First Results From Japanese Neutrino Experiment Hint at Big Things to Come

by Adrian Cho 

Dead ringer. In the water-filled Super-Kamiokande detector (sketch, above), an electron neutrino triggers an interaction that produces a high-energy electron. The electron then radiates a telltale circle of light (event display, below).

Credit: T2K Collaboration

The headlines were certainly dramatic. "Unlocking the Mysteries of Antimatter" read one, "Japanese Neutrino Find Could Explain Why There is Matter in the Universe" proclaimed another. The truth is somewhat less momentous yet nonetheless exciting for particle physicists. Scientists working with a massive subterranean particle detector in Japan have directly observed a hoped-for phenomenon in which particles called muon neutrinos transform into others called electron neutrinos.

In itself, that preliminary observation tells physicists nothing new about the relationship between matter and antimatter. However, the effect appears to be so large that it suggests future experiments now in the planning may have a far easier time spotting other phenomena. That includes a possible asymmetry between the behavior of neutrinos and antineutrinos that could explain why the universe evolved to contain so much matter and so little antimatter.

"If this result holds up, it would mean that we've got kind of a fun decade ahead of us in neutrino physics," says Mark Messier, a physicist at Indiana University, Bloomington, and co-spokesperson for the NOνA neutrino experiment at Fermi National Accelerator Laboratory in Batavia, Illinois.

Neutrinos are nearly massless particles that hardly interact with anything else, so that billions stream through each of us every second. They come in three different "flavors"—electron neutrinos, muon neutrinos, and tau neutrinos—and a neutrino can morph from one flavor into another as it zips along at light speed. That phenomenon was discovered in 1998 by physicists working with the 22,500-ton Super-Kamiokande particle detector in the Kamioka Mine in central Japan. Scientists knew that all around Earth, cosmic rays crashing into the atmosphere produce copious muon neutrinos. And they found that the number of muon neutrinos coming from the sky directly above the detector was greater than the number coming through the ground below it. That proved that some of those making the longer trip through Earth had changed into something else along the way, making them undetectable.

Since then, such "neutrino oscillations" have been confirmed using neutrinos generated in the sun, in nuclear reactors, and in high-energy collisions at particle physics labs. But those experiments have all relied on watching one flavor of neutrino disappear as it turns into something else—the only exception being a particle detector called OPERA in an underground lab in Italy that last year detected one tau neutrino appearing in a beam of muon neutrinos fired through the ground from Switzerland.

Physicists would like to observe electron neutrinos emerging from a beam of high-energy muon neutrinos. The rate of that transformation would reveal the last of three "angles" that mathematically describe the way the three flavors mix into one another. This emergence is exactly what scientists with the T2K experiment at the Kamioka mine have observed. Starting in January 2010, the researchers fired muon neutrinos from the Japan Proton Accelerator Research Complex in Tokai 295 kilometers away into the Super-Kamiokande detector, blasting some 150 million trillion protons into a target to produce a similar number of muon neutrinos.

From all those muon neutrinos, the researchers found six that appeared to have changed into electron neutrinos. That's not quite enough to declare a discovery, as the physicists would expect about 1.5 events from spurious "backgrounds," as they explain in a paper submitted to Physical Review Letters. Still, the researchers have taken less than 2% of the data that they hope to take, so the fact that a signal has already emerged suggests that the sought-after mixing angle, known as θ₁₃, is large, says Chang Kee Jung, a physicists at Stony Brook University in New York state and international co-spokesperson for the T2K team.

Others seem to think the preliminary data is pretty strong. "If you were a betting man, you'd put money on this effect being real," says Carl Bromberg, a physicist at Michigan State University in East Lansing and a member of the NOνA team. And if θ₁₃ is large, then subsequent neutrino experiments will see large effects. For example, physicists now know that two of the neutrinos have nearly the same mass while one is either considerably heavier or lighter. NOνA aims to nail down that mass hierarchy, a task that will be far easier if the T2K result pans out.

What does any of this have to do with matter and antimatter? If neutrinos mix into one another at different rates than antineutrinos do, then it would violate a kind of symmetry between matter and antimatter called charge-parity symmetry, or CP. According to some theories, violations of CP among neutrinos may explain why the infant universe came to be filled with so much matter and so little antimatter. So physicists are keen to try to compare the mixing of muon neutrinos and electron neutrinos with the mixing of muon antineutrinos and electron antineutrinos. That experiment may be doable if θ₁₃ is large. Even so, that will likely take an even more massive experiment than either T2K or NOνA. "Even if that moves very quickly," Messier says, "it would be the end of the decade before it starts."