 3%. The ${}^{235}$U and ${}^{239}$Pu antineutrino energy spectra are unfolded from the jointly deconvolved reactor spectra using the Wiener-SVD unfolding method, providing a data-based reference for other reactor antineutrino experiments and other applications. This is the first measurement of the ${}^{235}$U and ${}^{239}$Pu spectra based on the combination of experiments at low- and highly enriched uranium reactors.