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 Cambridge, 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'.
Their results, published in the journal Nature, 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 are among the most abundant particles in the universe, but 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.
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.
"The team saw flavour change on a length scale that is just not consistent with there only being three neutrinos," said Professor Justin Evans of the University of Manchester, MicroBooNE co-spokesperson. "The most popular explanation over the past 30 years to explain the anomaly is that there's a sterile neutrino."
The latest results 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.
"This result shows us that simply adding one additional light sterile neutrino can't explain the whole picture," said Magnus Handley from Cambridge's Cavendish Laboratory, and a co-author on the paper. "We need to continue the hunt using improved techniques and in conjunction with other experiments. While this result strongly restricts the single light sterile model, there are many interesting ways in which neutrino interactions can still allow us to probe new physics."
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.
Funded by the Science and Technology Facilities Council (STFC), scientists from Cambridge, as well as Edinburgh, Imperial College London, Lancaster, Manchester, Oxford, Queen Mary University of London and Warwick played a central role in this international programme.
The experiment brought together nearly 200 researchers from 40 institutions across six countries. In Cambridge, the High Energy Physics' Neutrino Group, based at the Cavendish Laboratory, was instrumental in creating event reconstruction software, which turns raw detector information into meaningful particle data for analysis; measuring the details of neutrino interactions with cross-section analyses; and training and mentoring the next generation of scientists.
Through these 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.
"These results mark an important milestone in our effort to understand some of the most elusive particles in the universe," said Professor Sinéad Farrington, STFC Director of Particle Physics. "The UK has played a critical role in this latest MicroBooNe result, providing leadership across the collaboration and developing the advanced technologies that made this breakthrough possible."
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 the next-generation Deep Underground Neutrino Experiment (DUNE) for which Cambridge researchers continue to lead development.
"In science, crossing a wrong answer off the list is often just as important as finding the right one," said Evans. "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."
"This result provides a powerful rejection of the 3+1 sterile neutrino model as the solution to a long-standing puzzle in particle physics," said Professor Leigh Whitehead, also from the Cavendish Laboratory. "We are excited to be members of DUNE, a next-generation experiment using similar technology to MicroBooNE, that will search for differences between neutrinos and antineutrinos, measure neutrino oscillation parameters with very high precision, and search for new physics beyond the Standard Model."
Reference:
The MicroBooNE Collaboration. 'Search for light sterile neutrinos with two neutrino beams at MicroBooNE.' Nature (2025). DOI: 10.1038/s41586-025-09757-7
