AILEA, HAWAII--The latest data about neutrinos have begun to eliminate competing theories about the nature of the elusive particles. Presented here last week at the first joint meeting of the nuclear physics divisions of the Japanese Physical Society and the American Physical Society, the findings mark the beginning of a new phase in understanding these mysterious particles. "It's a whole new frontier now," says Kevin Lesko, a neutrino physicist at Lawrence Berkeley National Laboratory in Berkeley, California.
The frontier has remained untamed mainly because neutrinos seldom interact with matter. But once in a while, a neutrino signals its presence. For instance, if a type of neutrino known as an electron neutrino strikes a deuterium atom in the 1000-ton sphere of heavy water at the Sudbury Neutrino Observatory (SNO) in Ontario, it splits the atom and leaves a telltale flicker of light. By gathering statistics on neutrinos, observatories such as SNO and Super-Kamiokande in Kamioka, Japan, have provided strong evidence that neutrinos oscillate from electron neutrinos to mu and tau neutrinos. (ScienceNOW, 18 June). That can happen only if the neutrinos have mass, a question that the standard model of particle physics leaves open.
With this big question all but settled, scientists are now zeroing in on an important quantity called the "mixing angle," which determines just how the neutrinos oscillate. The mixing angle has profound effects on neutrinos' behavior; for example, matter affects neutrinos with a small mixing angle much more than neutrinos with a large mixing angle. Most theorists had preferred small mixing angles, mainly because particles called quarks have them.
To solve the mixing-angle question, scientists at Super-K checked whether neutrinos coming from the sun behaved differently by day and at night, when they have to pass through Earth on their way to the detector. If the mixing angle were small, the physicists expected to see a distinct day-night variation. They didn't, says Yoichiro Suzuki of the Kamioka Observatory. Combined with data from other experiments, the results imply that the mixing angle is in fact large, says Suzuki. Although it's not an open-and-shut case, small mixing angles are clearly in trouble, he says.
The upshot, Lesko says, is that neutrino physics is coming of age. "We're getting a theoretical understanding of what must be, and we will have to expand the standard model to account for neutrinos."