Why have some of Fermilab’s neutrinos gone missing? (Image: Fermilab)

Now you see it, now you don’t: that’s neutrinos for you. The astounding ability of these subatomic particles to morph from one type to another may have created another crack in our understanding of nature.

It may point the way to new physics that could tell us why the universe appears to be made only of matter and not antimatter too.

Physicists on the Main Injector Neutrino Oscillation Search (MINOS) experiment at Fermilab in Batavia, Illinois, were studying a phenomenon called neutrino oscillation when they found a discrepancy between neutrinos and anti-neutrinos that cannot be explained by standard model physics.


Neutrinos and their antimatter counterparts oscillate between three types: electron, tau and muon. In the MINOS experiment, muon neutrinos and muon anti-neutrinos are beamed at two detectors: a “near” detector at Fermilab itself, and a “far” detector inside a mine in Soudan, Minnesota. The particles have to pass through 700 kilometres of earth to get to the far detector.

Missing neutrinos

The particles are most likely changing into their tau counterparts. MINOS is not sensitive to taus, but infers them by measuring a deficit of muon neutrinos and anti-neutrinos.

According to our current understanding of neutrino physics, MINOS should see a similar deficit for neutrinos and anti-neutrinos, but on Monday the MINOS collaboration announced that this may not be what is happening.

When physicists measured a specific parameter related to neutrino oscillations, it was about 40 per cent greater for anti-neutrinos than for neutrinos. They say this is tentative evidence of a greater deficit in the anti-neutrino beam than in the neutrino beam.

Jenny Thomas of University College London, a spokeswoman for MINOS, stresses that the results are preliminary. “It could be an unlucky statistical fluctuation,” she says. “Those things happen.”

But if the effect proves solid, it could help us solve one of the biggest mysteries in physics: how an imbalance of matter and antimatter arose in the early universe.

The discrepancy could be due to a difference in the way neutrinos oscillate compared with anti-neutrinos. Or the anti-neutrinos may be interacting with the 700 kilometres of rock in a way that is not understood.

“If the effect is real, then there is some physics that is not expected,” says Thomas. “Then there is something new that we don’t understand, and that’s fantastic.”

Antonio Ereditato at the University of Bern, Switzerland, a spokesman for the OPERA neutrino experiment in Italy says: “This is once more proof that neutrino physics is a privileged tool to assess new physics.” But he adds that statistically robust results are needed.