The US Department of Energy Office of Science and the National Science Foundation have approved funding for the SuperCDMS (Cryogenic Dark Matter Search) SNOLAB experiment, a hunt for light dark matter particles expected to start in the early 2020s.
Canada’s SNOLAB will host the experiment, with cryogenic and management support contributed respectively by DOE’s Fermi National Accelerator Laboratory and SLAC National Accelerator Laboratory. “We are eager to resume our search for dark matter particles and explore an entirely new region of their possible interactions with normal matter,” says Fermilab scientist Dan Bauer, spokesperson of the SuperCDMS collaboration.
SuperCDMS SNOLAB will be at least 50 times more sensitive to low-mass dark matter particles than CDMS, its predecessor at the Soudan Underground Laboratory, a Fermilab-led experiment that ended operation in 2015. SLAC is managing the SuperCDMS SNOLAB construction project for the international SuperCDMS collaboration of 111 members from 24 institutions in five countries.
Scientists have long known that ordinary matter accounts for only 15 percent of all matter in the universe. The rest is a mysterious substance called dark matter. Due to its gravitational pull on regular matter, dark matter is a key driver in the formation of galaxies (like our Milky Way) and is considered fundamental to our very existence.
However, researchers don’t know what particles make up dark matter. Astronomical observations suggest that dark matter particles barely interact with normal matter in the universe, though they could collide with an atom of our visible world every so often. Dark matter researchers are looking for these rare interactions, but they are difficult to spot in the presence of background interactions of normal matter particles from cosmic rays or small amounts of radioactivity in the environment.
In the SuperCDMS SNOLAB experiment, researchers will use silicon and germanium crystals in which collisions will trigger tiny vibrations. To measure the atomic jiggles, the detectors will need to be cooled to less than -459.6°F, a fraction of a degree above absolute zero temperature.
The experiment will be assembled and operated 6,800 feet underground at the Canadian laboratory SNOLAB—inside the Vale Creighton nickel mine near the city of Sudbury—where it will be protected from high-energy particles, called cosmic radiation, that can create unwanted background signals.
“SNOLAB is really looking forward to supporting the science delivery of the SuperCDMS SNOLAB experiment at our deep underground facility within the Vale Creighton mine, helping SuperCDMS SNOLAB fulfill its full potential as a leading dark matter search experiment,” says Nigel Smith, executive director of SNOLAB. “We were delighted to hear of the successful award of the DOE/NSF funding to SuperCDMS SNOLAB, and, with existing infrastructure support from the Canada Foundation for Innovation, look forward to the start of construction of SuperCDMS SNOLAB and deployment in Sudbury.”
Fermilab is responsible for the design and fabrication of the cryogenics system to produce the very cold temperatures required to operate the detectors. The design is based on that used for previous generations of the experiment but incorporates novel features that eliminate the need for large quantities of expensive liquid helium. The large copper vessels that will house the detectors must be as pure as possible to avoid radioactive backgrounds, and these must be shielded from environmental backgrounds by layers of lead, plastic and water. The cryogenics system must also be designed for remote operations, since the underground laboratory is not always accessible.
“It is quite a challenge to design such a large cryogenics system to reach such cold temperatures using only materials that are nearly free of radioactivity and without constant access to the system,” says Fermilab physicist Matt Hollister, main designer of the cryogenic system for SuperCDMS SNOLAB.
Fermilab engineers are also designing and fabricating electronics to control and read signals from the detectors. These electronics must have extremely low levels of noise in order to distinguish the small detector signals created by low-mass dark matter particles. The system for the new experiment is very compact, replacing a much larger chain of electronics modules from previous generations of CDMS. “Reaching the low levels of electronic noise required for SuperCDMS is an ongoing challenge,” says Fermilab engineer Sten Hansen. “The levels we have achieved are due to the hard work of many throughout the collaboration.”
The response of the detectors to known particles must be understood to calibrate their expected response to dark matter particles. The Fermilab team is designing a system to allow the detectors periodic exposure to particle sources for such calibration. “Previous generations of CDMS had to be calibrated manually,” says Fermilab scientist Lauren Hsu. “The new experiment is designed so that calibration can be done more frequently and easily.”
In addition to leading overall construction, SLAC National Accelerator Laboratory is building and testing the germanium and silicon detectors, and Pacific Northwest National Laboratory is helping to minimize and understand backgrounds for the experiment, a major challenge for the detection of faint signals from weakly interacting massive particles, also known as WIMPs, a candidate for dark matter.
Institutions in the United States, Canada, UK, France and India also play key roles in the experiment, working on all aspects of the experimental hardware as well as data analysis and simulation. The largest international contribution comes from Canada and includes the research infrastructure at SNOLAB.
“We’re pleased to be working closely with SNOLAB to manage the installation of this experiment,” says Fermilab scientist Pat Lukens, deputy project manager for SuperCDMS. “We’re eager to see the results of this next phase in the hunt for dark matter particles.”