Does an eV-scale sterile neutrino exist?
In plain words
A fourth neutrino that feels no force except gravity, with mass near 1 eV, could explain some anomalies. It would show up as extra appearance and disappearance at short distances.
Precise statement
Determine whether a fourth mass state with $\Delta m^2_{41} \sim 0.1 \text{ to } 10\,\mathrm{eV}^2$ and mixings |U_e4|, |U_mu4| large enough to produce sin^2(2 theta_mue) ~ 1e-3 exists. An answer is a $5\,\sigma$ oscillation signal or exclusion of the region preferred by LSND and gallium in both appearance and disappearance.
What would settle it
Combined SBN-program oscillation fits with near and far detectors, plus reactor (PROSPECT, STEREO) and atmospheric (IceCube) disappearance limits.
Status in the literature
Unverified note
Appearance-disappearance tension in global fits and the 2025 MicroBooNE result disfavor the simple 3+1 model.
Related problems
- Special case of What causes the short-baseline neutrino anomalies?