An international team of physicists has achieved a critical milestone in the search for dark matter, cooling the SuperCDMS experiment at SNOLAB to just thousandths of a degree above absolute zero. This ultracold state enables the experiment to detect faint signals from elusive dark matter particles, which make up about 85% of all matter in the universe.
The SuperCDMS (Super Cryogenic Dark Matter Search) experiment, located two kilometers underground in Canada’s SNOLAB, has reached an ultracold operating temperature, a breakthrough that marks a pivotal step in the hunt for dark matter. This temperature—thousandths of a degree above absolute zero—allows the experiment to eliminate thermal noise, enabling it to detect the faintest signals from dark matter particles, which interact so weakly with ordinary matter that they have stealthily evaded direct detection.
The Ultracold Breakthrough
The experiment’s success hinges on its ultra-pure silicon and germanium crystals, which are equipped with superconducting sensors. These sensors can detect tiny vibrations and electrical signals produced if a dark matter particle collides with the crystals. However, achieving this sensitivity required the team to cool the detectors to an extremely low temperature, a feat accomplished through a combination of advanced cryogenics and shielding from cosmic rays.
Reaching this ultracold temperature means our experiment has crossed a major threshold,
said Enectali Figueroa-Feliciano, a professor of physics and astronomy at Northwestern’s Weinberg College of Arts and Sciences and Northwestern’s SuperCDMS lead. The detectors are now cold enough to operate, so we can begin calibrating them to prepare for the first search for dark matter.
Calibration and Collaboration
To ensure the detectors can distinguish dark matter signals from background noise, the team uses a unique setup at the Northwestern Experimental Underground Site (NEXUS), located 106 meters below Fermilab. NEXUS employs a neutron beam to simulate dark matter interactions, allowing researchers to measure how the detectors respond to known particle interactions. This calibration is critical for interpreting data from SNOLAB, where the true dark matter search will take place.
This combination allows NEXUS to use the neutrons from the beam as a stand-in for dark matter events in the detector,
Figueroa-Feliciano explained. 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.
A New Era of Sensitivity
The upgraded SuperCDMS experiment features more sensors per detector than its predecessor, the SuperCDMS Soudan experiment in Minnesota, along with advanced simulation tools and AI-enabled data reconstruction. These improvements will generate richer data, potentially revealing new particle interactions or confirming the existence of light dark matter particles, which have about half the mass of a single proton.
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 Broader Implications
Dark matter, which makes up about 23% of the universe, remains one of the greatest mysteries in physics. While visible matter accounts for about 4%, the gravitational effects of dark matter are evident in galaxy rotational velocity curves and gravitational lensing. The search for dark matter has spurred a range of experimental approaches, including the Axion Dark Matter Experiment (ADMX), which aims to convert axions into microwave photons using a strong magnet.
PNNL scientists contribute to the SuperCDMS Collaboration, an international research effort based on experiments that deploy detectors highly sensitive to ionization and phonon signals from collisions of dark matter with atomic nuclei in silicon and germanium crystals.
The next phase of the experiment involves turning on the detectors and beginning the first search for dark matter. Researchers will analyze the data to determine if any signals originate from dark matter particles or from ordinary background particles. The results could redefine our understanding of the universe’s composition and open new avenues in particle physics.
However, uncertainties remain. The experiment’s success depends on the existence of light dark matter particles, which have not yet been observed. If no signals are detected, the team may need to refine their methods or explore alternative theories. For now, the ultracold milestone represents a significant step forward, but the true test lies ahead.
