PROSPECT-II: Physics goals with an upgraded precision reactor oscillation and spectrum neutrino experiment

The Precision Reactor Oscillation and Spectrum Experiment (PROSPECT) has made a series of world-leading short-baseline (SBL) measurements of neutrinos from the highly enriched High Flux Isotope Reactor (HFIR) at Oak Ridge National Lab. In its first phase of operation, PROSPECT has produced the strongest limits on the existence of eV-scale sterile neutrinos in the high-mass range and the first modern measurement of the ²³⁵U neutrino energy spectrum. The detector was a pathfinder, demonstrating effective neutrino detection with signal-to-background greater than 1:1 without significant overburden. The collaboration is preparing an upgraded detector, PROSPECT-II, leveraging the successes of the first phase to increase robustness and extend sensitivity beyond that already achieved. In a proposed two-year deployment at HFIR, PROSPECT-II would substantially improve the measurement of the ²³⁵U antineutrino spectrum and make a measurement of the absolute ²³⁵U antineutrino flux allowing for detailed studies of discrepancies between theoretical models and experimental data.  The increased sensitivity of the SBL sterile neutrino sensitivity extends beyond the Reactor Antineutrino Anomaly and reaches into the range of importance for long-baseline neutrino oscillation experiments.  As a robust, movable detector, PROSPECT-II could also be deployed at a low-enriched reactor enabling the separate determination of ²³⁹Pu. PROSPECT-II is an opportunity for continued probes of neutrino physics utilizing nuclear reactors while training the next generation of neutrino physicists.