SuperCDMS SNOLAB cools down to near absolute zero, setting the stage for one of the world’s most sensitive dark matter searches

Deep underground in Canada, a refrigerator about a hundred times colder than outer space – designed to detect dark matter, the mysterious substance that makes up 85% of all matter in the universe – has just reached a critical milestone.

Super Cryogenic Dark Matter Search (SuperCDMS) SNOLAB – a second-generation dark matter search experiment – is an international collaboration of 24 institutions, with the Department of Energy’s SLAC National Accelerator Laboratory serving as the lead laboratory.

Scientists working on SuperCDMS have successfully cooled the experiment to its base temperature, the temperature required for the superconducting detectors to become operational. For SuperCDMS, that temperature is just tens of millikelvins, or thousandths of a degree above absolute zero.

The SuperCDMS experiment is housed at SNOLAB, a research facility located about 2 kilometers (roughly 6,800 feet) underground in an active nickel mine near Sudbury, Ontario. This depth shields the experiment from cosmic rays and other background particles that could otherwise obscure the faint signals scientists are searching for. 

The experiment is designed to detect dark matter particles that are passing through Earth.

“We know from astrophysical observations that the Milky Way sits inside a halo of dark matter,” Stifter said. “Dark matter is going through us all the time. Our challenge is to build a detector quiet and sensitive enough to notice when one of those particles interacts.”

SuperCDMS will be sensitive to dark matter particles that weigh so little that their tiny interactions with normal matter have so far escaped direct detection.  The experiment will be the first to explore this uncharted territory.

“We expect world-leading sensitivity between about half a proton mass and five times the proton mass,” Stifter said. “That’s a region not many searches have really explored before.”

At the heart of SuperCDMS are detectors made from ultra-pure silicon and germanium crystals, each about the size of a hockey puck. When a dark matter particle strikes one of these crystals, it produces a tiny vibration called a phonon, along with a small electrical signal. To detect those minuscule signals, the crystals are outfitted with superconducting sensors that only work when they are extremely cold.

“The detectors simply don’t function unless they’re cold enough to enter the superconducting transition,” said SLAC scientist Richard Partridge, who manages the experiment’s installation. “For us, that means roughly 15 to 30 millikelvins.”

Cooling the experiment reduces thermal noise, the random motion of atoms that can mask faint signals. 

“When everything is that cold, the crystals are basically quiet,” Partridge said. “Even very small energy deposits become detectable.”

Reaching base temperature is the culmination of years of preparation and months of detailed planning. Over the last year, the team developed a step-by-step cooldown plan, working closely with cryogenics experts responsible for different parts of the system.

“It’s more complicated than just hitting the 'go' button and watching the temperature drop,” Stifter said. “For the past two years, we’ve been installing the experiment in anticipation of this moment.”

With base temperature achieved, the collaboration has now moved into detector commissioning, a months-long process of turning on, calibrating and optimizing each detector channel.

“We have 24 detectors, each with multiple readout channels,” Stifter said. “Commissioning is about making sure as many of them as possible behave the way we expect.” 

Once commissioning is complete, SuperCDMS will begin its first science run, expected to last about a year. Even the first few months of data could be enough to discover dark matter particles – if they have masses near that of a proton and interact frequently enough with ordinary matter to be detected.