After ten years and billions of collisions, US physicists rule out a long-suspected ghost particle

After ten years and billions of collisions, US physicists rule out a long-suspected ghost particle

For more than a decade, physicists in the United States have been chasing what came to be known as a “ghost particle” — the sterile neutrino. It was a hypothetical addition to the Standard Model of particle physics, proposed to explain puzzling signals seen in earlier experiments. Now, after years of careful work, researchers say the particle almost certainly does not exist, at least not in the form they were searching for.

The conclusion comes from the MicroBooNE experiment at the US Department of Energy’s Fermi National Accelerator Laboratory (Fermilab) near Chicago. MicroBooNE was designed to investigate unexplained excess signals detected in earlier neutrino experiments, particularly the MiniBooNE experiment, which had hinted at something beyond known physics.

Neutrinos themselves are already strange. They are extremely light, barely interact with matter and pass through the Earth in vast numbers every second. The sterile neutrino, if real, would have been even more elusive — interacting only through gravity and not through the weak nuclear force like normal neutrinos.

MicroBooNE used a powerful beam of neutrinos and a large detector filled with liquid argon to track how these particles interacted. The experiment recorded and analysed millions of events between 2015 and 2024, allowing scientists to reconstruct interactions in unprecedented detail.

In its final set of results, released this year, the MicroBooNE collaboration reported no evidence of sterile neutrinos. The data ruled out the most popular sterile neutrino explanations with about 95 percent confidence, effectively closing the door on a theory that had occupied physicists for years.

Importantly, the experiment also solved another mystery. Earlier detectors had trouble distinguishing between electrons and photons, leading to confusion about what was actually being observed. MicroBooNE’s high-resolution imaging showed that the excess signals seen previously were not caused by new particles, but by known background processes that older experiments could not fully separate.

“This is a definitive result,” researchers said in technical briefings, noting that while it may feel disappointing to rule something out, it is a crucial part of how science advances. Knowing what does not exist is just as important as discovering something new.

The findings narrow the search for physics beyond the Standard Model but do not end it. Other experiments at Fermilab, including SBND and ICARUS, will continue probing neutrinos for surprises. For now, however, one of particle physics’ most persistent ghosts has finally been laid to rest.