Takaaki Kajita of Japan and Arthur McDonald of Canada won the Nobel Prize in physics on Tuesday for discovering the "chameleon-like" nature of neutrinos, work that yielded the crucial insight that the tiny particles have mass.
The Royal Swedish Academy of Sciences said the two researchers had made key contributions to experiments showing that neutrinos change identities as they whiz through the universe at nearly the speed of light.
McDonald told a news conference in Stockholm by telephone that the eureka moment was when it became clear that his experiment had proven with great accuracy that neutrinos changed from one type to another in traveling from the sun to Earth.
The University of Tokyo said in a statement congratulating Kajita that he was one of the students of 2002 Nobel physics winner Masatoshi Koshiba, who also has contributed to Japan's neutrino research.