McDonald, Arthur Bruce (1943-…), a Canadian physicist , won a share of the 2015 Nobel Prize in physics for his discovery that tiny particles called neutrinos have mass . He shared the prize with the Japanese physicist Takaaki Kajita . Neutrinos are so small that they pass readily through solid matter , moving near the speed of light. Scientists had doubted that neutrinos have mass. McDonald and Kajita independently helped confirm a remarkable property of the neutrino. As the particle moves through space, it can oscillate (shift back and forth) among three different types. For neutrinos to oscillate, they must have mass.
McDonald was born on Aug. 29, 1943, in Sydney, Nova Scotia. He attended Dalhousie University in Halifax, Nova Scotia. In 1969, he received his Ph.D. degree in physics from the California Institute of Technology in Pasadena. In 1989, McDonald became the director of the Sudbury Neutrino Observatory near Greater Sudbury , Ontario. This neutrino detector was built in an underground mine. Neutrinos pass readily through the overlying rock, enabling the facility to isolate their signals from those of other particles.
Physicists refer to three types or flavors of neutrinos. Each flavor is generated by a different physical process. In the sun’s core, nuclear reactions create one flavor, electron-neutrinos. Since the 1960’s, however, physicists had been puzzled by the fact that the sun only seemed to generate about a third as many neutrinos as predicted by theory.
Earlier neutrino detectors could only sense one particular flavor of neutrino. The Sudbury detector, in contrast, could sense signals from all three flavors of neutrinos, while isolating signals from electron-neutrinos. In 2001, McDonald’s team confirmed that the total number of neutrinos streaming from the sun matched theoretical predictions, but only a third of them were electron-neutrinos.
Because the sun’s core only generates electron-neutrinos, McDonald’s team concluded that about two-thirds of these particles must have oscillated into other flavors of neutrinos on their long journey to Earth. Oscillation is described by the field of quantum mechanics. In quantum mechanics , particles exist as “waves of probability,” with properties that can vary over space and time. McDonald’s carefully gathered data fit the quantum theory of neutrino oscillation. It thus showed that at least two flavors of neutrino have mass, although the exact amount and nature of their mass remains a mystery. Kajita gathered similar data showing the oscillation of muon-neutrinos generated in Earth’s atmosphere.
