Hypothesis
A fourth, non-interacting 'sterile' neutrino state would explain the anomalous muon-to-electron neutrino transitions reported by LSND and MiniBooNE.
What failed
Using data from two accelerator beams in the MicroBooNE liquid-argon detector, the collaboration saw no electron-neutrino appearance or disappearance beyond the three-flavour picture and excluded the single-sterile-neutrino explanation of the LSND and MiniBooNE anomalies at 95% confidence, along with much of the parameter space for the gallium anomaly.
Broken assumption
The short-baseline neutrino anomalies are caused by oscillation into one extra light sterile neutrino.
Why
Two beams broke the degeneracy between appearance and disappearance that had weakened earlier tests; with that resolved, the data simply show no non-standard oscillation.
From the published paper: Abratenko et al. Nature, 2025. ↗CC BY