After Decades of Mystery, Scientists Debunk a Major Theory About a Bizarre Particle

After a decade of precision measurements, scientists have overturned a long-standing theory about a mysterious particle once thought to hide beyond the Standard Model. Using the MicroBooNE experiment’s powerful liquid-argon detector, researchers found no evidence for the elusive sterile neutrino. Credit: Shutterstock

A decade-long investigation into puzzling neutrino behavior has now ruled out one of the most widely discussed explanations: the sterile neutrino.

After a decade of gathering and studying data, scientists, including researchers from Rutgers, have overturned a long-standing theory about a mysterious type of particle.

The results, reported in Nature, were produced by the MicroBooNE experiment at the U.S. Department of Energy’s Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois. (The acronym MicroBooNE stands for “Micro Booster Neutrino Experiment.”)

By studying two neutrino beams with a highly sensitive liquid-argon detector, the team was able to rule out the presence of a single sterile neutrino with 95 percent confidence.

Andrew Mastbaum, an associate professor in the Department of Physics and Astronomy in the Rutgers School of Arts and Sciences and a member of the MicroBooNE leadership team, described the outcome as a major moment for the field.

“This result will spark innovative ideas across neutrino research to understand what is really going on,” he said. “We can rule out a great suspect, but that doesn’t quite solve a mystery.”

Rutgers University physicist Andrew Mastbaum. Credit: Andrew Mastbaum/Rutgers University

Neutrinos are extremely small subatomic particles that interact so weakly with matter that they can move straight through entire planets. According to the Standard Model, which serves as the central framework of particle physics, there are three known varieties of neutrinos: electron, muon, and tau. Each type can shift into another through a phenomenon known as oscillation.

The Sterile Neutrino Hypothesis

Earlier experiments, however, revealed neutrino behavior that did not align with the predictions of the Standard Model. To account for these unexpected results, scientists suggested the possibility of a fourth kind of neutrino called the sterile neutrino. This particle would be even harder to observe than the others because it would not interact with matter in any way other than through gravity.

MicroBooNE scientists tested this idea by observing neutrinos from two different beams and measuring how they oscillate. After ten years of data collection and analysis, the team found no sign of sterile neutrinos, closing the door on one of the most popular explanations for strange neutrino behavior.

Members of the MicroBooNE experiment, an international physics collaboration, meet at the Rutgers Inn in May 2022 for an annual meeting. Credit: Andrew Mastbaum/Rutgers University

Mastbaum helped lead the experiment’s analysis program as co-coordinator for analysis tools and techniques, overseeing how scientists turned raw data into meaningful physics results. He previously led the team that worked out what the research team refers to as systematic uncertainties, which are the possible sources of error in the measurements. This includes understanding how neutrinos interact with atomic nuclei, how many neutrinos are in the beam, and how the detector responds.

Getting these uncertainties right is critical because it allows scientists to make strong, reliable statements about what the data really shows, Mastbaum said.

Contributions From Rutgers Scientists

Panagiotis Englezos, a doctoral student in the Department of Physics and Astronomy at the Rutgers School of Arts and Sciences, served on the MicroBooNE Data Management Team, helping to process data and produce supporting simulations. Keng Lin, also a doctoral student in the department, helped validate the neutrino flux from Fermilab’s NuMI (Neutrinos from the Main Injector) beam, one of the two neutrino beams used in this analysis.

These efforts ensured the accuracy and reliability of the experiment’s findings.

This result is important, Mastbaum said, because it rules out a major theory about new physics. The Standard Model doesn’t explain everything, including dark matter, dark energy, or gravity, he said, so scientists are searching for clues that point beyond the model. Eliminating one possibility helps focus the search on other ideas that could lead to breakthroughs in understanding the universe.

Rutgers scientists played a crucial role in analyzing the data and improving techniques for measuring neutrino interactions in liquid argon. These advances will help future experiments, including the Deep Underground Neutrino Experiment (DUNE).

“With careful modeling and clever analysis approaches, the MicroBooNE team has squeezed an incredible amount of information out of this detector,” Mastbaum said. “With the next generation of experiments, such as DUNE, we are already using these techniques to address even more fundamental questions about the nature of matter and the existence of the universe.”

Reference: “Search for light sterile neutrinos with two neutrino beams at MicroBooNE” by The MicroBooNE Collaboration, 3 December 2025, Nature.

DOI: 10.1038/s41586-025-09757-7

Funding: U.S. Department of Energy, U.S. National Science Foundation, Science and Technology Facilities Council, Royal Society, UK Research and Innovation

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