First antineutrino energy spectrum from $^{235}$U fissions with the STEREO detector at ILL

These data support the measurement of the $^{235}$U-induced antineutrino spectrum shape by the STEREO experiment. 43 000 antineutrinos have been detected at about 10 m from the highly enriched core of the ILL reactor during 118 full days equivalent at nominal power. The measured inverse beta decay spectrum is unfolded to provide a pure $^{235}$U spectrum in antineutrino energy. A careful study of the unfolding procedure, including a cross-validation by an independent framework, has shown that no major biases are introduced by the method. A significant local distortion is found with respect to predictions around $E_\nu = 5.3$ MeV. A gaussian fit of this local excess leads to an amplitude of 12.1 $\pm$ 3.4% (3.5$\sigma$).