2015 Nobel Prize in Physics: Canadian Arthur B. McDonald shares win with Japan's Takaaki Kajita

Arthur McDonald, a professor emeritus at Queen's University in Kingston, Ont., is the co-winner of the 2015 Nobel Prize in Physics.

McDonald will share the prize with Takaaki Kajita of the University of Tokyo.

The winners were announced by a committee at the Royal Swedish Academy of Sciences in Stockholm on Tuesday.McDonald andKajitawill split the eight millionSwedishkronor(almost$1.3 million Cdn) prize.

The academy said the two men won the prize for theircontributions to experiments demonstratingthat subatomic particles called neutrinos change identities, also known as "flavours." The neutrinos transform themselves between three types: electron-type, muon-type and tau-type.

Themetamorphosis requires that neutrinos have mass, dispelling the long-held notion that they were massless.The academy said thediscovery "has changed our understanding of the innermost workings of matter."

• Arthur McDonald, Nobel Prize winner: the man, neutrinos, why his work matters

Kajita said his work was important because it showed theremust be a new kind of physics beyond the so-called StandardModel of fundamental particles, which requires neutrinos to bemassless.

Working at the Super-Kamiokandedetector in Japan, Kajita, in 1998, showed thatneutrinoscaptured at the detector underwent a metamorphosis in the atmosphere.

Meanwhile, researchers at theSudbury Neutrino Observatory, as also known asSNOLAB, were looking at neutrinos that come from the sun.McDonald, who has been director of the observatory since 1989, discovered in 2001 that those neutrinos from the sun also changed their identities.

'Eureka moment'

"Yes,there certainly was a Eureka moment in this experiment when we were able to see that neutrinos appeared to change from one type to the other in travelling from the Sun to the Earth," McDonald told reporters by telephone from his home in Kingston.

Neutrinos, along with quarks and electrons, are the most basic particles that make up matter "particles that we don't know how to subdivide any further," McDonald told CBC News in an interview. Among those three types of basic particles, neutrinos are the hardest to detect.

McDonald managed to study them with a detector the size of a 10-storey building deep underground at SNOLAB, where most other particles that could cause false signals can't penetrate.

"By our measurements we were able to confirm with great accuracy calculations of how the sun burns by fusionprocesses," McDonald said.

He added thatscientists still want to know what the actual mass of the neutrino is, and whether there are other types beyond the threecurrently known.

McDonald said being named winneris a "very daunting experience, needless to say."

"Fortunately, I have many colleagues as well who share thisprize with me," he added.

McDonald is retired from teaching, but he is still involved in research.

"In fact, we're just about to turn on an experiment to attemptto observe particles called darkmatter particles. We'll have ten times bettersensitivity than otherexperiments have had so far...andthat may lead to another Eurekamoment, we hope," he said in an interview with CBC's Heather Hiscox.

Born in Sydney, N.S., in 1943, McDonald earned bachelor's and master's degrees from Dalhousie University. He got his PhD in physics from California Institute of Technology in 1969.

He worked for Atomic Energy of Canada from the late 1960suntil 1982, when he moved to Princeton University for seven years.

He has been at Queen's since 1989and has been a professor emeritus since 2013.

McDonald was invested as an Officer of the Order of Canada in 2008.

Three other Canadians have previously won the Nobel Prize in Physics:

• Richard Taylor, in1990 for discoveries about subatomic particles called quarks.

• Bertram Brockhousein1994 for the development of an analytical technique called neutron spectroscopy.

• Willard Boyle in2009 for inventing the charge-coupled device (CCD)that makesdigital cameras work.

On Monday the 2015 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.