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 and Kajita will split the eight million Swedish kronor (almost $1.3 million Cdn) prize.
The academy said the two men won the prize for their contributions to experiments demonstrating that subatomic particles called neutrinos change identities, also known as "flavours." The neutrinos transform themselves between three types: electron-type, muon-type and tau-type.
The metamorphosis requires that neutrinos have mass, dispelling the long-held notion that they were massless. The academy said the discovery "has changed our understanding of the innermost workings of matter."
Kajita said his work was important because it showed there must be a new kind of physics beyond the so-called Standard Model of fundamental particles, which requires neutrinos to be massless.
Working at the Super-Kamiokande detector in Japan, Kajita, in 1998, showed that neutrinos captured at the detector underwent a metamorphosis in the atmosphere.
Meanwhile, researchers at the Sudbury Neutrino Observatory, as also known as SNOLAB, 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.
"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 fusion processes," McDonald said.
He added that scientists still want to know what the actual mass of the neutrino is, and whether there are other types beyond the three currently known.
McDonald said being named winner is a "very daunting experience, needless to say."
"Fortunately, I have many colleagues as well who share this prize 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 attempt to observe particles called dark matter particles. We'll have ten times better sensitivity than other experiments have had so far...and that may lead to another Eureka moment, 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 1960s until 1982, when he moved to Princeton University for seven years.
He has been at Queen's since 1989 and 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, in 1990 for discoveries about subatomic particles called quarks.
Bertram Brockhouse in 1994 for the development of an analytical technique called neutron spectroscopy.
Willard Boyle in 2009 for inventing the charge-coupled device (CCD) that makes digital 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.
An earlier version of this story said only one Canadian had won the Nobel Prize in Physics before Arthur McDonald. In fact, three Canadians had won it before McDonald did.Oct 06, 2015 10:59 AM ET