Dark matter mysteries (and our plan to solve them)

Dark matter mysteries (and our plan to solve them)

By Catarina Chagas

February 28, 2023


[A photo depicting the reflection of the photographer]
A photographer gets up close to a photomultiplier tube (PMT) in the SNO+ control room at SNOLAB.

It’s been almost 100 years since astronomers first observed that galaxies move in unexplainable ways. Scientists back then hypothesized this was due to the existence of dark matter, a type of matter that does not reflect or produce light (making it very hard to spot).

[Photo of Dr. Tony Noble]
Dr. Tony Noble, Scientific Director at the Arthur B. McDonald Canadian Astroparticle Physics Research Institute

We now know that dark matter is out there and it makes up a staggering 85 per cent of the universe. Its gravitational effects are thought to be the glue that holds galaxies together. But the absolute proof remains somewhat elusive as research groups worldwide have, for decades, tried to directly observe dark matter and study its properties. To this day, dark matter remains one of the biggest mysteries of science – one that astroparticle physicists are eager to uncover.

"Observing dark matter directly would confirm that it is a particle by nature and open a new window into understanding cosmology and the formation and evolution of structure in the universe," says Tony Noble, Queen’s professor and Scientific Director at the Arthur B. McDonald Canadian Astroparticle Physics Research Institute. "Establishing the properties of this new form of matter would help complete our understanding of the fundamental building blocks of nature."


A record of research success

Since the 1980s, Queen’s has invested in developing the research infrastructure and expertise needed to allow breakthrough discoveries in astroparticle physics, the area of science investigating the building blocks of the universe and the objects within it, including dark matter.

[People walking through the Vale mine to SNOLAB]
Dr. Nancy Ross, Vice-Principal (Research) accompanies Queen's Emeritus Professor and Nobel Laureate Arthur McDonald, Minister François-Philippe Champagne, local Members of Parliament, and SNOLAB administration on their way to the facility 2 km underground to announce the Canada Foundation for Innovation's 2022 Major Science Initiatives Fund. [Credit: Gerry Kingsley; CFI]

A fundamental step in this journey has been the creation of the Sudbury Neutrino Observatory (SNO), situated in a nickel mine in Sudbury, Ontario and led by Queen’s in partnership with other Canadian and international research groups.

Twenty years ago, SNO obtained its first groundbreaking results. The team, led by Queen’s professor Arthur McDonald, was investigating neutrinos originating in the core of the Sun. They proved that neutrinos have mass – a discovery that changed our understanding of the Sun and of the universe. For this, in 2015, Dr. McDonald was co-awarded the Nobel Prize in Physics.

[Photo of Dr. Jodi Cooley]
Dr. Jodi Cooley, Executive Director at SNOLAB

SNO has since evolved from a single experiment to a major international facility, SNOLAB, and is now the deepest clean lab in the world. The underground location provides an ideal environment for highly sensitive experiments, such as the hunt for dark matter.

"The 2 kilometres of rock above the facility shield the equipment from cosmic rays that constantly hit Earth’s surface, reducing the ‘noise’ in the experiments, and ensuring they capture the specific particles and interactions the researchers want to observe," says Jodi Cooley, Executive Director at SNOLAB and Queen’s professor. Because dark matter doesn’t have any appreciable interaction with ordinary matter like rocks, researchers believe it will just pass through the Earth.


Building the future of dark matter research

[Photo of the SNO+ neutrino detector]
The 12-metre diameter acrylic SNO+ detector is filled with 780 tonnes of liquid scintillator and surrounded by 10,000 photomultiplier light detectors. [Credit: Dr. Alex Wright for the SNO+ Collaboration]

One of the challenges of cutting-edge research like dark matter science is building the best equipment to support the experiments. It’s an ongoing task, and SNOLAB has a facility dedicated to testing each component of the dark matter detectors. Now is an exciting time: the next generation experiments are up and running, constantly collecting data.

SNOLAB uses different approaches in the search for dark matter. One of them, for example, uses a bubble chamber – a container filled with refrigerant fluid that creates a bubble whenever interacting with a particle. Another experiment uses liquid argon, that responds to interactions emitting ultraviolet light. A third has a copper vessel filled with a noble gas, producing measurable electric charges when hit by a particle.

[Photo of a researcher adjusting a coil in SNOLAB]
Data from the detectors is transferred to lab computers through a temperature cooling process. [Credit: Jonathan Corbett]

"Having multiple experiments employing a variety of techniques is essential as the nature of dark matter interactions is unknown, and they may interact differently according to the nature of the target," explains Dr. Noble. "In addition, with a signal that occurs so rarely, having different techniques will enable results from one experiment to be confirmed by another where the possible backgrounds mimicking the signal will be vastly different in the disparate approaches."

Together with dark matter research, neutrino science remains a top priority for SNOLAB and for physics research overall. Just as scientists have been invested in observing dark matter particles, they are excited about examining phenomena like neutrinoless double beta decay, a rare but theoretically possible occurrence that, if confirmed, will revolutionize our understanding of how the universe evolved.


International effort for broad impact

Groundbreaking scientific discoveries take time, resources, and expertise. While Queen’s and Canada have established leadership in astroparticle physics, these experiments are international and leverage world-class infrastructure to achieve global impact.

"This area of science thrives on international cooperation because it is very expensive to do," says Dr. Cooley. "It takes large teams and multiple countries to make the necessary investments to get to the big discoveries. Canada has made the strategic decision to invest in SNOLAB and in world-leading astrophysics talent at Canadian universities, and this has placed us at the foreground of underground science today."

[Researcher atop the SNO+ neutrino detector]
Dr. Peter Skensved is the only person to ever access the top of the acrylic SNO+ detector in order to perform a small modification to its surface. [Credit: Dr. Mark Chen]
[Photo of Dr. Arthur McDonald]
Dr. Arthur B. McDonald, Nobel Laureate and Queen's Emeritus Professor

Everything at SNOLAB illustrates the importance of collaboration. Experiments combine the work of more than 1,100 researchers from 164 research institutions in 24 countries and across six continents. Under Queen’s leadership, Canadian and international universities and research institutes are working together, benefitting from support from both government and private industry – starting with Brazilian mining company Vale, who owns the mine in which SNOLAB operates and provides invaluable support so all the 150 staff members can focus on providing a clean lab for its science program.

In 2016, with $63.7 million in support from the Canada First Research Excellence Fund, Queen’s led the creation of the McDonald Institute, the largest network for astroparticle physics in the country. Recently, in August 2022, the Canada Foundation for Innovation announced $102 million in infrastructure funding for SNOLAB.

"These investments create the conditions wherein the international community is attracted to Canada to benefit from the world’s premier facility, SNOLAB, and where the scientific teams at Canadian universities are taking significant leadership roles on major international experiments in this field," says Dr. Noble.

Images taken by Queen's researchers showing behind the scenes at SNOLAB are courtesy of the Queen's Art of Research photo contest.


Multidisciplinary facility

Beyond astroparticle physics research, SNOLAB hosts experiments and research projects in other areas, such as biology and nuclear science. SNOLAB is one of the places on Earth with the least radioactivity, allowing groups to study what happens with our cells in the total absence of radiation. When it comes to nuclear energy, the SNOLAB team is working on how to develop nuclear detectors to verify compliance with the Comprehensive Nuclear Test Ban Treaty. Soon, SNOLAB might also facilitate research on quantum computing, in a way that will both advance the field of quantum technology and support the development of quantum sensors to further the potential of SNOLAB’s dark matter experiments.

Arts and Science