Neutrinos are fascinating particles with unique properties. They are at least one million times lighter than electrons and are able to pass through light-years of matter undisturbed. While you have been reading this article, billions of neutrinos from the sun have traversed every square inch of your body unnoticed.
Neutrinos are produced in one of three types, or “flavors,” called electron, muon and tau. However, neutrinos can change their flavor, “oscillating” back and forth among the three types as they move through space. It’s as if a pitcher threw a baseball that then turned into a cricket ball before getting to the batter. This is possible in the weird quantum world of physics because neutrinos of well-defined flavor do not have a well-defined mass; it takes a mixture of masses to make a neutrino with a well-defined flavor and vice versa. This mixing of masses and flavors is what allows neutrinos to shape-shift as they travel through space.
Both the 2015 Nobel Prize in physics and the 2016 Breakthrough Prize were awarded for the discovery and study of neutrino oscillations, but many critical questions remain. Among these is whether one of the mass states, known as the third mass state, contains an equal mixture of muon and tau neutrinos. Previous experimental results are consistent with this “maximal mixing” scenario, but with our current understanding there is no fundamental reason that the mixture should be exactly maximal. If true, it would be evidence of a new conservation law.
The NOvA long-baseline neutrino oscillation experiment is exposed to the world’s most powerful neutrino beam, which sends muon neutrinos through the Earth from Fermilab to a building-size detector in northern Minnesota. These give NOvA an outstanding sensitivity to measure neutrino oscillations, both by analyzing the number of muon neutrinos that remain after traveling through the Earth and by finding electron neutrinos that were not in the beam when it was produced.
The figure above shows the latest counts of muon neutrinos versus energy reported at the Neutrino 2016 conference in London. In the absence of oscillations we would expect to see 473 muon neutrinos, but only 78 were actually observed — very strong evidence of oscillation. But the most important highlight of this result is that the shape of the data is best described by a nonmaximal scenario.
This is a very intriguing hint, but the case against maximal mixing is not iron-clad. With current data, the result has 2.5 sigma significance, which means that there is still a 1.2 percent chance that maximal mixing could have produced the data seen by NOvA. Particle physicists do not like to gamble and typically require probabilities as small as 0.3 percent (or 3 sigma) to claim evidence and as small as 0.00006 percent (5 sigma) for discovery.
However, NOvA is really just getting started: The data reported today is just one-sixth of the total planned — and it will be exciting to see if future updates reach those higher degrees of certainty.
Gregory Pawloski is a physicist at the University of Minnesota. Bruno Zamorano is a physicist at the University of Sussex.