First search for short-baseline neutrino oscillations at HFIR with PROSPECT

TL;DR

PROSPECT is the first experiment to observe antineutrinos from $^{235}$U at HFIR, providing new constraints on sterile neutrino oscillations with rapid detection capabilities and a reactor-model independent analysis.

Contribution

This work presents the first observation of reactor antineutrinos at HFIR and constrains sterile neutrino parameters using a novel segmented liquid scintillator detector.

Findings

Detected over 25,000 inverse beta decay events in 33 days

Achieved 5σ detection significance within two hours of data collection

Disfavored the Reactor Antineutrino Anomaly best fit at 2.2σ confidence level

Abstract

This Letter reports the first scientific results from the observation of antineutrinos emitted by fission products of $^{235}$U at the High Flux Isotope Reactor. PROSPECT, the Precision Reactor Oscillation and Spectrum Experiment, consists of a segmented 4 ton $^6$Li-doped liquid scintillator detector covering a baseline range of 7-9 m from the reactor and operating under less than 1 meter water equivalent overburden. Data collected during 33 live-days of reactor operation at a nominal power of 85 MW yields a detection of 25461 $\pm$ 283 (stat.) inverse beta decays. Observation of reactor antineutrinos can be achieved in PROSPECT at 5$\sigma$ statistical significance within two hours of on-surface reactor-on data-taking. A reactor-model independent analysis of the inverse beta decay prompt energy spectrum as a function of baseline constrains significant portions of the previously allowed sterile neutrino oscillation parameter space at 95% confidence level and disfavors the best fit of the Reactor Antineutrino Anomaly at 2.2$\sigma$ confidence level.