An international collaboration, including Northwestern University, has reached a critical milestone in the search for dark matter, the mysterious substance that makes up about 85% of all matter in the universe.
Located two kilometers below ground in Canada, the Super Cryogenic Dark Matter Search (SuperCDMS) at SNOLAB has cooled to its operating temperature, the collaboration announced today.
Just thousandths of a degree above absolute zero, the cryogenic experiment is about 100 times colder than the temperature of deep space. This extreme cold enables scientists to eliminate thermal noise from vibrating atoms, potentially isolating dark matter's incredibly tiny signals.
With this milestone, the project transitions from building the experiment to preparing for the search. Researchers now can turn on the dark matter detectors, whose superconducting sensors only function when cooled to extremely low temperatures. If the equipment operates correctly, it should achieve the highest level of sensitivity yet for detecting low-mass particles, which have about half the mass of a single proton.
"Reaching this ultracold temperature means our experiment has crossed a major threshold," said Northwestern's Enectali Figueroa-Feliciano. "The detectors are now cold enough to operate, so we can begin calibrating them to prepare for the first search for dark matter. Detecting dark matter would not only reveal the identity of most of the mass of the universe, and it would likely be the key to a new realm of particle physics."
Figueroa-Feliciano is a professor of physics and astronomy at Northwestern's Weinberg College of Arts and Sciences and Northwestern's SuperCDMS lead. The collaboration is led by the Department of Energy's SLAC National Accelerator Laboratory.
The collaboration, which comprises 24 institutions, designed SuperCDMS to detect light dark matter particles, a category of dark matter particles that interact so weakly with ordinary matter that they have stealthily evaded direct detection. To catch these elusive particles, the experiment employs ultra-pure silicon and germanium crystals equipped with superconducting sensors. If a dark matter particle collides with one of these crystals, it should produce tiny vibrations and electrical signals that the sensors can detect.
But to recognize a real dark-matter interaction, scientists first must understand exactly how the detectors respond when particles hit them. That's where Northwestern's contribution comes in.
Northwestern and Fermi National Accelerator Laboratory (Fermilab) lead an experiment to measure how the detectors respond to known particle interactions - essential measurements for interpreting the data.
To perform these measurements, Figueroa-Feliciano and his team built the Northwestern Experimental Underground Site (NEXUS), located 106 meters below Fermilab. NEXUS' underground location and thick lead shield safeguards it from cosmic rays, which could interfere with tiny signals from dark matter particles.
Just last fall, Figueroa-Feliciano and his collaborators used NEXUS to investigate how radiation affects superconducting qubits, the fragile quantum devices used in quantum computers and advanced particle detectors. Now, the team uses the site's unique position underground, neutron beam and dedicated neutron detector to simulate interactions expected in the SNOLAB experiment.
"This combination allows NEXUS to use the neutrons from the beam as a stand-in for dark matter events in the detector," Figueroa-Feliciano said. "This special setup allows us to calibrate the detectors and measure a quantity called the ionization yield, which is essential to the dark matter analysis done at SNOLAB. Without these measurements, we would not be able to determine if a detected signal is from dark matter or ordinary background particles."
Beyond dark matter, SuperCDMS will allow scientists to probe previously inaccessible energy scales thanks to its unprecedented sensitivity, and maybe even uncover entirely new kinds of particle interactions.
"With many more sensors per detector than in the previous SuperCDMS Soudan experiment (in Minnesota), along with new simulation tools and AI-enabled reconstruction, the data will be far richer than we originally planned," said SLAC scientist Noah Kurinsky, who helped design the detectors. "Every day will be new; this is new science from day one."
The SuperCDMS SNOLAB experiment is a joint project of 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).
