Dissertation Defense: Danielle Norcini, Yale University, “First search for eV-scale sterile neutrinos and precision measurement of the U-235 antineutrino spectrum with the PROSPECT experiment”

Event time: 
Friday, May 31, 2019 - 10:00am to 11:00am
Wright Lab (WNSL), 216 See map
272 Whitney Avenue
New Haven, CT 06511
Event description: 

Reactor experiments have been devoted to establishing the properties of the weakly-interacting neutrino. Recent neutrino oscillation experiments at low-enriched uranium (LEU) reactors suggest a disagreement between the observed electron antineutrino flux and energy spectrum when compared to leading model predictions. The ~6% flux deficit, known as the Reactor Antineutrino Anomaly, measured by detectors with baselines <500 m can be explained by the addition of a beyond-the-Standard-Model particle, an eV-scale sterile neutrino. This new type of matter would have a profound impact on fundamental physics and cosmology. The spectral deviations may be attributed to an incomplete understanding of antineutrino emission from fissile material. However, the insufficiencies in the nuclear models and the specific isotopic contributions are not yet clear.

PROSPECT is a newly constructed, short-baseline experiment observing antineutrinos from the 85 MWth highly-enriched uranium High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory. Positioned 7-9 m from the HFIR core, the main science goals are to unambiguously search for eV-scale sterile neutrino oscillations and to precisely measure the antineutrino emission from U-235 fission. PROSPECT uses the inverse beta decay interaction to identify neutrino events, but must do so in a near-surface environment with harsh reactor-related and cosmogenically-induced backgrounds. To overcome this challenge, PROSPECT developed a novel, compact 6Li-loaded scintillator detector with efficient neutron tagging and particle identification capabilities. These features, combined with event topology reconstruction made possible by optical segmentation, allow an unprecedented level of background suppression to observe antineutrinos with high energy resolution within a few hours of reactor-on operation.

This thesis details the detector design, instrumentation development, and construction of the PROSPECT experiment at the Yale Wright Laboratory from 2014–2018. The detector calibration, event reconstruction, and performance are also discussed. With 33 (28) days of reactor-on data at HFIR, the first search for eV-scale sterile neutrinos is presented. No sterile neutrino oscillations are observed within the sensitive (Δm41^2, sin^22ϴ14) parameter space and PROSPECT excludes the best-fit sterile neutrino hypothesis to the Reactor Antineutrino Anomaly at 2.2σ confidence level. Using a slightly larger data set containing >31,000 antineutrino candidates, PROSPECT has produced the world-leading measurement of the U-235 spectrum with a signal-to-background of 1.7 and 5%/√E(MeV) energy resolution. The experimental data shows discrepancies with the leading Huber reactor model, with further tests disfavoring U-235 as the sole isotope responsible for the spectral deviations observed by LEU experiments at 2.1σ confidence level.

Thesis Advisor: Karsten Heeger (karsten.heeger@yale.edu)