Two weeks ago, experimental results seemed to indicate that we're getting a handle on the low-mass particles called neutrinos. Today, Fermilab announced results generated using antineutrinos that suggests we may need to make major revisions to the Standard Model of physics. The textbook description of antimatter is that it's like a mirror image of more familiar particles. But new work from Fermilab indicates that the mass differences among antineutrinos aren't the same as those for regular neutrinos. If the findings hold up, they would call for some new physics to explain the discrepancy.

Like the earlier results, the new data relies on observations of neutrino flavor changes. Neutrinos and antineutrinos, unlike other particles, appear not to have a fixed nature. They exist as a mixture of three identities—electron, muon, and tau—and a given particle oscillates among these identities in a probabilistic manner that depends in part on the mass differences among the three classes. So, if we can observe these oscillations, we can get some indication of the relative masses of these extremely light particles.

Observing the oscillations is a serious challenge, since it requires a large distance between the source of the neutrinos and the detector. In this case, the work was done by Fermilab's MINOS team, which produces neutrinos and antineutrinos using the Tevatron's injector ring. The resulting beam is sent to a detector in a mine in Minnesota, over 700km away, a trip that takes about 2.5 milliseconds.

Scientists from the MINOS team presented some preliminary results of their work at a meeting being held in Athens (the catchy meeting name: Neutrino 2010). The release announcing them cautions that the data is only just approaching statistical significance, and will require extensive confirmation work, not to mention a trip through peer review before publication. Those cautions aside, the results are pretty intriguing, since they suggest something very strange is up with the neutrinos' masses.

Right now, we can't directly measure the masses of the neutrinos very precisely, but we can get a sense of how the masses differ among the three flavors. The MINOS experiment only produces muon neutrinos and their antimatter counterparts, but on their way to Minnesota some of these will change flavor. This oscillation is governed in part by the relative masses of the different flavors, providing researchers with a glimpse into the particles' masses.

As shown in the graph above, the MINOS team has generated a fairly tight estimate of the mass difference (Δm) for regular muon neutrinos, but the errors for antineutrinos, shown in red, remain quite large. Nevertheless, it's pretty obvious that the best fits aren't very close to each other; should further data narrow down the existing errors without changing the best fit, it will clearly indicate that neutrinos and their antimatter partners are not simply mirror images of each other.

Not surprisingly, that will leave physicists scratching their heads. "If this result holds up, it would signal a fundamentally new property of the neutrino-antineutrino system," said Rob Plunkett, a MINOS spokesperson. "The implications of this difference for the physics of the Universe would be profound.”