UK scientists have contributed to resolving a long-standing question in particle physics by helping rule out the existence of a fourth type of neutrino, known as the 'sterile neutrino.' Researchers from the University of Cambridge were part of an international team working on the MicroBooNE experiment at the Fermi National Accelerator Laboratory in the United States.
The results, published in Nature, indicate with 95% certainty that there is no evidence for a single sterile neutrino. This finding narrows down possibilities for new physics beyond the Standard Model and provides more clarity about neutrinos and their role in fundamental physics.
Neutrinos are abundant but difficult to detect because they rarely interact with matter. The Standard Model describes three types—electron, muon, and tau—that can change between these forms. However, previous experiments observed behavior that did not fit this framework, leading some scientists to propose a hypothetical sterile neutrino as an explanation.
Professor Justin Evans of the University of Manchester, co-spokesperson for MicroBooNE, 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."
The recent findings close off this line of inquiry. Magnus Handley from Cambridge’s Cavendish Laboratory stated: "This result shows us that simply adding one additional light sterile neutrino can’t explain the whole picture. 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 used a liquid argon detector to study neutrinos from two different particle beams over six years. By analyzing data from both sources together, researchers were able to examine theories about sterile neutrinos more thoroughly than before.
UK involvement was supported by funding from the Science and Technology Facilities Council (STFC), with contributions from multiple universities including Cambridge. Nearly 200 researchers from 40 institutions across six countries participated in MicroBooNE. In Cambridge specifically, teams developed software for event reconstruction and carried out detailed analyses of how neutrinos interact with matter.
Professor Sinéad Farrington, STFC Director of Particle Physics, commented: "These results mark an important milestone in our effort to understand some of the most elusive particles in the universe. 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."
Despite ruling out one possibility for unexplained phenomena involving neutrinos, questions remain about their true nature. Work continues through MicroBooNE and future projects such as DUNE (Deep Underground Neutrino Experiment), where Cambridge researchers maintain leading roles.
"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."
Professor Leigh Whitehead from Cavendish Laboratory added: "This result provides a powerful rejection of the 3+1 sterile neutrino model as the solution to a long-standing puzzle in particle physics. 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