ALBUQUERQUE, NEW MEXICO--The publicity-shy particles known as neutrinos are back in the spotlight. At a joint meeting here of the American Physical Society and the High Energy Physics Division of the American Astronomical Society, physicists released much-anticipated measurements of the flow of neutrinos from the sun and other sources. The results put the final nail in the coffin of the decades-old solar neutrino paradox and eliminate a once-favored assumption about a key property of the particles.
Neutrinos are incredibly hard to detect because they seldom interact with matter; they can zip through Earth without noticing it. When they do trigger physicists' detectors, not all neutrinos are equally easy to catch. Neutrinos come in three "flavors," named after the subatomic particle that each is associated with: the electron neutrino, the muon neutrino, and the tau neutrino. Electron neutrinos are the easiest to detect, because they participate in reactions involving the very common electron; tau and muon neutrinos are hard to spot. That reticence seemed to explain a puzzling deficit of electron neutrinos created in the sun's nuclear furnace: If electron neutrinos changed flavors into muon or tau neutrinos, they could escape detection.
On 21 April, scientists from the Sudbury Neutrino Observatory (SNO) in Ontario revealed their measurements of a reaction, called the "neutral current," in which a neutrino of any flavor slams into deuterium, a heavy isotope of hydrogen, and breaks it apart into a neutron and a proton. By measuring the relative ratios of electron, muon, and tau neutrinos coming from the sun, SNO scientists saw that solar neutrinos, which began their journey to earth as electron neutrinos, have changed into muon or tau neutrinos by the time they reach the detector. "This is strong evidence for flavor change," says team member Andre Hamer of the Los Alamos National Laboratory.
Not only do the new measurements provide the strongest evidence yet for flavor change, but they also pin down the properties of neutrinos with unprecedented precision. In addition, the results show that another property of neutrinos related to how they interact with matter, known as the "mixing angle," must be large rather than small, contrary to what physicists believed until quite recently. Although the case isn't closed, says neutrino physicist Bruce Berger of Lawrence Berkeley National Laboratory in Berkeley, California, "it's extremely unlikely, given current measurements, that it's a small mixing angle." Little by little, the inscrutable neutrino is finally revealing its secrets.