A Canadian-led collaboration of physics researchers is getting ready to deploy a new dark matter detector deep underground in Sudbury, Ontario. The assembly, which includes the building of an enormous 17,000-kilogram vessel, is expected to be completed this December. Data collection should start a few months later, once the equipment calibration and tests have been completed.

The new experiment, named PICO-500, builds on decades of research and three previous iterations of this type of detector, known as a bubble chamber. PICO-500 is the largest and most sensitive yet.

Queen’s lead researcher for the PICO collaboration and scientific director of the Arthur B. McDonald Canadian Astroparticle Physics Research Institute, Anthony Noble (Physics, Engineering Physics, and Astronomy) says PICO-500 is a unique experiment, and the best in the world to detect spin-dependent interactions – one of the ways dark matter may interact with regular matter.

Dark matter is the key piece of a sophisticated theory explaining how our universe became what it is, including why galaxies move the way they do and why light from distant galaxies bends as if pulled by invisible mass. It is also one of science’s greatest mysteries: for a century, researchers have been trying to observe dark matter using ingenious detectors, out-of-this-world telescopes, and even attempting to produce dark matter within cutting-edge particle accelerators. There is unequivocal evidence for dark matter, which is known to comprise about 85 per cent of all matter in the Universe, but we have no idea what it is. With each new piece of equipment with ever increasing sensitivity, scientists hope to get closer to finally seeing it.

While traditional dark matter experiments around the world are looking to detect tiny amounts of energy produced by dark matter particles, PICO has been built to allow very weak interactions to trigger a fiery cascade of events.

The core part of the detector is a large synthetic quartz cylinder filled with 250 liters of refrigerant fluid (similar to the one used in household refrigerators). The liquid is maintained at a superheated state, meaning it is kept above its boiling temperature, but won’t spontaneously start to boil, unless it is disturbed by something – like a tiny little particle coming from space. Bubbles will then start to form and be captured by always-on cameras and microphones.

"Imagine a rock precariously balanced on the edge of a cliff. With just a little push, it would fall over and make a very large impression," explains Dr. Noble. "And that’s how PICO is set up. If dark matter hits the liquid within our bubble chamber, it will explode and boil immediately.  We will be able to see and hear the bubbles."

A tricky detail is that dark matter isn’t the only particle that could trigger such reaction. If bubbles do come up, how will researchers know what hit the fluid? Well, to start, PICO-500 is located at SNOLAB, a deep underground research laboratory. Two kilometers of solid rock above the lab shield the experiment from almost anything. Nevertheless, something other than dark matter could trigger the bubble chamber, and scientists are prepared to discern most probable background "noise," including radioactive elements, using visual and acoustic cues.

PICO-500 will run and collect data for five years.

The PICO experiment is located at SNOLAB, a world-leading deep underground research laboratory in Sudbury, 2km below the surface.

A history of collaboration and resilience

SNOLAB is named after the Sudbury Neutrino Observatory (SNO), built in the 1990s and famous for its groundbreaking results in solar neutrino science, recognized by the 2015 Nobel Prize in Physics. Since the original experiment, SNOLAB has evolved to become an even larger facility hosting several high-profile astroparticle physics experiments.

The PICO collaboration started in 2013 as a merger of the Canadian PICASSO experiment (created in the late 1990s and led by Viktor Zacek at the University of Montreal) and the U.S.-led COUPP experiment. It has since grown into a team of 60 researchers from 17 institutions in seven countries – a modest-sized program, if compared to other physics experiments operated by teams of hundreds to a thousand researchers.

PICO provides an ideal training opportunity for students and postdocs, who can get involved in all aspects of the experiment, from designing and building the detector to running it and doing the data analysis. "It’s very exciting to be part of something like this," says Derek Cranshaw, a Queen’s postdoctoral researcher participating in the experiment. "Finding dark matter would be a field-changing discovery."

Deploying a new experiment is also a refreshing opportunity for veterans like Dr. Noble, who has been searching for dark matter for over 25 years.

"Every now and then I have one of these existential crises where I wander down the hall and start talking to the astronomers and the theorists about why we should believe dark matter exists in a form detectable by terrestrial experiments," he says. "We have, of course, extremely strong astronomical evidence for its existence. Thinking of the field of physics as a whole, finally observing dark matter would solve many of our theoretical problems. We’re all motivated by that and, in the past century, we’ve seen many different ideas about what dark matter could be and where to look for it. There is an explosion of effort around the world to figure this out, and I’m excited to be part of it."