Scientists spent 10 years chasing a particle that wasn’t there

The results were published in Nature and come from 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.")

A Decade-Long Test at Fermilab

MicroBooNE relies on a large liquid-argon detector and observations from two separate neutrino beams. By carefully tracking how neutrinos behave, scientists were able to rule out the existence of a single sterile neutrino with 95% certainty.

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 finding as a major shift 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."

Why Neutrinos Matter

Neutrinos are extremely small particles that rarely interact with matter. They can travel straight through entire planets without slowing down. According to the Standard Model, which is the leading framework in particle physics, there are three known types of neutrinos: electron, muon and tau. These particles can transform from one type to another through a phenomenon known as oscillation.

In earlier experiments, however, scientists observed neutrino behavior that did not fully match the predictions of the Standard Model. To explain those results, researchers suggested the existence of a fourth type of neutrino called the sterile neutrino. Unlike the known types, a sterile neutrino would not interact with matter at all, except through gravity, making it exceptionally difficult to detect.

Putting the Sterile Neutrino to the Test

To investigate this idea, the MicroBooNE team measured neutrinos produced by two different beams and analyzed how they changed as they traveled. After ten years of collecting and interpreting data, the researchers found no evidence supporting the sterile neutrino hypothesis. This effectively shuts down one of the most widely discussed explanations for unusual neutrino behavior.

Mastbaum played a central role in guiding the experiment's analysis efforts as co-coordinator for analysis tools and techniques. His work focused on how raw detector signals were converted into meaningful scientific conclusions. He also previously led efforts to understand what the team calls systematic uncertainties, which are possible sources of error in the measurements.

These uncertainties include how neutrinos interact with atomic nuclei, the exact number of neutrinos in the beam and how the detector itself responds to incoming particles. Accurately accounting for these factors is essential for drawing firm conclusions from the data.

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

Graduate Researchers and Data Accuracy

Graduate students from Rutgers also contributed to the project. Panagiotis Englezos, a doctoral student in the Department of Physics and Astronomy in the Rutgers School of Arts and Sciences, worked on the MicroBooNE Data Management Team, helping process experimental data and create simulations that supported the analysis.

Keng Lin, another doctoral student in the department, focused on validating the neutrino flux from Fermilab's NuMI (Neutrinos from the Main Injector) beam, which was one of the two neutrino sources used in the study. Together, these efforts helped ensure the precision and reliability of the final results.

What This Means for Physics

According to Mastbaum, the finding is significant because it removes a major candidate for new physics beyond the Standard Model. While the Standard Model has been highly successful, it does not explain phenomena such as dark matter, dark energy or gravity. Researchers continue to search for clues that point beyond the model, and eliminating one possibility helps narrow the field.

Rutgers scientists also helped advance methods for measuring how neutrinos interact in liquid argon. These improved techniques will benefit future projects, 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."