Joint measurement of the pure-235 U reactor antineutrino spectrum by STEREO and PROSPECT experiments

Abstract

STEREO and PROSPECT are two high-precision very-short-baseline experiments studying U-235 antineutrinos produced by highly-enriched nuclear fuel. Located at about 10 meters from reactor cores at the research facilities of Institut Laue-Langevin (Grenoble, France) and Oak Ridge National Laboratory (U.S.A.), respectively, they investigate the data-to-prediction distortions on the U-235 antineutrino spectrum in terms of normalization ("Reactor Antineutrino Anomaly") or shape ("5-MeV bump"). Here, we report a joint spectral analysis performed by the STEREO and PROSPECT collaborations. The two experimental energy spectra have been found to be compatible ($\chi^2/\mathrm{ndf}=24.1/21$), allowing a simultaneous unfolding of the prompt spectra into antineutrino energy, which provides a new reference spectrum for the U-235 isotope. When compared to the shape of the Huber model, an excess of events around 5-6 MeV is observed with 2.4$\sigma$ significance. This measurement also proves to be consistent and complementary with the results from experiments using low-enriched nuclear fuel, where several isotopes contribute to the antineutrino spectrum.