Neutrinos have always caused the physics community a bit of grief. It took decades to go from the first hints of their existence to actually detecting their presence. Then, when studying solar neutrinos, scientists were stumped by an absence—far fewer showed up in the detectors than the Sun should be producing. This was eventually explained by what are called flavor oscillations, which cause neutrinos to shift among the three known types: electron, muon, and tau. Now, researchers are facing yet another enigma: antineutrinos undergo flavor oscillations at a different rate than their regular counterparts.

Flavor oscillations were big news when they were first discovered. The ability of a single neutrino to shift identity over the course of its travels implied that these particles have mass, something that was a bit unexpected. It was only this year that a detector in Italy provided a direct confirmation of a flavor oscillation taking place in a beam of neutrinos that originated at CERN, in Switzerland.

The new work comes from a detector at Fermilab called MiniBooNE, which creates neutrinos by siphoning off protons from the Tevatron's booster system and smashing them into a stationary target. MiniBooNE was intended to confirm some odd results that had originated a decade ago at a scintillation detector in Los Alamos. Those results had suggested that muon antineutrinos oscillated into electron antineutrinos at a higher than expected rate, which is a bit perplexing.

Initially, the MiniBooNE experiment looked at regular muon neutrinos, and found no sign of an excess of electron neutrinos. The new paper reports data on its search with muon antineutrinos, and the result is quite different: an excess of 43.2 +/- 22.5 detection events for the antineutrinos. (For those wondering how they got fractional events, it's a product of the process by which the team eliminated sources of noise, which produced probabilities that were more precise than a single event.) These numbers, and the energy distribution observed, appear to be compatible with the ones from the Los Alamos experiment.

This isn't the first time something strange has been observed for muon antineutrinos. A different detector group at Fermi has preliminary hints that the masses of these antiparticles may not be the same as their muon nuetrino counterparts.

All of this is exciting in part because none of it is compatible with the Standard Model, which means that neutrinos appear to be engaging in some sort of physics we don't understand yet. The question is what—the authors note that a number of possibilities have been proposed to explain the excess flavor oscillations, including a coupling between neutrinos and photons.

However, what seems to have caught everyone's attention is the suggestion that this might be evidence of what are called sterile neutrinos. 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.

It all sounds appealing, but so far the evidence for sterile neutrinos remains very indirect. However, by confirming the earlier results from Los Alamos, MiniBooNE has at least indicated that there's something worth looking into here. Which means that a full scale detector, named BooNE, might now be built in order to follow up on this data.

Physical Review Letters, 2010. DOI: 10.1103/PhysRevLett.105.181801 (About DOIs).

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