Mystery of this elusive particle has been solved leading to a surprising answer

MicroBooNE ruled out sterile neutrinos using precise data, leaving neutrino mysteries unresolved for future research.

Scientists working on the Micro Booster Neutrino Experiment (MicroBooNE) have published results in Nature that rule out the existence of the sterile neutrino (a hypothetical type of neutrino that would not interact via the weak nuclear force, only gravity), a particle that had been suggested for decades as a possible explanation for puzzling behavior in neutrinos. This closes the door on one of the most popular theories in the field, while leaving the mystery itself still unsolved.

Neutrinos (tiny, hard-to-detect fundamental particles that are among the most abundant in the universe) are everywhere in the universe. “Neutrinos are elusive fundamental particles that are difficult to detect experimentally, yet are among the most abundant particles in the universe,” said David Caratelli, assistant professor of physics at UC Santa Barbara. Past experiments had shown results that did not match expectations, sparking speculation about a fourth type of neutrino, called the sterile neutrino. MicroBooNE’s data, however, did not support this idea.

The Standard Model of particle physics (the theoretical framework describing known fundamental particles and forces, but not including gravity, dark matter, or dark energy) explains most known particles and forces, but it does not cover everything. “We know that the Standard Model does a great job describing a host of phenomena in the natural world,” said Fermilab senior scientist Matthew Toups. “And at the same time, we know it’s incomplete. It doesn’t account for dark matter, dark energy or gravity.” Neutrinos are one of the areas where the model falls short. They were once thought to have no mass, but experiments showed they change between three “flavors” (electron, muon, and tau types that neutrinos can transform between through a process called neutrino oscillation), which can only happen if they do have mass.

In the 1990s, experiments at Los Alamos and Fermilab found odd results suggesting muon neutrinos were turning into electron neutrinos in ways that could not be explained with just three flavors. “The most popular explanation to these anomalies for the past 30 years has been a hypothetical sterile neutrino,” said Justin Evans, professor at the University of Manchester.

MicroBooNE was built to study these anomalies more closely. Between 2015 and 2021, the team collected data using neutrino beams at Fermilab and a liquid-argon detector (a device using ultra-cold liquid argon to record charged particle tracks through ionization). The recent paper reports the first analysis using all five years of data, with an exposure of 1.11×10²¹ protons on target (a measure of how many particles in the accelerator beam hit the target material), a 70% increase over earlier results. The study looked at low-energy electron-neutrino interactions, focusing on two samples: one with visible protons and one without. The data matched predictions fairly well, with combined p-values (statistical measures of how likely observed results could occur by chance) of at least 26.7%, though predictions were slightly higher than the data in some regions.

The team also tested two models designed to explain the MiniBooNE excess of electron-neutrino-like events. One model unfolded results by neutrino energy, while the other matched shower energy and angle distributions. MicroBooNE excluded an electronlike interpretation of the MiniBooNE excess based on these models at greater than 99% confidence level (a statistical measure indicating extremely strong evidence against a hypothesis being true). “Basically, what we were looking for is the effect of the appearance of new electron neutrinos caused by this oscillation phenomenon,” Caratelli explained. The absence of such signals ruled out the sterile neutrino.

Although this closes off one explanation, the puzzle remains. “I think it’s a bit of a paradigm shift for us,” Caratelli said. Researchers are now exploring other possibilities, such as whether photons or other new physics could explain the anomalies. Larger experiments are on the horizon, including the Deep Underground Neutrino Experiment (DUNE) in South Dakota, which will be the biggest neutrino detector ever built. “MicroBooNE is big — it’s the size of a school bus. But DUNE is football field-scale,” Caratelli said. Its sensitivity could help answer not only questions about neutrinos but also why the universe has more matter than antimatter.

Caratelli added that the lessons learned from MicroBooNE will guide future work. “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.”

Source: UC Santa Barbara, APS

This article was generated with some help from AI and reviewed by an editor. Under Section 107 of the Copyright Act 1976, this material is used for the purpose of news reporting. Fair use is a use permitted by copyright statute that might otherwise be infringing.

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Posted Monday 25 May 2026 at 7:58 am AEST (my time).

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