Neutrinos are devious little particles. Only in the late 1990s were they shown to have mass, after decades of head-scratching hints to that effect. They can oscillate between three neutrino types, or "flavors," changing their identity on the fly. And, perhaps most famously, they were accused just last year of breaking cosmic law by traveling faster than light. (The jury is out, but an acquittal appears imminent.)

Now investigators are just a bit closer to figuring out the neutrino's modus operandi. A collaboration of physicists says it has measured one of the key descriptors of the neutrino's flavor-changing behavior—a number called theta 13 (pronounced "theta one three"). That number, known as a mixing angle, describes the probability that an electron neutrino's antiparticle, the electron antineutrino, will oscillate into another flavor over a relatively short distance. (Each of the three neutrino flavors—electron, tau and muon—has its own antiparticle partner.) Two other neutrino oscillation parameters, or mixing angles, have already been measured, but theta 13 is relatively small compared with the other two and has proved harder to pin down.

Since last year a group of physicists has been trying to measure theta 13 by tracking antineutrinos given off by a large Chinese nuclear power plant. The Daya Bay Reactor Neutrino Experiment collaboration built a series of six detectors, some near the reactors and some more than a kilometer farther away, to track how electron antineutrinos morph into other flavors as they travel through space. Because the detectors are tuned to identify only electron antineutrinos, any oscillation means that the neutrinos will escape detection—that is, they will seem to disappear. Other experiments have taken the opposite tack, looking for the appearance of electron neutrinos in a beam carrying other types of neutrinos.

In just two months of data, the distant set of detectors registered more than 10,000 hits by electron antineutrinos. But that is only 94 percent as many as would be naively expected by extrapolating from the detectors closest to the nuclear reactors. That means that a substantial fraction had oscillated to another flavor on their relatively short journey. "What we're seeing now is this disappearance of [electron antineutrinos] is at the 6 percent level," says neutrino physicist Karsten Heeger of the University of Wisconsin–Madison, a member of the Daya Bay collaboration. "It's a fairly large effect." Heeger presented the experimental results March 8 at a symposium at Duke University, and the group has submitted its study to Physical Review Letters.

The experiment is not even fully built yet—a seventh and eighth detector are in the works—but already the Daya Bay team has observed enough disappearances to quantify how the process works. The new estimate, which falls within previous limits set by other experiments, establishes that theta 13 is not equal to zero, and in fact is relatively large compared with what was plausible in light of other recent results. A zero value for theta 13 would mean that electron neutrinos would not appear in beams of muon neutrinos or, in the Daya Bay case, that electron antineutrinos would not disappear by the time they reached the far detectors. Another reactor experiment, called KamLAND, has also registered the disappearance of antineutrinos over much larger distances, where the oscillation is described by the mixing angle theta 12, rather than theta 13.

"We are the first experiment that measures it and shows that it is nonzero," Heeger says of theta 13. "There have been recent indications, but none of the other results were significant enough to match what we physicists call a discovery." The Daya Bay group claims better than 5-sigma evidence in support of a nonzero value for theta 13. 5 sigma, or five standard deviations, implies that the finding has only a one-in-several-million chance of being caused by a statistical fluke.

A nonzero theta 13 is good news for physicists hoping to explore the differences between neutrinos and antineutrinos, should any such differences exist. (In fact, in another bit of neutrino slipperiness, it may be that neutrinos are their own antiparticles.) Any such differences may bear on the lingering question of why there is so much matter around, and so little antimatter, when both should have come out of the big bang on equal footing. One kind of matter–antimatter bias, a phenomenon known as CP violation, has been observed in other particles, and the new findings indicate that it might be demonstrable in neutrinos as well. "Theta 13 is the key parameter governing whether we can explore CP violation or not," says physicist Kam-Biu Luk of Lawrence Berkeley National Laboratory and the University of California, Berkeley, who serves as co-spokesperson for the experiment. "Now, with theta 13 being nonzero, there's a chance that we may find CP violation in the neutrino sector."

Future experiments, Luk says, should be able to investigate that possibility by sending neutrinos and antineutrinos from one lab to another, across hundreds of kilometers, to compare how they oscillate. "Certainly now, with our first result, it should give them a lot of the ammunition to push ahead," he says.

Fermilab physicist Rob Plunkett, co-spokesperson of the MINOS neutrino experiment, agrees that having a firm grasp on theta 13 is important to the field. "The number actually controls how much of other phenomena you may get," he says. "A large theta 13 is good, because it causes a lot of these other phenomena." Plunkett notes that other research groups, including his own, have been closing in on theta 13 and will continue to publish their findings to help improve the consensus estimate of its value. "I think that things are converging, but rather more quickly than people had anticipated," he says.