Final Spectral Shape and Sterile Neutrino Oscillation Analyses from the PROSPECT-I Experiment

The discovery of the neutrino stands as a pivotal milestone in the annals of modern physics. Following the initial detection of neutrinos, a diverse array of experiments employing both natural and artificial sources of neutrinos have played a crucial role in shedding light on the elusive nature of these particles, including their intriguing behavior, such as neutrino oscillations. PROSPECT is a reactor antineutrino experiment consisting of a 4-ton liquid scintillator antineutrino detector divided into an 11x14 array of optically separated segments. The detector was designed to probe the existence of sterile neutrino oscillations and precisely measure the antineutrino spectrum resulting from U-235 fission. Data was taken in 2018 and 2019 with a first-generation detector called PROSPECT-I located on the Earth’s surface roughly 7 m from the 85 MW, compact, highly-enriched High Flux Isotope Reactor at Oak Ridge National Laboratory. This dataset has already had a substantial impact by placing stringent limits on sterile neutrino oscillations at the eV scale, setting new direct limits on boosted dark matter models, providing a precision U-235 spectral measurement, and demonstrating unique neutrino detection capabilities. A new analysis of the PROSPECT-I data has been performed, incorporating innovative event selection tools, resulting in a multi-period dataset marked by a significant enhancement in statistical power and improved signal-to-background ratios. This work reports the conclusive findings regarding the measurement of the antineutrino energy spectrum from the fission of U-235, as well as the search for eV scale sterile neutrino oscillations using the final PROSPECT-I multi-period data set.