An international team of scientists, including researchers from the University of Michigan, has reported that the MicroBooNE experiment at Fermilab found no evidence for the existence of a “sterile neutrino.” This particle had been proposed as a possible explanation for anomalies observed in earlier neutrino experiments since the late 1990s.
The results, published in Nature, indicate with 95% certainty that sterile neutrinos do not account for these experimental discrepancies. The MicroBooNE experiment uses two different neutrino beams and a single detector to increase sensitivity and reduce systematic uncertainties.
“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, professor of physics at the University of Michigan and long-time collaborator on MicroBooNE.
“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.”
Previous experiments such as LSND at Los Alamos National Laboratory and MiniBooNE at Fermilab had observed behaviors in neutrinos inconsistent with predictions from the Standard Model of particle physics. These findings led some physicists to propose a fourth type of neutrino—the sterile neutrino—beyond the three known types: muon, electron, and tau.
“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.”
Justin Evans, professor at the University of Manchester and co-spokesperson for MicroBooNE, explained: “They saw flavor change on a length scale that is just not consistent with there only being three neutrinos. And the most popular explanation over the past 30 years to explain the anomaly is that there’s a sterile neutrino.”
Now that MicroBooNE has ruled out this possibility, researchers are considering other explanations. According to Spitz of U-M: “One line of thinking is that there are ‘unknown unknowns’ in the design, operation or interpretation of 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.”
Benjamin Bogart, doctoral student at U-M and co-author on this study, added: “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.”
The Short-Baseline Neutrino Program (SBN) builds upon these efforts by using multiple detectors—a near detector (SBND) which began taking data in 2024 and a far detector (ICARUS) operational since 2021—to further investigate whether more complicated models could explain previous anomalies.
The collaboration also plays an important role in training new scientists; nearly half its members are students or postdoctoral researchers. Bogart commented: “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. But I’ve also been able to help with some of assembly SBND. So I’m basically at opposite ends of lifetimes both these experiments which is unique and special opportunity that I’m really quite thankful for.”
