Neutrinos are lightweight, elusive subatomic particles, and current theories recognize three different types.
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What is a sterile neutrino?
Today, neutrinos are an integral part of the Standard Model of particle physics.
Not all fermions interact with all the forces.
The 17 particles of the Standard Model.
The 12 fermions on the left make up matter, while the 5 bosons on the right carry forces.
The three known neutrinos are on the bottom row.MissMJ / Wikimedia.
All of these neutrinos are left-handed with respect to the weak force.
All other known fermions come in both left- and right-handed versions.
This encourages us to think that right-handed neutrinos should also exist in nature.
Being right-handed, these hypothetical neutrinos are blind even to the weak force and are in this sense sterile.
But like all known particles, they should still feel gravity.
Hunting for sterile neutrinos
If sterile neutrinos exist, how would we find them?
Results from the MiniBooNE experiment hinted at the possible existence of a light sterile neutrino.Fred Ullrich / Fermilab.
At LSND and MiniBooNE, they saw more electron-neutrinos than expected.
We know from other experiments that muon-neutrinos cannot oscillate directly into electron-neutrinos over these distances.
This is the sterile neutrino hypothesis.
What if there are sterile neutrinos?
There is but one problem.
The light sterile neutrino we started with is a headache for cosmologists.
This could mean the sterile neutrino hypothesis is incorrect.
But it is also possible that something else in our understanding of the Universe is amiss.
It also raises the question of exactly what caused the results observed by the earlier experiments.
Further analysis of the MicroBooNE data may help unravel this mystery.
The IceCube experiment in Antarctica has found no evidence in favour of sterile neutrinos.
Neither of these saw evidence of disappearing muon-neutrinos as the sterile neutrino hypothesis predicts.
Elsewhere, there have been claims of disappearing neutrinos in nuclear reactor experiments.
Future searches
The MicroBooNE collaboration has so far analysed only half of its collected data.
Technically, we need measurements at two distances or more to definitively establish oscillations or otherwise.
The prospects for a final verdict on light sterile neutrinos in the next decade are therefore good.
