Searching for Sterile Neutrinos

In an effort to solve several fundamental mysteries about the cosmos, physicists have been searching for more than two decades for a proposed elementary particle known as the sterile neutrino. The existence of this hypothetical particle could explain why the universe contains more matter than antimatter. It might also be a source of dark matter, the invisible material that accounts for most of the mass in the universe.

Researchers at the National Institute of Standards and Technology (NIST) brought their expertise in studying another elementary particle, the neutron, when they joined the hunt for sterile neutrinos in 2012. NIST developed high-energy neutron detectors to monitor exposure to nuclear radiation and safeguard personnel at medical accelerators, the crew on commercial airlines, and spaceflight travelers. These detectors are strikingly similar to those needed for neutrino studies.

In a study published April 14 in Physical Review Letters, Hans Pieter Mumm of NIST and a large team of collaborators from academia and government laboratories have set stringent new limits on the existence and mass of sterile neutrinos. While the scientists have yet to find the particles, they have now expanded the range of energies and masses for which the particle cannot exist.

The researchers analyzed data from the PROSPECT detector, stationed just outside the High Flux Isotope Reactor at Oak Ridge National Laboratory in Tennessee. Nuclear reactors produce enormous numbers of antineutrinos, the antiparticles of neutrinos.

Neutrinos and their antiparticles rarely interact with matter and come in three known “flavors”—the tau, electron, and muon neutrino. Curiously, each flavor can morph into any other as the neutrinos travel through space. The sterile neutrino would be a new particle that would also participate in the transformation.

Sterile neutrinos can’t be observed directly because they interact with matter even more rarely than the three known types. But their existence would result in changes in the number of anti-electron neutrinos recorded by PROSPECT.

That’s because some of the electron antineutrinos generated by the Oak Ridge nuclear reactor would temporarily change, or “oscillate,” into unobservable sterile neutrinos before reaching PROSPECT, effectively vanishing into thin air and then reappearing again. The number of times an electron anti-neutrino would oscillate into a sterile neutrino and transform back depends on two main factors: The detector’s distance from the nuclear reactor and the difference in the mass squared between the electron anti-neutrino and the proposed sterile neutrino.

The researchers saw no evidence for such oscillations when they recorded the number of electron anti-neutrinos at six different distances from the reactor, ranging from 7 meters to 9 meters, and at several energies.

PROSPECT’s results rule out the existence of sterile neutrinos over a range of masses that other experiments, which lie farther from a nuclear reactor, cannot examine. Specifically, the experiment excludes sterile neutrinos whose difference in mass squared from an electron antineutrino is between 1 and 7 eV².

The new paper is the tenth physics publication based on data collected by PROSPECT in 2018. The detector provided the first precision measurement of neutrinos detected above ground, paving the way for a less expensive way to study neutrinos. Most neutrino experiments lie kilometers beneath Earth’s surface, relying on layers of rock to shield them from cosmic rays and other particles that bombard our planet and can confound results. These underground studies, however, are both costly and require huge detectors.

In addition to exploring fundamental physics, the PROSPECT detector could be adapted to monitor in real time the activity of remote nuclear reactors. Such a surveillance instrument could be a tool for ensuring that countries are honoring nuclear nonproliferation agreements.

Paper: Andriamirado, M., Balantekin, A.B., Bass, C.D., Benevides Rodrigues, O., Bernard, E.P., Bowden, N.S., Bryan, C.D., Carr, R., Classen, T., et al. (PROSPECT Collaboration*) Physical Review Letters, 134, 151802 – Published April 14, 2025. DOI: https://doi.org/10.1103/PhysRevLett.134.151802