After years of careful investigation, researchers working on the Micro Booster Neutrino Experiment (MicroBooNE) have determined that a long-hypothesized particle known as the sterile neutrino does not exist. This proposed particle had been widely discussed as a possible answer to unresolved problems in particle physics. Reporting their findings in the journal Nature, the team's results significantly narrow the range of explanations for one of the most persistent mysteries involving neutrinos.
"Neutrinos are elusive fundamental particles that are difficult to detect experimentally, yet are among the most abundant particles in the universe," said UC Santa Barbara assistant physics professor David Caratelli, who served as physics coordinator for the experiment during the analysis. Earlier experiments, he explained, produced results that did not match existing knowledge, leading scientists to speculate about the presence of a fourth neutrino -- a "sterile" neutrino. MicroBooNE's new measurements, however, show that this idea does not align with the data.
According to Caratelli, eliminating the sterile neutrino hypothesis represents a major step forward. The result clears the way for exploring new possibilities and helps prepare the field for larger and more advanced neutrino experiments.
This research received partial support from the U.S. Department of Energy's Office of Science and the National Science Foundation.
Why Neutrinos Still Puzzle Physicists
The Standard Model provides a well-tested framework for understanding the fundamental forces and particles that shape the universe. Even so, it leaves some major questions unanswered.
"We know that the Standard Model does a great job describing a host of phenomena in the natural world," said Matthew Toups, a senior scientist at Fermilab 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."
Neutrinos represent one of these gaps. When the Standard Model was first developed, neutrinos were assumed to have no mass. That assumption began to unravel in the late 20th century, when experiments observing neutrinos arriving from space revealed unexpected behavior. Certain types of neutrinos seemed to vanish as they traveled.
Scientists realized that neutrinos come in three forms, known as electron, muon, and tau flavors, and that these flavors can change as neutrinos move through space. This process, called oscillation, implies that neutrinos must have mass.
"The only way this oscillation can happen is if neutrinos have mass," Caratelli explained. "This is something that the Standard Model did not predict."
The Sterile Neutrino Hypothesis
In the 1990s, further experiments deepened the mystery. Studies at the Liquid Scintillator Neutrino Detector (LSND) at Los Alamos National Laboratory and later at the MiniBooNE experiment at Fermilab observed muon neutrinos transforming into electron neutrinos in ways that could not be explained using only the three known neutrino types.
"The most popular explanation to these anomalies for the past 30 years has been a hypothetical sterile neutrino," said Justin Evans, a professor at the University of Manchester and co-spokesperson for MicroBooNE.
Unlike the known neutrinos, which interact with other particles through the electroweak force, a sterile neutrino would not interact with matter in the same way. This made it extremely difficult to detect directly.
How MicroBooNE Tested the Theory
To examine these anomalies more closely, scientists built MicroBooNE at Fermilab, a detector designed to capture neutrino interactions in unprecedented detail.
Between 2015 and 2021, the experiment recorded neutrinos produced by two beams at the Fermilab site. These beams sent neutrinos into a liquid-argon time projection chamber, where their interactions could be observed with high precision.
"We produce neutrinos of one kind and place our detectors at optimal positions so that we could maximize the probability of finding this sterile neutrino," Caratelli said. "In practice, what we did is produce muon neutrinos and if a sterile neutrino were to exist, we would see an appearance of electron neutrinos."
The team compared the number of electron neutrinos detected with predictions based on models that included a sterile neutrino and models that did not. "Basically, what we were looking for is the effect of the appearance of new electron neutrinos caused by this oscillation phenomenon."
The results showed no such effect. The data matched expectations for a universe without sterile neutrinos, effectively ruling out the particle's existence. This conclusion builds on earlier work led by the UC Santa Barbara group and published in Physics Review Letters in the summer of 2025, which also found no excess of electron neutrinos.
A Turning Point for Neutrino Research
Although the sterile neutrino explanation has been set aside, the original anomalies observed by LSND and MiniBooNE have not been fully resolved.
"I think it's a bit of a paradigm shift for us," Caratelli said. With the decades-old hypothesis no longer viable, researchers are now exploring a broader set of ideas that could explain the strange observations and potentially shed light on deeper questions, including the nature of dark matter.
"We have a much more varied menu of options that we're investigating," Caratelli said. The tools and techniques refined during the MicroBooNE experiment are now being applied to more complex, multi-detector studies.
One alternative idea involves photons that may have been misidentified in earlier experiments or could point to new physics. UC Santa Barbara physics professor and MicroBooNE collaborator Xiao Luo has recently published an initial analysis examining this possibility. Future work within Fermilab's Short Baseline Neutrino program is expected to explore these questions in greater detail.
Looking Ahead to the Next Generation of Experiments
At the same time, construction is moving forward on the Deep Underground Neutrino Experiment (DUNE). Built a mile beneath the surface at the Sanford Underground Research Facility in South Dakota, DUNE will be the largest neutrino detector ever created. It will receive an intense beam of high energy neutrinos sent through the Earth from Fermilab, 800 miles away.
"MicroBooNE is big -- it's the size of a school bus. But DUNE is football field-scale," Caratelli said. The scale and precision of DUNE could help answer questions not only about neutrino behavior but also about why the universe contains more matter than anti-matter.
According to Caratelli, MicroBooNE played a critical role in preparing scientists for what comes next.
"One of the key things that MicroBooNE did was give us all confidence and teach us how to use this technology to measure neutrinos with high precision," he said. "What we learned with MicroBooNE on how to analyze the data that comes to the detector all directly applies to DUNE."
