UK scientists have played a key role in solving a neutrino mystery that has challenged researchers for decades.
An international team, including researchers from the University of Oxford’s Department of Physics, working on the MicroBooNE experiment at the US Department of Energy’s Fermi National Accelerator Laboratory, has found no evidence of a long-suspected fourth type of neutrino, known as the ‘sterile neutrino’. The new results, published in Nature [link to], rule out the single sterile neutrino model with 95% certainty.
Ruling out this long-suspected particle sharpens the search for physics beyond the Standard Model, bringing scientists a step closer to uncovering the true nature of neutrinos and the fundamental laws that govern the universe.
Neutrinos: the ghost particles of the universe
Neutrinos are among the most abundant particles in the universe, yet they are notoriously difficult to study. They rarely interact with matter, with trillions passing through our bodies every second without a trace.
According to the Standard Model, neutrinos come in three types, or ‘flavours’: electron, muon and tau. They can change, or oscillate, between these flavours, a behaviour predicted by the Standard Model.
The Standard Model remains the best framework for understanding the universe, but it is incomplete. For decades, some experiments have hinted at unexpected behaviour that challenged this framework.
Unpredictable neutrino behaviour
Previous experiments observed neutrino behaviour that didn’t fit the three-neutrino framework, prompting scientists to suggest a hypothetical ‘sterile’ neutrino, one that would interact only through gravity, might explain the anomalies.
Professor Justin Evans of the University of Manchester, MicroBooNE co-spokesperson, said:
‘The team saw flavour change on a length scale that is just not consistent with there only being three neutrinos. The most popular explanation over the past 30 years to explain the anomaly is that there’s a sterile neutrino.’
Nowhere to hide
The latest results from the MicroBooNE experiment rule out the existence of a sterile neutrino with 95% confidence, shutting down one of the strongest explanations for the for the mysterious behaviour of these ghostly particles. MicroBooNE, a state-of-the-art detector filled with liquid argon, studied neutrinos coming from two separate particle beams at Fermilab over a period of six years; by combining data from both beams, the experiment probed the theory more deeply than ever, leaving almost no room where a single sterile neutrino could be hiding.
Critical UK contribution
The experiment brought together nearly 200 researchers from 40 institutions across six countries. The UK played a central role in the international programme thanks to the involvement of scientists from Cambridge, Edinburgh, Imperial College London, Lancaster, Manchester, Oxford, Queen Mary University of London and Warwick, funded by the Science and Technology Facilities Council (STFC).
In Oxford, Associate Professor Kirsty Duffy as Physics Coordinator for MicroBooNE, has been involved in the analysis from the start providing leadership and guidance, reviewing the result and inputting into the publication itself; she also developed the neutrino interaction uncertainty model used in the analysis. Dr Patrick Green was pivotal in validating the flux prediction for the NuMI beam which was a vital piece of the analysis and also in generating a lot of the simulated data samples used for this result.
Through the UK’s contributions, the UK has shaped MicroBooNE’s scientific impact and strengthened the foundations for future flagship experiments such as the Deep Underground Neutrino Experiment (DUNE), currently under construction, ensuring continued UK leadership in neutrino physics.
Next steps in the neutrino hunt
With sterile neutrinos now ruled out, the mystery of neutrinos remains. MicroBooNE is continuing the search for new physics and delivering vital data on how neutrinos behave in liquid argon, crucial knowledge for future experiments, including DUNE.
Professor Evans added: ‘While this new result doesn’t reveal what is behind the neutrino mystery, it does eliminate what many believed was the most promising explanation. In science, crossing a wrong answer off the list is often just as important as finding the right one. By narrowing the field, MicroBooNE brings scientists closer to uncovering the true physics behind neutrinos, particles that may ultimately help explain why the universe looks the way it does. In the search for new physics, even a closed door is progress.’
Exciting prospects ahead
Dr Duffy concludes: ‘Neutrino physics has always held fantastic potential for discovery. Past experiments challenged our understanding of particle physics, and some anomalies even hinted at a new type of neutrino, the “sterile neutrino”. MicroBooNE was built to investigate this, and it’s incredibly exciting to publish the results of this decade-long search. We find no disagreement with the Standard Model, ruling out the simple sterile neutrino explanation at 95% confidence. At the same time, this work demonstrates the precision and power of liquid-argon detectors—a capability we will carry forward into future experiments like DUNE. It’s a major step forward, with even more exciting prospects ahead.’
Search for light sterile neutrinos with two neutrino beams at MicroBooNE, MicroBooNE Collaboration, Nature 648, 64-69 (2025)