Measurement of the Antineutrino Spectrum from $^{235}$U Fission at HFIR with PROSPECT

This Letter reports the first measurement of the $^{235}$U antineutrino energy spectrum by PROSPECT, the Precision Reactor Oscillation and Spectrum experiment, operating 7.9m from the 85MW$_{\mathrm{th}}$ highly-enriched uranium (HEU) High Flux Isotope Reactor. With an exposure of 3426 MW-days, PROSPECT has detected 31678$\pm$304 (stat.) $\overline{\nu}_{e}$-induced inverse beta decays, the largest sample from HEU fission to date, 99% of which are attributed to $^{235}$U. When compared to model predictions, PROSPECT disfavors $^{235}$U as the sole source of the $\overline{\nu}_{e}$ energy spectrum discrepancy observed at nuclear power reactors at $\sim$3$\sigma$ C.L.