SuperCDMS

The international SuperCDMS (Super Cryogenic Dark Matter Search) collaboration is attempting to make the first direct detection of a dark matter particle by sensing the tiny vibrations caused by its collision with ordinary matter.

Based at SNOLAB, SuperDMS is one of five TRIUMF-SNOLAB collaborations involving some of the world’s most advanced experiments in DM detection (also DEAP-3600) and neutrino science (SNO+, nEXO and HALO). SNOLAB is the deepest underground laboratory in North America, based two-kilometers under Sudbury, Ontario.

The origin and nature of dark matter is one of the great mysteries in cosmology, astrophysics and particle physics. Through detailed observations of the gravitational behaviour of galaxies, scientists have inferred the existence of dark matter and that it makes up about a quarter of cosmic mass. Baryonic matter, the stuff of the Periodic Table – including people, planets and stars – makes up less than 5% of the cosmos.

As Earth orbits through the Milky Way galaxy, physicists believe we move through a sea of dark matter particles, though there’s never been a direct detection of one.

The SuperCDMS experiment is designed to detect dark matter by measuring the collision energy imparted to a germanium or silicon atom nucleus in a collision with a dark matter particle. The experiment will be particularly sensitive to particles with masses less than a few times that of a proton.

The detection and characterization of dark matter would revolutionize particle physics and cosmology. By confirming aspects of beyond-Standard Model theories, it would guide the work of generations of physicists, paving the way to new physics and its practical applications.

Similarly, non-detection by SuperCDMS will significantly narrow the possible characteristics of hypothesized dark matter particles.

SuperCDMS is a next-generation, scaled-up stage of the long-term international CDMS collaboration. Initially based at Stanford University’s Underground Facility and then the Soudan mine in Minnesota, USA, the project is moving to SNOLAB to benefit from the cosmic ray shielding provided by the greater depth. SuperCDMS is expected to begin measurements in 2022.

Physicists theorize that dark matter particles have a small and measurable cross-section with regular matter: if they get close enough to one another they’ll physically interact. Thus, SuperCDMS is uniquely designed to capture the tiny vibrations of such a dark matter-baryonic matter interaction, with the lowest energy threshold for the detection of such a collision.

To achieve this, the SuperCDMS experiment relies on three key components: specialized germanium and silicon crystal detectors; a powerful cryogenic system; and an environment shielded from background particle noise both from the surroundings and the experimental apparatus itself.

1. SuperCDMS Germanium Detectors

The SuperCDMS collaboration is pioneering the use of low-temperature, germanium and silicon detectors in the search for collisions between dark matter particles and atomic nuclei.

The new SuperCDMS detector modules consist of a collection of puck-shaped 3.3 cm high and 10 cm in diameter germanium and silicon crystals. When a particle collides with a germanium or silicon nucleus it will create vibrations (phonons) in the crystal lattice which are absorbed by sensors and turned into an electrical signal, providing information on the interaction energy.

In addition, another set of sensors will collect any electrical charge produced by the interaction. The coincident measurement of charge and phonons separates collisions of normal matter from those of dark matter since the latter doesn’t interact electromagnetically. SuperCDMS will also use a second type of phonon-only detector able to detect interactions depositing much lower amounts of energy. The sensitivity of the experiment will be optimized by combining the information from the different detector types and materials.

2. Cryogenic cooling

In order to distinguish a dark matter particle interaction from background vibrations, the detector crystals will be cooled to just a few tenths of a degree above absolute zero (0 Kelvin). This extreme cooling greatly reduces the background thermal vibrations of the germanium and silicon crystals.

To keep the detectors cold, they are placed in the centre of a cryostat which consists of a set of nested copper cans attached through a cold-finger to graded thermal stages of a dilution refrigerator, each layer shielding the lower one inside it.

3. Reducing background detections

SuperCDMS is based at SNOLAB to shield the experiment from the detection of high-energy cosmic rays which would cause background noise and might mimic dark matter detections. Similarly, the entire SuperCDMS detector is surrounded by tanks of ultra-pure water and encased in separate layers of lead and polyethylene shielding to further shield the detectors from environmental radiation, such as neutrons from radioactive decays (for example, radon) in the surrounding rock. Finally, SuperCDMS is constructed from ultra-pure materials to minimize radiation from the structure itself.

SuperCDMS and TRIUMF

As part of SuperCDMS, TRIUMF is involved in the installation, commissioning and operation of a Cryogenic Underground TEst facility (CUTE) located next to the SuperCDMS setup.

As part of the cryogenics program, a dilution refrigerator system will be installed at TRIUMF for SuperCDMS research and development activities, potentially including detector characterization, testing of new detector concepts, operating modes or calibration concepts. And TRIUMF is supporting tentative plans by SuperCDMS to install a muon or neutron veto detector during the next major upgrade of the experiment.

TRIUMF is also heavily involved in the University of British Columbia-led development of the new MIDAS-based data acquisition software for SuperCDMS.