When neutrinos change from one phase to another, they tell us something about their mysterious nature. These ghostly subatomic particles come in three flavors, physicists say: muon, tau, and electron. Just this summer, a team caught a neutrino in the act of changing from muon to tau, a finding that backed up the argument that these particles do, in fact, have mass. This week, a new study of neutrino oscillation—the changing of flavors—suggests an deeper mystery, and implies that these three flavors of neutrino may not be enough to account for these particles' behavior. In Physical Review Letters, a large group of physicists published their study from the MiniBooNE experiment at Fermilab in Illinois. When the physicists looked at oscillations of muon antineutrinos into electron antineutrinos, they found the process happening faster than known physics predicts. Neutrinos followed the rules, but antineutrinos didn't behave the same way did. So what does it mean? We asked physicist Silvia Pascoli at the U.K.'s Durham University to explain:
These oscillations are faster than expected, i.e. they require a mass squared difference (which is one of the two parameters which control the probability of oscillation, the other being the "mixing angle") which is much larger than what required by the other evidences we have for neutrino oscillations.... In order to explain the existence of this large mass squared difference, we need four masses. This cannot be accommodated if we have only three neutrinos, as predicted by the Standard Model. We need a fourth one.
