Neutrinos are tough to detect

Update: The MiniBooNE collaboration reported fresh evidence for the sterile neutrino at a conference in June. Now the work has been published in Physical Review Letters (DOI: 10.1103/PhysRevLett.105.181801)

A ghostly particle given up for dead is showing signs of life.

Not only could this “sterile” neutrino be the stuff of dark matter, thought to make up the bulk of our universe, it might also help to explain how an excess of matter over antimatter arose in our universe.


Neutrinos are subatomic particles that rarely interact with ordinary matter. They are known to come in three flavours – electron, muon and tau – with each able to spontaneously transform into another.

In the 1990s, results from the Liquid Scintillator Neutrino Detector (LSND) at the Los Alamos National Laboratory in New Mexico suggested there might be a fourth flavour: a “sterile” neutrino that is even less inclined to interact with ordinary matter than the others.

Hasty dismissal

Sterile neutrinos would be big news because the only way to detect them would be by their gravitational influence – just the sort of feature needed to explain dark matter.

Then in 2007 came the disheartening news that the Mini Booster Neutrino Experiment (MiniBooNE, pictured) at the Fermi National Accelerator Laboratory in Batavia, Illinois, had failed to find evidence of them.

But perhaps sterile neutrinos were dismissed too soon. While MiniBooNE used neutrinos to look for the sterile neutrino,

LSND used antineutrinos – the antimatter equivalent. Although antineutrinos should behave exactly the same as neutrinos, just to be safe, the MiniBooNE team decided to repeat the experiment – this time with antineutrinos.

Weird excess

Lo and behold, the team saw muon antineutrinos turning into electron antineutrinos at a higher rate than expected – just like at LSND. MiniBooNE member Richard Van de Water reported the result at a neutrino conference in Athens, Greece, on 14 June.

The excess could be because muon antineutrinos turn into sterile neutrinos before becoming electron antineutrinos, says Fermilab physicist Dan Hooper, who is not part of MiniBooNE. “This is very, very weird,” he adds.

Although it could be a statistical fluke, Hooper suggests that both MiniBooNE results could be explained if antineutrinos can change into sterile neutrinos but neutrinos cannot – an unexpected difference in behaviour.

The finding would fit nicely with research from the Main Injector Neutrino Oscillation Search, or MINOS, also at Fermilab, which, the same day, announced subtle differences in the oscillation behaviour of neutrinos and antineutrinos.

Antimatter and matter are supposed to behave like mirror versions of each other, but flaws in this symmetry could explain how our universe ended up with more matter.