For two decades, anomalies in short-baseline neutrino experiments have hinted at a tantalising possibility: a fourth, sterile neutrino beyond the three known flavours. University of Manchester physicists, as part of the international MicroBooNE collaboration at Fermi National Accelerator Laboratory in the United States, have now delivered the strongest evidence yet against the simplest version of that idea, published in Nature.
The experiment's liquid-argon detector recorded neutrinos simultaneously from two beams, Fermilab's Booster Neutrino Beam and the NuMI beam, using data collected between 2015 and 2021. It is the first MicroBooNE search to combine one detector with two beams at once, a configuration that substantially reduces the systematic uncertainties that have clouded previous claims. The analysis rules out a single sterile neutrino explanation with 95 per cent certainty and excludes nearly all of the favoured parameter region where such a particle could hide.
The result does not close the case entirely: the short-baseline anomaly that motivated the sterile neutrino hypothesis remains unexplained, and analysis of remaining data and complementary short-baseline experiments continues. But by cutting away the most popular explanation, the measurement forces theorists to look elsewhere, whether to more exotic new physics or to subtler experimental effects.