Not all detectives wear trench coats and carry a notepad and magnifying glass to solve mysteries.
Gilles Gerbier is using a helium-filled copper sphere, containing a tiny ball at the centre attached to a rod, to search for an elusive signal from an enigmatic, invisible particle that might rule the universe.
Wolfgang Rau is using detectors made of germanium and silicon crystals, cooled close to absolute zero, to detect tiny increases in temperature that may indicate a rare, very weak interaction with this elusive particle, which has yet to be found.
Tony Noble is using the world’s most sophisticated bubble chamber, filled with a superheated, fluorocarbon fluid, to look for a bubble formation pattern that signifies fluorine interacting with this extraordinary particle – one that doesn’t shine like regular matter but is the most abundant form of matter in the universe.
These three Queen’s particle astrophysics researchers are detectives and leaders in the international hunt for dark matter, which makes up about 85 per cent of matter in the universe, although no one knows what dark matter particles look like or their physical properties. Gerbier, Rau and Noble are each directing or playing key roles in large collaborative teams of Canadian and international researchers conducting three competing, but complementary experiments that use different tools to seek, find and ultimately understand the nature of dark matter.
Mystery of the missing mass
Dark matter is one of the great mysteries of physics and the cosmos. The first evidence for dark matter emerged in the 1930s from calculations by astronomers that clusters of galaxies in our universe are moving so fast that the gravitational pull generated by their observable matter could not possibly hold them together. Astronomers then found that stars also move much faster around galactic centres than expected given their observable mass.
The hunt for dark matter is a quest to find the missing mass of the universe. “We’re searching for the dominant matter in the universe. Without that matter, galaxies would fall apart, and the world would not be conducive to sustaining life as we understand it. If we want to understand why we’re here or why the universe works the way it does, dark matter is a fundamental component,” says Rau, leader of the Canadian research teams in the international SuperCDMS (Cryogenic Dark Matter Search) collaboration.
Queen’s researchers are at the forefront of a global search for the most promising candidate for a dark matter particle, called a Weakly Interacting Massive Particle (WIMP). They are attempting to distinguish it from other weakly interacting particles, like neutrinos, which have very little mass, and do not explain the missing mass of dark matter. Queen’s, through its collaborations with SNOLAB and the McDonald Institute, has the largest university research group investigating dark matter in Canada and globally, with about 40 research scientists, post-doctoral fellows and students working in the area. Their findings to date in setting new detection limits to narrow the search for dark matter follow many cutting-edge discoveries in particle astrophysics, including the ground-breaking research at SNOLAB by Queen’s Professor Emeritus Arthur McDonald, co-recipient of the 2015 Nobel Prize in Physics, demonstrating that neutrinos have mass.
New tools to probe unexplored subatomic space
In recent years, Canada and Queen’s researchers have leapt to centre stage in the global hunt for dark matter by moving forward with plans to build and launch three next-generation detectors, which will operate 6,800 feet (2,100 metres) underground at SNOLAB, outside Sudbury, Ontario. SNOLAB is the world’s deepest clean lab, and its great depth provides excellent shielding from background cosmic rays that interfere with dark matter signal detection.
The three science sleuths will be using new tools with much greater detection capabilities and sensitivities to probe new territory in parameter space that has never been searched before, and pursue the most promising leads to solve the case of the missing mass in the universe.
Gerbier, a world-leading astrophysicist from France known for pioneering new techniques aimed at detecting dark matter, joined Queen’s as the prestigious Canada Excellence Research Chair (CERC) in Particle Astrophysics in 2014. He’s leading the NEWS-G (New Experiments with Spheres-Gas) project, an entirely new type of spherical gaseous detector he and a French colleague developed to be sensitive at the unexplored lower end of the particle mass range, comparable in mass to a proton. (The mass of a proton is about one atomic mass unit, or a mass in energy units of about 938.27 million electron volts.) It began operating mid-2019. “Our aim with NEWS-G is to explore new territory by looking at lower mass ranges than other experiments,” says Gerbier, who has built a large international collaboration with European, U.S. and Canadian researchers, based at Queen’s.
Gerbier and Rau are also working together prepare for the next generation of the SuperCDMS experiment, which is operated by a collaboration involving more than 100 researchers from the US, Canada, Europe and Asia and is moving from an underground lab in Minnesota to the much deeper SNOLAB facility. It’s scheduled to launch in 2020. “What’s exciting is that we’re building a new generation of cryogenic detectors optimized to detect low-mass dark particles, with lower background and much better sensitivity for dark matter interactions than our previous detectors,” says Rau.
Tony Noble is leading the PICO 500 project, a new, much larger bubble chamber detector that will begin running at SNOLAB in 2020. The PICO collaboration, which includes researchers from 17 institutions in Canada, the US, Europe and India, leads the world in setting new limits and narrowing the search for spin-dependent dark matter particle interactions.
PICO 500 will push the limits for detection of spin-dependent interactions with its much greater sensitivity and enhanced discrimination capabilities. “With this suite of three large international experiments, we’re trying to cover a wider range of possibilities to detect dark matter particles and their interactions,” explains Noble, also director of the Arthur B. McDonald Canadian Astroparticle Physics Research Institute (McDonald Institute). “Our experiments each have a sweet spot where we’re most sensitive. If one type of experiment detects a signal that appears to be dark matter, we’ll need to confirm the signal is truly dark matter with a different experiment.”
Unraveling the universe
Dark matter is the elusive, cosmic glue that holds everything in the universe together. These three detectives are devising and deploying super-sensitive detection tools deep underground to crack the code of dark matter’s distinctive particle signature. Their advanced detection tools – ultra high-tech versions of Sherlock Holmes’ magnifying glass – may also lead to practical commercial spinoff applications. Gerbier’s gaseous spherical detector, for example, is being developed to measure specific types of radiation levels at nuclear facilities more cheaply and safely than existing devices.
On a cosmic scale, unlocking the secrets of dark matter particles will undoubtedly change the Standard Model of particle physics used by scientists to explain how the universe works. Discovering the unknown properties of the missing mass could also provide exciting new clues to help predict the size, shape and fate of the universe in the future.
One Billion Neutrinos
Arthur B. McDonald Canadian Astroparticle Physics Institute
The Arthur B. McDonald Canadian Astroparticle Physics Research Institute is a national hub for astroparticle physics research, uniting researchers, theorists, and technical experts within one organization.
Queen’s University led 13 Canadian institutions in creating the centre’s predecessor organization in 2015. The McDonald Institute, officially launched in 2018, works to enhance Canada’s global leadership in the field, which includes dark matter and neutrino research.