Joint Measurement of the U 235 Antineutrino Spectrum by PROSPECT and STEREO

The PROSPECT and STEREO collaborations present a combined measurement of the pure U235 antineutrino spectrum, without site specific corrections or detector-dependent effects. The spectral measurements of the two highest precision experiments at research reactors are found to be compatible with χ2/ndf=24.1/21, allowing a joint unfolding of the prompt energy measurements into antineutrino energy. This ν¯e energy spectrum is provided to the community, and an excess of events relative to the Huber model is found in the 5-6 MeV region. When a Gaussian bump is fitted to the excess, the data-model χ2 value is improved, corresponding to a 2.4σ significance.