A Canadian scientist who helped solve a mystery surrounding one of the fundamental particles that make up the universe will receive the prestigious Benjamin Franklin Medal at a ceremony Thursday.

Queen's University physicist Art McDonald, the director of the Sudbury Neutrino Observatory (SNO) Institute, will share the Benjamin Franklin Medal for Physics with Yoji Totsuka from the University of Tokyo.

Queen's University professor Art McDonald has been working in the field of particle physics for over 40 years. He'll be honoured Thursday for his role in discovering how neutrinos have mass and oscillate between different states as they travel long distances.
(CBC)

They are being honoured for discovering that the three known types of neutrinos can change into one another when travelling long distances and that they have mass.

Neutrinos belong to the same group of particles as electrons, although they are much smaller and carry no electromagnetic charge. Instead, they are involved in the weak nuclear force — which shows itself during the decay of atomic nuclei.

They are an essential part of the Standard Model of physics and help explain everything from radioactive decay to the processes that occurred during the Big Bang.

It's not the first award McDonald has received for the discovery, but it is the most prestigious, he told CBC News Online.

Previous winners of the Benjamin Franklin Medals — which date back to 1824 — include Albert Einstein, Alexander Graham Bell and Orville Wright, with more than 100 Franklin winners going on to win Nobel Prizes.

"It's one of the highest awards, and it means a lot to me and my team," said McDonald. He will receive the award at a ceremony in Philadelphia on Thursday.

He and his team at SNO solved a 30-year-old problem that had plagued physicists studying neutrinos, the case of the so-called "missing solar neutrinos."

Theories held that the nuclear processes at work in the sun should produce a certain number of electron neutrinos. But testing revealed the number of electron neutrinos reaching the Earth from the sun was always smaller than the models suggested.

To study the particles, McDonald and his team had to conduct their tests in a laboratory two kilometres below the surface in Creighton Mine in Sudbury, Ont. While other particles like cosmic rays can't penetrate the rocky underground, the chargeless neutrinos pass through matter and so could be studied in isolation.

What the researchers discovered was that the solar electron neutrinos weren't disappearing; they were changing into either of the two other types of neutrinos, the tau neutrino or the muon neutrino, each of which had a slightly different velocity.

This change in velocity between the three particles also implied neutrinos have mass, refuting one of the original predictions of the Standard Model and confirming some theories.

The neutrino is extremely small. The largest neutrino has a mass about 1,000 times smaller than that of an electron.

"We set about trying to understand this puzzle and in the process we revealed something about both the sun and one of the basic particles of physics," said McDonald.

He said it has been particularly rewarding to see other scientists take the new information and make connections to other areas of physics, such as the high-energy processes that are thought to have taken place during the first moments of the universe.

"It's been great fun," said the native of Sydney, N.S., who has been involved in particle physics since graduating from Dalhousie in 1964.

"This field inspires you to participate in continual learning."