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.

The hypothetical fourth one, which physicists have imagined before, is called the "sterile" neutrino. Ars Technica elaborates

:

Although regular neutrinos barely interact with matter, sterile neutrinos can only interact via gravity, which (if they exist) is what has allowed them to escape our detection to date. Since they'd also be heavier than the regular neutrinos, they would make good dark matter candidates. They could also potentially explain these new results, because having an additional neutrino for flavor oscillations to target might account for some of the unusual behavior. [Ars Technica]

Pascoli explains that this phenomenon has been nagging neutrino researchers for a while. The weird oscillation effect was picked up indirectly back in the 1990s by an experiment called the Liquid Scintillator Neutrino Detector

(LSND). MiniBooNE, the experiment that produced this new result, was built to confirm the findings that antineutrinos oscillated differently than neutrinos, and now it appears to have achieved this goal (though "the statistical significance of the MiniBooNE results is not very strong," Pascoli says—there's a three percent chance the effect comes from background noise). So MiniBooNE scientists have more work to do to produce even more convincing data. But if the findings of these strange oscillations are true, it would be a jolt to physicists who doubted those effects because they should have shown up in other oscillation experiments, and therefore sought more complicated answers—or, simply, a jolt to those who like their neutrinos in three shades. The news release

from Fermilab puts it bluntly: "The result is difficult for scientists to reconcile in a world with only three active neutrinos." And, Pascoli says, the physics upheaval could go further:

Moreover, the MiniBooNE results would be evidence of the fact that neutrinos and antineutrino behave differently. This would be the first evidence we have for this behavior in the leptonic sector and poses an even greater challenge to the theorists. If the MiniBooNE results are confirmed in the future, our picture of the fundamental laws of Nature and of its fundamental constituents will need to greatly change. While Tevatron and the LHC promise to unveil the physics which lies at high energies, discovering very heavy particles, it might be possible that LSND and MiniBooNE (and other neutrino experiments) are giving us hints towards new physics at much lower energies. What this physics is is still a mystery.

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Image: Fermilab (MiniBooNE Detector)