Two kilometres beneath the Earth’s surface in a mine near Sudbury, Ont., scientists are searching for the enigmatic dark matter that constitutes 80 per cent of the material in the universe.
The Super Cryogenic Dark Matter Search, or SuperCDMS, is located in SNOLAB, one of the world’s deepest scientific laboratory. The experiment is being conducted by an international collaboration formed over ten years ago that includes physicists from the Faculty of Arts & Science’s Department of Physics.
“It's really fun going down into the mine to the lab,” says Madeleine Zurowski, a postdoctoral fellow in the department and a member of the group building the SuperCDMS detector. “It's definitely not a place I thought I'd find myself when I started studying physics — but it's really cool.”
The U of T team includes Zurowski, along with SuperCDMS co-principal investigators Professors Miriam Diamond, Ziqing Hong and Pekka Sinervo. Diamond works on computer simulations which predict what a detection will look like; Hong heads the team building the experiment’s cryogenic components; and Sinervo focuses on organizing the underground work, statistical modelling and data analysis. The team is rounded out by another four postdoctoral fellows and ten graduate students.
Dark matter is the hypothetical constituent of the universe which — as the name suggests — cannot be observed the way normal matter in stars, nebulae and planets can. In fact, physicists expect that millions or billions of dark matter particles pass through our bodies undetected every second.
Dark matter belies its presence only from the gravitational pull it exerts; when astronomers observe the speed of rotation of galaxies, they can infer that the galaxies contain much more matter than meets the eye.
According to Diamond, “Research in these two originally disparate fields — astrophysics and particle physics — converged a few decades ago when we realized that, while dark matter is not predicted or described by the Standard Model — which describes three of the four known forces in the universe and all of the known elementary particles — it’s discovery would lead to extensions or revisions to that theoretical framework.”
SuperCDMS is hidden underground to shield its super-sensitive detectors from the radiation from the sun and everything else in the cosmos that constantly bathes us in sub-atomic particles. What’s more, the researchers must minimize the radiation from nearby everyday objects — including themselves — because the experiment is so incredibly sensitive.
“People are radioactive, dust is radioactive — bananas are relatively very radioactive because they contain so much potassium,” says Zurowski. “Everything has a low level of radioactivity. These low levels don’t present any danger to us but if that radioactivity gets into the detector, it will drown out the extremely weak dark-matter signal.”
Hence, researchers shower before entering in the underground lab, wear personal protective equipment, and keep their equipment immaculately dust-free.
At the heart of SuperCDMS are ultra-sensitive germanium and silicon crystals. When a dark matter particle interacts with an electron or nucleus within a crystal, the detector signals the hit as a minute increase in temperature.
“We’re talking about a temperature change of a few thousandth of a degree,” says Hong. “So, in order to sense this tiny amount of heat, we first need to cool the detector down to very close to absolute zero. In effect, when a particle is detected, SuperCDMS is acting as a really, really sensitive thermometer.”
Physicists are searching for a variety of dark matter particles with varying properties and whimsical names: Weakly Interacting Massive Particles or WIMPs, dark photons, axions, right-handed neutrinos, and Lightly Ionizing Particles or LIPs. The strength of SuperCDMS is that, unlike other experiments, it is designed to detect more than one kind of dark matter particle.
“With SuperCDMS, we have the ability to detect dark photons in certain mass ranges, axion-like particles, LIPs, as well as low mass WIMP-like particles,” says Diamond.
How significant will a first-ever detection be by one of the teams hunting dark matter?
“It's hard to overstate how exciting the discovery would be,” says Zurowski. “We have literally no idea what dark matter is and the fact that we don't is so unsatisfying to me. It's like how so much of the ocean remains unmapped or unexplored even today. Finding this huge piece of the dark matter puzzle would be extremely exciting.”
The team and their collaborators continue to work on their experiment, embedded deep in the rock of the Canadian Shield, and aim to commence the search in 2025.
“I would call us optimistic, but humble,” says Diamond on the prospects of the success of SuperCDMS. “Humble in that we have to realize nature does not particularly care what we think. But we are optimistic and there’s no doubt it would be a huge leap forward in our understanding of the universe.”
“All we can do is work as hard as we have been,” say Hong. “And ask nature for data. Nature will tell us what it wants to tell us.”