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The Nobel Prize-winning work

This neutrino breakthrough resolved a long-standing puzzle in the Standard Model of particle physics: far fewer neutrinos were detected on Earth than the number predicted to come from the sun. There are three known types, or “flavours,” of neutrinos: electron, muon, and tau. Earlier detectors could largely register only electron neutrinos, and since the sun was thought to produce only that flavour, the shortfall appeared to suggest something was wrong in theory.

As director of the subterranean Sudbury Neutrino Observatory (SNO) in Sudbury, Ontario, McDonald led a team that built one of the most sensitive neutrino detectors ever constructed. The SNO detector is the size of a ten-storey building and sits two kilometres underground in an ultraclean laboratory, considered one of the world’s lowest-radioactivity locations. By 2002, data collected by the international SNO Collaboration members showed that many of the neutrinos from the sun had changed into the other two flavours by the time they reached Earth. This finding explained the mystery of the missing neutrinos and confirmed that neutrinos have mass, because such a transformation is possible only if neutrinos have mass. Solving that problem opened the door to deeper mysteries that McDonald says remain at the forefront of physics and are now being explored at an expanded underground facility called SNOLAB.

“We are studying existential questions, not just how humans evolved but how everything evolved,” says McDonald. “Even my six-year-old granddaughter asks where it all came from, and I tell her about the Big Bang, the stars, and the long chain of events that led to us. Human curiosity has always reached for these answers, and we continue to uncover fascinating new insights that help refine our questions and advance our knowledge of the cosmos.”

From discovery to new questions

Since that discovery, scientists have had to rethink some major cosmic questions. The neutrino results ruled out a theory that dark matter, an invisible substance never directly observed and known only through its gravitational effects on galaxies and other cosmic structures, could be comprised of neutrinos.

“Once neutrinos were found to have mass, it soon became clear they did not have nearly enough to account for all the dark matter theorized to exist,” says McDonald, who is retired but remains closely involved in SNOLAB’s work. “Several of our experiments are now searching for dark matter with different techniques, and we have an ideal location for that research.”

Neutrinos may not explain dark matter, but they could help explain why any matter exists at all. According to the Standard Model, the Big Bang should have produced equal amounts of matter and antimatter. When these counterparts encounter one another, they annihilate into pure energy. By that logic, nothing should have remained. Yet matter endured, and scientists suspect neutrinos may be part of the reason. The success of SNO led to SNO+, an evolution of the detector with a new aim.

“SNO+ is an excellent repurposing of the detector,” says McDonald. “It is designed with greater sensitivity to search for a very rare radioactive decay that could reveal more about the properties of neutrinos and help determine what happened to all the anti-matter after the Big Bang.”

The implications of this research extend beyond cosmology. By studying the neutrino-producing reactions that power the sun, scientists are gaining insights that are helping advance the global effort to harness fusion energy on Earth as a clean and reliable source of power.

Expertise gained in SNOLAB’s ultra-low-radioactivity environment is inspiring advances beyond physics. The lab has contributed to innovations in water purification, material science, and medical imaging technologies like positron emission tomography (PET) scanners.

The underground facility is also used to test quantum computing and sensing devices, where shielding from cosmic rays makes it possible to eliminate flaws caused by them at the surface.

Building Canada’s physics leadership

These days, very few astroparticle physics experiments at SNOLAB or other Canadian sites take place without involvement from the Arthur B. McDonald Canadian Astroparticle Physics Research Institute, based at Queen’s. Named in honour of the Nobel laureate and directed by Queen’s professor Tony Noble, the institute connects scientists across the country, strengthening national collaboration and fostering international projects that push the field forward.

“Canada is home to extraordinary scientific infrastructure, but it’s the people who transform those resources into discovery,” says Noble. “The McDonald Institute brings together some of the most curious, driven, and talented researchers, engineers, technicians, and students to take on the some of the most complex scientific questions, and to build collaborations that shape the breakthroughs of tomorrow.”

The McDonald Institute launched in 2016 with a $63.8 million federal investment that supported major new hires. It has since partnered with 11 universities and six research institutes, helping to double the number of faculty in Canada active in astroparticle physics from 22 to 48, twelve of whom are at Queen’s. McDonald Institute also secured a Canada Excellence Research Chair, a competitive national program that recruits international field-leading researchers. This growth has positioned Canada and the McDonald Institute at the forefront of shaping the future of particle astrophysics.

“Tony’s insight and creativity have led the development of the Institute since its inception and we are very grateful for the work he has done to establish astroparticle physics as an area in which Canada is a world leader,” says McDonald.

Projects linked to the McDonald Institute are also building momentum across Canada. The $55 million US-Canada SuperCDMS collaboration is searching for dark matter particles at SNOLAB using semi-conductor detectors, the P-ONE project is building a massive neutrino telescope underwater in the Pacific Ocean, and the PICO experiment, where Noble plays a leading role, is hunting for dark matter using a bubble chamber filled with superheated liquid that captures rare particle interactions.

Students are a major part of this success too. Graduate students, postdoctoral fellows, and undergraduate students at Queen’s, along with those working with McDonald Institute partners, are gaining the skills to tackle complex problems and lead innovation. Drawn from Canada and abroad, this young, highly skilled community represents the next generation of researchers. Many go on to careers at universities, research labs, government agencies, and in sectors such as finance and technology worldwide, where they apply their training in problem solving, critical thinking, and evidence-based decision making to lead and innovate.

The institute also engages communities across Canada in STEM through camps and school visits, with a focus on promoting equity and broadening access to science education.

In recognition of its impact, the Government of Canada recently awarded the McDonald Institute $45.5 million to expand its research and training programs, a major investment that strengthens both national capacity and Queen’s leadership in astroparticle physics. The university community will celebrate the investment and the 10-year anniversary of the Nobel Prize on Oct. 17, 2025.

Together, Queen’s, the McDonald Institute, and affiliate institutions nationwide, are ensuring Canada continues to play an outsized role in deepening our understanding of the Sun, the distant stars, and everything in between.