Scientists at Fermilab have delivered a major update on a problem that has puzzled the physics community for three decades. After six years of observations, the MicroBooNE experiment has found no sign of a hidden “sterile” neutrino, dismissing the simplest explanation for strange results reported in earlier studies. The new findings, published in four papers in Nature, challenge a long-running idea that once seemed like an easy fix to a stubborn mystery.
The story began in the 1990s when the LSND experiment, and later the MiniBooNE detector in the 2000s, picked up odd behavior from beams of neutrinos. These tiny particles usually switch between three known types as they move, but both experiments saw changes that happened much faster than expected. The simplest explanation was a fourth type of neutrino that does not interact with matter.
It would mix with the other three and cause the rapid flavor changes the earlier detectors reported. The idea caught on quickly, partly because it promised new physics beyond the standard picture and was straightforward to test.
MicroBooNE was built to run that test with much greater detail. The detector sits at Fermilab near Chicago and contains a large tank filled with liquid argon. When a neutrino hits an argon atom, it creates a spray of charged particles that leaves crisp tracks inside the detector.
Those tracks give researchers a clear view of each interaction. MicroBooNE observed two separate neutrino beams, which helped reduce uncertainty and gave the team a reliable picture of what was happening inside the tank. If a sterile neutrino existed, the detector should have seen an unusual excess of electron-like events.
Instead, it saw nothing out of the ordinary. The analysis reported no excess of any kind and ruled out almost all of the region where the simple one-sterile-neutrino model was expected to show up. The result surprised many physicists because the earlier MiniBooNE signal is still present.
That older detector continues to record an unexplained bump in its data. Since both detectors sit along the same beamline, MicroBooNE should have confirmed the finding if the sterile neutrino idea was correct. The disagreement has now become even sharper, raising new questions about whether MiniBooNE is seeing an unseen background effect or whether something more complex is happening.
Researchers are not short on ideas. Some are looking at more exotic explanations involving extra particles, new interactions, or even unusual behavior inside the beam. Others expect the anomaly to fade once more precise measurements arrive. Two other Fermilab detectors, ICARUS and SBND, are already gathering data and will give the field more clarity soon.
MicroBooNE has also pushed forward the technology behind liquid argon detectors, which sets the stage for the much larger DUNE project now being built in South Dakota. That future experiment will track neutrinos over a long distance and relies on the detailed measurements that MicroBooNE has helped refine.
Source: Search for light sterile neutrinos with two neutrino beams at MicroBooNE