Kajita, Takaaki (1959-…), a Japanese 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 Canadian physicist Arthur B. McDonald . Neutrinos are so small that they pass readily through solid matter , moving near the speed of light . Scientists had doubted that neutrinos have mass. Kajita and McDonald 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.
Kajita was born on March 9, 1959, in Higashimatsuyama, near Saitama, Japan. He received a Ph.D. degree from the University of Tokyo in 1986. Afterward, Kajita worked at the university’s Institute for Cosmic Ray Research, at an underground complex called the Super-Kamiokande detector . The detector was built deep underground in an unused mine near Gifu. Neutrinos pass readily through the overlying rock, enabling the underground 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. The Super-Kamiokande detector was designed to sense muon-neutrinos, which are created when high-energy particles called cosmic rays strike Earth’s atmosphere. The detector could also sense from which direction the muon-neutrinos came.
In 1998, Kajita’s research team confirmed a puzzling imbalance in the detector’s measurements. The number of muon-neutrinos coming from directly above the detector roughly matched the number predicted by theory. But far fewer muon-neutrinos from the opposite side of Earth were striking the detector. In theory, the number of muon-neutrinos produced in Earth’s atmosphere should be roughly the same from all directions.
Kajita concluded that the neutrinos from the other side of Earth were not missing. Instead, they had oscillated into other flavors. 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. Because muon-neutrinos from the opposite side of Earth had traveled farther, the properties of other neutrino flavors had a greater chance to arise in the particle’s wave function.
Kajita’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 remained a mystery. McDonald gathered similar data showing the oscillation of electron-neutrinos generated by the sun.
