STOCKHOLM (AP) – Takaaki Kajita of Japan and Arthur McDonald of Canada won the Nobel Prize in physics on Tuesday for discovering that tiny particles called neutrinos change identities as they whiz through the universe, proving that they have mass.

By uncovering the ``chameleon-like'' nature of neutrinos, the laureates had solved a long-standing puzzle in particle physics that could alter our grasp of the cosmos, the Royal Swedish Academy of Sciences said.

``The discovery has changed our understanding of the innermost workings of matter and can prove crucial to our view of the universe,'' the academy said.

Kajita, 56, is director of the Institute for Cosmic Ray Research and professor at the University of Tokyo. McDonald, 72, is a professor emeritus at Queen's University in Kingston, Canada.

Asked how he felt when he realized Tuesday that his work was suddenly going to receive the world's focus, McDonald said, ``It's a very daunting experience, needless to say.''

Neutrinos are miniscule particles created in nuclear reactions, such as in the sun and the stars.

For decades the neutrino remained a hypothetical particle until American researchers proved that it was real in 1956.

There are three kinds, or flavors, of neutrinos and the laureates showed they oscillate from one flavor to another, dispelling the long-held notion that they were massless.

Kajita showed in 1998 that neutrinos captured at the Super-Kamiokande detector in Japan underwent a metamorphosis in the atmosphere, the academy said.

Three years later, while working at the Sudbury Neutrino Observatory in Canada, McDonald found that neutrinos coming from the sun also switched identities.

``A far-reaching conclusion of the experiments is that the neutrino, for a long time considered to be massless, must have mass,'' the academy said.

McDonald said that scientists would still like to know the actual masses of the various forms of neutrino. And experiments are looking at whether there are other types of neutrinos beyond the three clearly observed.

The idea that neutrinos could transform from one type into another was first put forward by the Italian physicist Bruno Pontecorvo in the late 1950s, but scientists' understanding of the process was rather vague until Kajita announced his discovery in 1998, said Antonio Ereditato, director of the Albert Einstein Center for Fundamental Physics at the University of Bern, Switzerland.

``This was a big shock because he proved in a statistically significant manner ... that neutrinos oscillate,'' said Ereditato. ``Then Art McDonald explored another channel using solar neutrinos. It came after Kajita but he also proved neutrino oscillation in another channel. The two deserved this award.''

Neutrinos are the second most abundant particles in the universe after photons, ``so any property of neutrinos can have dramatic repercussions on the life of the universe and on its evolution,'' he said. ``This is really one of the milestones in our understanding of nature.''

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. 

The winners will split the 8 million Swedish kronor (about $960,000) prize money. Each winner also gets a diploma and a gold medal at the prize ceremony on Dec. 10.

On Monday the Nobel Prize in medicine went to scientists from Japan, the U.S. and China who discovered drugs that are now used to fight malaria and other tropical diseases.

The prize announcements continue with chemistry on Wednesday, literature on Thursday, the Nobel Peace Prize on Friday and the economics award next Monday.