Deep in a Sudbury, Ont. mine, scientists have reached a critical milestone in their efforts to detect dark matter - the mysterious substance that makes up 85 per cent of matter in the universe.
Scientists working on the Super Cryogenic Dark Matter Search (SuperCDMS) experiment have successfully cooled their "refrigerator" to the temperature required for their superconducting detectors to become operational. For SuperCDMS, that temperature is just tens of milliKelvin, or thousandths of a degree, above absolute zero - about a hundred times colder than outer space.
"Reaching this base temperature now allows us to turn on the detectors, make sure they are all working and start collecting data that potentially is coming from dark matter particles hitting our detectors," says Miriam Diamond, a co-principal investigator in the international collaboration and an assistant professor in the University of Toronto's department of physics in the Faculty of Arts & Science.
Reaching base temperature marks a major transition for SuperCDMS - from construction and installation to commissioning and science operations.
The SLAC National Accelerator Laboratory serves as the lead laboratory, while the SuperCDMS experiment is housed at SNOLAB, a research facility located about two kilometres underground in an active nickel mine near Sudbury. This depth shields the experiment from cosmic rays and other background particles that could otherwise obscure faint signals.
The experiment is designed to detect dark matter particles that are already passing through Earth.
"Dark matter makes up about 75 per cent of the matter in our universe, with each galaxy like our own Milky Way galaxy embedded in a large dark matter cloud. But we don't know exactly what it is," explains Diamond, whose fellow co-principal investigators from U of T's department of physics are Assistant Professor Ziqing Hong and Professor Pekka Sinervo.
"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 among the first to explore this uncharted territory.
"Our experiment is able to have this level of sensitivity because we have worked very hard to eliminate all other possible sources that could mimic a dark matter particle hitting our detectors," says Hong.
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.
Cooling the experiment reduces thermal noise, the random motion of atoms that can mask faint signals.
"U of T has been taking a lead role in assembling the experiment and starting the operations to get down to base temperature," says Hong. "Our team of graduate students and postdoctoral fellows have been working both underground at SNOLAB and here at the university for the last three years to help make this happen. Reaching this milestone is a reflection of their expertise and commitment."
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.
The process involves multiple cooling stages. First, cooling from room temperature to 50 Kelvin, then down through four Kelvin, one Kelvin, and finally into the milliKelvin range. A separate cooling system chills the experiment's readout cables, preventing them from injecting unwanted heat or noise into the detectors.
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.
Once commissioning is complete, SuperCDMS will begin its first science run, which is expected to last about a year. Even the first few months of data could be enough to discover dark matter - if particles are around the mass of a proton and if they are attracted strongly enough to ordinary matter. Or it could reveal something entirely new.
Beyond dark matter, SuperCDMS will also allow scientists to study rare isotopes, probe feeble particle interactions with unprecedented precision and possibly uncover entirely new kinds of particle interactions.
The SuperCDMS collaboration consists of 100 researchers from 25 institutions located in the U.S., Canada, Germany, Spain and the UAE. It is jointly funded by the U.S. Department of Energy Office of Science, the U.S. National Science Foundation, the Canada Foundation for Innovation, the Natural Sciences and Engineering Research Council of Canada and the Arthur B. McDonald Institute (Canada).
With files from SLAC National Accelerator Laboratory
