MicroBooNE experiment finds no evidence for the long-sought ‘sterile neutrino’

Scientists are closing the door on one explanation for a mystery that has plagued particle physics for decades. An international collaboration of scientists, which includes researchers from the University of Michigan, announced that it found no evidence for a so-called “sterile neutrino.”

This new particle was proposed to explain results from experiments happening since roughly the turn of the 21st century that couldn’t be reconciled with just the three known neutrinos. Now, however, the cutting-edge MicroBooNE experiment at the U.S. Department of Energy’s Fermi National Accelerator Laboratory, or Fermilab, has ruled out the sterile neutrino as a solution to that problem with 95% certainty. The research team published its results in the journal Nature.

“MicroBooNE is exposed to two different neutrino beams while using the same detector. This provides an extra, enhanced sensitivity because you don’t have the systematic uncertainties that would come with using different detectors,” said Joshua Spitz, a professor of physics at U-M and a collaborator who’s been working on MicroBooNE since its inception.

“And the punchline is, basically, we don’t see anything we didn’t expect. We’re able to rule out the sterile neutrino as the explanation for the anomalies in earlier experiments.”

Anomalies

The spots and tracks here are particles that emanate from a collision between a neutrino and a liquid argon atom in MicroBoone’s detector. MicroBooNE can pinpoint these particles with millimeter precision. Image credit: MicroBooNE Collaboration

MicroBooNE’s predecessors had revealed neutrinos behaving in a way inconsistent with the Standard Model of particle physics, which successfully accounts for many of the fundamental particles and interactions at work in the universe. And, while the Standard Model is a triumph of modern physics, scientists know it still has some holes.

“The Standard Model does a great job describing a host of phenomena in the natural world,” said Matthew Toups, Fermilab senior scientist and co-spokesperson for MicroBooNE. “And at the same time, we know it’s incomplete. It doesn’t account for dark matter, dark energy or gravity.”

When neutrino experiments began showing inconsistencies with the Standard Model, they opened exciting pathways to potentially discover new physics and address deficits of the Standard Model.

According to the Standard Model there are three types, or flavors, of neutrino: muon, electron and tau. Neutrinos can switch or oscillate between these flavors, changing, for instance, from a muon neutrino to an electron neutrino. Scientists have been studying how neutrinos oscillate for decades, providing a strong foundation for understanding how often neutrinos naturally change flavor.

The Liquid Scintillator Neutrino Detector, or LSND, at Los Alamos National Laboratory raised the first hints that our understanding wasn’t quite aligning with reality in 1995. Fermilab then launched an experiment called MiniBooNE to verify the LSND results. Both experiments made observations suggesting that muon neutrinos were oscillating into electron neutrinos over shorter distances than are possible with only three neutrino flavors.

“They saw flavor change on a length scale that is just not consistent with there only being three neutrinos,” said Justin Evans, a professor at the University of Manchester and co-spokesperson for MicroBooNE. “And the most popular explanation over the past 30 years to explain the anomaly is that there’s a sterile neutrino.”

Now MicroBooNE has ruled out that explanation.

Looking ahead

Researchers have offered other hypotheses to explain the anomalies, said Spitz of U-M. One line of thinking is that there are “unknown unknowns” in the design, operation or interpretation of the previous experiments that give rise to the appearance of too-short oscillations.

“The other path is that you can start thinking about the existence of more than one sterile neutrino participating in oscillations,” Spitz said.

There could also be explanations aside from sterile neutrinos, said Benjamin Bogart, a doctoral student at U-M and co-author of the new study. MicroBooNE and the newer Short-Baseline Neutrino Program, or SBN, could help explore those possibilities.

“Though we closed the door on a single light sterile neutrino, MicroBooNE and SBN continue to open doors on a whole host of other scenarios—sometimes more complex and more interesting—beyond the Standard Model,” Bogart said.

The SBN Program adds a powerful multidetector approach with a near detector and a far detector to determine whether a more complicated model could explain the LSND and MiniBooNE anomalies. ICARUS, the far detector in the program, began taking beam data at Fermilab in 2021 and the Short-Baseline Near Detector, or SBND, started taking data in 2024. Both Bogart and Spitz are also collaborators on the SBND experiment.

While these experiments provide unparalleled insights into the workings of the universe, they’re also preparing the next generation of experts. For example, half of the researchers at MicroBooNE—which involves nearly 200 from 40 institutions in six countries—are students or postdocs.

That includes Bogart.

“I’m very grateful for how things have fallen in the timeline of my Ph.D., where I have lots and lots of data from MicroBooNE for analysis,” said Bogart, who noted MicroBooNE started collecting data when he was in 10th grade. “But I’ve also been able to help with some of the assembly of SBND. So I’m basically at opposite ends of the lifetimes of both these experiments, which is a unique and special opportunity that I’m really quite thankful for.”