Physicists from the PROSPECT experiment have completed an analysis of data from the compact nuclear reactor in Oak Ridge and have found no evidence of electron antineutrinos turning sterile. In doing so, the scientists were able to exclude 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 could theoretically temporarily transform into ordinary neutrinos and vice versa. Such a scenario would explain several anomalies in the data from various neutrino experiments, including the reactor and gallium anomalies, which showed a deficit of neutrinos compared to predictions. Searching for these transformations at short distances from the source is one of the most direct ways to find sterile states. In particular, scientists from the Neutrino-4 experiment have previously reported that they have observed oscillations that are highly likely to indicate the presence of sterile neutrinos.
However, physicists from the PROSPECT experiment failed to see such oscillations. They analyzed all the data collected until 2018 at a distance of 6.7–9.2 meters from the HFIR compact reactor in the United States using a detector filled with liquid scintillator with added 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 use more data compared to the previous work, including from segments with partially operational photodetectors, and to achieve high accuracy due to more reliable separation of signals and background.
The analysis did not reveal any significant deviation from the oscillation-free model. In particular, the point previously claimed by Neutrino-4 was excluded at a level above 5σ. Also, the entire region up to 10 electronvolts squared, which is allowed by the gallium anomaly, was completely excluded. Thus, the PROSPECT results call into question the interpretation of these experiments in terms of sterile neutrinos.
For more information about sterile neutrinos and experiments to search for them, read our article “Pure Anomaly.”