Like a magician pulling a rabbit from a hat, an international team of physicists has made one kind of neutrino appear from a beam of another type of neutrino. Their odd observation clinches the case that these elusive particles can transform into another in a process called neutrino oscillations, an idea that had gained wide acceptance in recent years but had not been directly observed.
In the past, physicists inferred the oscillation from the apparent disappearance of neutrinos in certain situations, explains Antonio Ereditato of the University of Bern, who is spokesperson for the 170 physicists working with the 1800-ton OPERA particle detector in the subterranean Gran Sasso National Laboratory in central Italy. But they wanted to confirm the effect by actually seeing another type of neutrino appear. “We know there was a murder, now we want to find the body,” Ereditato says.
Neutrinos may be the oddest of subatomic particles. Uncharged and almost massless, they emerge from a certain type of radioactive decay and some nuclear reactions, but they interact with matter so feebly that they’re nearly undetectable. Even stranger, they come in three types—electron neutrinos, muon neutrinos, and tau neutrinos—that physicist believe can transform into one another.
For example, nuclei of boron-8 in the sun decay by spitting out an antielectron and an electron neutrino, and theorists can predict the number of such low-energy solar neutrinos. Researchers measured the actual number in the 1960s, counting rare events in which a chlorine nucleus in a tank of dry-cleaning fluid absorbed an electron neutrino and emitted an electron. They found only one-third as many electron neutrinos as predicted, suggesting that the particles were turning into something else during their trip from the sun to Earth.
Further evidence came in 1998, when researchers measured neutrinos generated in the atmosphere. When cosmic rays crash into the air, the particle interactions should produce about twice as many muon neutrinos as electron neutrinos. But physicists in Japan found far fewer muon neutrinos, suggesting that they had changed into another type, presumably tau neutrinos. Still, nobody had actually seen neutrinos switch from one type to another.
Now, the OPERA team at Gran Sasso has found evidence that muon neutrinos do indeed become tau neutrinos. The team uses a beam of muon neutrinos generated 730 kilometers away at the European particle physics laboratory, CERN, near Geneva, by blasting a proton beam into a chunk of metal. Flying in the same direction as the original proton beam, the high-energy neutrinos whiz through the ground to Gran Sasso, and every second, a trillion high-energy neutrinos flow into OPERA's detector, which consists of 150,000 “bricks” of lead sheets interleaved with sheets of photographic emulsion. A lead nucleus can absorb a muon neutrino and eject a muon, a heavier cousin of the electron that leaves a straight track in a brick’s films. A system of detectors pinpoints a hit brick, which is then removed and analyzed robotically.
If, however, the muon neutrino first turns into a tau neutrino, the lead nucleus will spit out a tau lepton, an even more massive cousin of the electron. The tau lepton lasts only long enough to travel a couple of millimeters before it decays into something else, creating a telltale kinked track. So far, the team has analyzed one-third of the data they’ve collected since 2008, and among a total of 6000 neutrinos, they have found one tau neutrino, Ereditato says. It's the corpse he had hoped to find.
“It’s a tremendous achievement,” says Maury Goodman, a neutrino physicist at Argonne National Laboratory in Illinois, although he adds, “It’s probably what everyone expects.” The next step for the OPERA team will be to observe enough events to see if the rate of oscillation agrees with predictions. It would be even more exciting if it doesn’t, Ereditato says, as that would present scientists with their next mystery.