Physicists from the PROSPECT experiment have completed their analysis of data from the Oak Ridge compact nuclear reactor and found no evidence of electron antineutrinos becoming sterile. Furthermore, the scientists were able to rule out an entire range of parameters, including the point previously reported by the Neutrino-4 experiment as an indication of the existence of sterile neutrinos. The work was published in Physical Review Letters.
Sterile neutrinos are hypothetical particles that do not participate in the weak interaction but can theoretically temporarily transform into ordinary neutrinos and vice versa. This scenario would explain several anomalies in the data from various neutrino experiments—in particular, the reactor and gallium anomalies, which detected a neutrino deficit compared to predictions. Searching for these transformations at short distances from the source is one of the most direct ways to detect sterile states. Specifically, scientists from the Neutrino-4 experiment previously reported observing oscillations that are highly likely to indicate the presence of sterile neutrinos.
However, physicists from the PROSPECT experiment failed to observe such oscillations. They analyzed all data collected until 2018 at distances of 6.7–9.2 meters from the HFIR compact reactor in the United States using a detector filled with liquid scintillator doped with lithium-6. The scientists selected events of inverse beta decay—the interaction of an antineutrino with a proton. They applied a new data analysis technique that allowed them to utilize more data compared to previous work, including data from segments with partially operational photodetectors, and achieved high accuracy through more reliable separation of signals from background.
The analysis revealed no significant discrepancies with the oscillation-free model. Specifically, the point previously claimed by the Neutrino-4 experiment was excluded at a level above 5σ. The entire range of up to 10 electronvolts squared, which is allowed by the gallium anomaly, was also completely excluded. Thus, the PROSPECT results call into question the interpretation of these experiments in terms of sterile neutrinos.
For more information on sterile neutrinos and experiments searching for them, read our article "Pure Anomaly."
