Improved short-baseline neutrino oscillation search and energy spectrum measurement with the PROSPECT experiment at HFIR

We present a detailed report on sterile neutrino oscillation and U-235 (nu) over bar (e) energy spectrum measurement results from the PROSPECT experiment at the highly enriched High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory. In 96 calendar days of data taken at an average baseline distance of 7.9 m from the center of the 85 MW HFIR core, the PROSPECT detector has observed more than 50,000 interactions of (nu) over bar (e) produced in beta decays of U-235 fission products. New limits on the oscillation of (nu) over bar (e) to light sterile neutrinos have been set by comparing the detected energy spectra of ten reactor-detector baselines between 6.7 and 9.2 meters. Measured differences in energy spectra between baselines show no statistically significant indication of (nu) over bar (e) to sterile neutrino oscillation and disfavor the reactor antineutrino anomaly best-fit point at the 2.5 sigma confidence level. The reported U-235 (nu) over bar (e) energy spectrum measurement shows excellent agreement with energy spectrum models generated via conversion of the measured U-235 beta spectrum, with a chi(2)/d.o.f . of 31/31. PROSPECT is able to disfavor at 2.4 sigma confidence level the hypothesis that U-235 (nu) over bar (e) are solely responsible for spectrum discrepancies between model and data obtained at commercial reactor cores. A data-model deviation in PROSPECT similar to that observed by commercial core experiments is preferred with respect to no observed deviation, at a 2.2 sigma confidence level.