The elusive neutrino has just become a little less mysterious. At a press conference in Japan on 5 December, physicists announced the first results from an experiment that measures particles streaming away from Japanese nuclear reactors. The new data show that antineutrinos behave just like their mirror opposites, neutrinos. The results eliminate certain nagging doubts about other experiments that use neutrinos from the sun.
Neutrinos are damnably hard to study--they're the least social of the beasts in the particle zoo. Because they seldom deign to interact with matter, they tend to pass through Earth without pausing at all. But over the past few years, neutrino hunters have managed to trap neutrinos with underground detectors, usually vast tubs of material surrounded by sensors. Based at Kamioka, Japan, the KamLAND detector fits that profile, but it is unlike most other modern neutrino experiments, which detect neutrinos coming from the sun and from the atmosphere. KamLAND is designed to spot antineutrinos that are created by nuclear reactors that dot the Japanese and Korean landscape. When a proton in the detector's bath is struck by an electron antineutrino, both particles change identities. The proton becomes a neutron, and the antineutrino becomes an antielectron. The scientists detect the flashes caused by the newborn antielectron and neutron and are able to conclude that an antineutrino has met its demise.
After about 150 days of observations, the KamLAND detected a mere 54 electron antineutrinos when about 87 were expected--a significant deficit that implies that the electron antineutrinos change into muon or tau antineutrinos, just as electron neutrinos from the sun change into muon or tau neutrinos. Because the antineutrinos are humanmade rather than naturally created by the sun, physicists no longer need to worry about whether incorrect assumptions about the sun would mess up their conclusions. For example, there was a slight possibility that magnetic fields could flip the spins of neutrinos, says John Learned, a KamLAND collaborator at the University of Hawaii, Manoa.
The study "dots the i's and crosses the t's for the interpretation of what happens with solar neutrinos. It's an incredible achievement," says John Bahcall, a physicist at the Institute for Advanced Study in Princeton, New Jersey. Furthermore, showing that neutrinos and antineutrinos behave the same way shows that physicists understand the basic symmetries between matter and antimatter.