- LSND (Liquid Scintillator Neutrino Detector) anomaly: This detector observed electron antineutrinos appearing in a beam of muon antineutrinos 
- MiniBooNE (Mini boosted Neutrino Experiment) found excesses of both electron neutrinos and antineutrinos 
- Reactor anomaly: the anomalous disappearance of electron antineutrinos 
How to hunt for “sterile" neutrinos?
The effect of “sterile” neutrino existence can be searched via a deviation from the standard neutrino oscillation paradigm since they can mix with the standard neutrinos and thus change the amplitude of oscillation probability of standard neutrinos. In the simplest model of sterile neutrino, a fourth mass state with mass m4 is added into the mixing matrix of the standard neutrinos. Based on the LSND and MiniBooNE results, a mass range of “sterile” neutrino is predicted. The neutrino oscillation cannot tell the absolute mass of neutrinos but the mass squared difference between two flavor neutrinos. Three standards neutrinos have two mass squared differences of ~7.5x10-5 eV2 and 2.4x10-3 eV2. The sterile neutrino mass is hypothesized much heavier than the standard neutrinos and the mass squared difference of mixing between a sterile neutrino and a normal neutrino to be 0.1-10 eV2.
So far, there are two methods to search “sterile” neutrino
- Search deviation in the oscillation probability by fitting with the modified oscillation probability with “sterile” neutrino taken into account
- Search for the disappearance of neutral-current neutrinos. In principle, the neutral-current neutrinos are not under standard oscillation, only the charged-current neutrinos do. Thus a deficit in total number of neutral-current neutrinos comparing to the predicted one might indicate the existence of sterile neutrino.
Recent results in “sterile" neutrino search
Recent result published by both MINOS and IceCube collaborations doesn’t show any evidence. However the search is still going on.