"A new search for dark matter 6800 feet underground"

Prototype tests of the future SuperCDMS SNOLAB experiment are in full swing.

From left: SLAC's Tsuguo Aramak, Paul Brink and Mike Racine are performing final adjustments to the SuperCDMS SNOLAB engineering

When an extraordinarily sensitive dark matter experiment goes online at one of the world’s deepest underground research labs, the chances are better than ever that it will find evidence for particles of dark matter—a substance that makes up 85 percent of all matter in the universe but whose constituents have never been detected.

The heart of the experiment, called SuperCDMS SNOLAB, will be one of the most sensitive detectors for hypothetical dark matter particles called WIMPs, short for “weakly interacting massive particles.” SuperCDMS SNOLAB is one of two next-generation experiments (the other one being an experiment called LZ) selected by the US Department of Energy and the National Science Foundation to take the search for WIMPs to the next level, beginning in the early 2020s.

“The experiment will allow us to enter completely unexplored territory,” says Richard Partridge, head of the SuperCDMS SNOLAB group at the Kavli Institute for Particle Astrophysics and Cosmology, a joint institute of Stanford University and SLAC National Accelerator Laboratory. “It’ll be the world’s most sensitive detector for WIMPs with relatively low mass, complementing LZ, which will look for heavier WIMPs.”

The experiment will operate deep underground at Canadian laboratory SNOLAB inside a nickel mine near the city of Sudbury, where 6800 feet of rock provide a natural shield from high-energy particles from space, called cosmic rays. This radiation would not only cause unwanted background in the detector; it would also create radioactive isotopes in the experiment’s silicon and germanium sensors, making them useless for the WIMP search. That’s also why the experiment will be assembled from major parts at its underground location.

A detector prototype is currently being tested at SLAC, which oversees the efforts of the SuperCDMS SNOLAB project.

Colder than the universe

The only reason we know dark matter exists is that its gravity pulls on regular matter, affecting how galaxies rotate and light propagates. But researchers believe that if WIMPs exist, they could occasionally bump into normal matter, and these collisions could be picked up by modern detectors.

SuperCDMS SNOLAB will use germanium and silicon crystals in the shape of oversized hockey pucks as sensors for these sporadic interactions. If a WIMP hits a germanium or silicon atom inside these crystals, two things will happen: The WIMP will deposit a small amount of energy, causing the crystal lattice to vibrate, and it’ll create pairs of electrons and electron deficiencies that move through the crystal and alter its electrical conductivity. The experiment will measure both responses.

“Detecting the vibrations is very challenging,” says KIPAC’s Paul Brink, who oversees the detector fabrication at Stanford. “Even the smallest amounts of heat cause lattice vibrations that would make it impossible to detect a WIMP signal. Therefore, we’ll cool the sensors to about one hundredth of a Kelvin, which is much colder than the average temperature of the universe.”

These chilly temperatures give the experiment its name: CDMS stands for “Cryogenic Dark Matter Search.” (The prefix “Super” indicates that the experiment is more sensitive than previous detector generations.)

The use of extremely cold temperatures will be paired with sophisticated electronics, such as transition-edge sensors that switch from a superconducting state of zero electrical resistance to a normal-conducting state when a small amount of energy is deposited in the crystal, as well as superconducting quantum interference devices, or SQUIDs, that measure these tiny changes in resistance.

The experiment will initially have four detector towers, each holding six crystals. For each crystal material—silicon and germanium—there will be two different detector types, called high-voltage (HV) and interleaved Z-sensitive ionization phonon (iZIP) detectors. Future upgrades can further boost the experiment’s sensitivity by increasing the number of towers to 31, corresponding to a total of 186 sensors.

Completed fabrication of four SuperCDMS SNOLAB iZIP detectors at the Stanford Nanofabrication Facility (SNF).

Four SuperCDMS SNOLAB iZIP detectors at the Stanford Nanofabrication Facility

Matt Cherry

SLAC's Tsuguo Aramaki is inspecting an electrical readout section of the SuperCDMS SNOLAB engineering tower.

Electrical readout section of SNOLAB Engineering Tower

Paul Brink

Dilution fridge's mixing chamber manufactured by BlueFors Cryogenics and installed in Building 33 at SLAC.

Mixing chamber for the dilution fridge manufactured by BlueFors Cryogenics and installed in Building 33 at SLAC

Paul Brink

Dilution fridge manufactured by BlueFors Cryogenics installed in Building 33 at SLAC.

Dilution fridge

Paul Brink

Mechanical test fit of the SuperCDMS SNOLAB engineering tower at the dilution fridge test facility in Building 33 at SLAC.

Mechanical test-fit assembly of SNOLAB Engineering Tower into dilution fridge test facility in Building 33 at SLAC

Paul Brink

SuperCDMS SNOLAB enginnering tower being prepared for the installation into the dilution fridge test facility in Building 33 at

SNOLAB Engineering Tower prepared for installation into dilution fridge test facility

Paul Brink

SLAC's Tsuguo Aramaki is completing the assembly of the dilution fridge test facility in Building 33 at SLAC.

Tsuguo Aramaki completes the assembly of the dilution fridge test facility at SLAC.

Paul Brink

SLAC's Mike Racine is inspecting the SuperCDMS SNOLAB engineering tower, which has been installed into the dilution fridge test

Mike Racine of SLAC inspects the SNOLAB Engineering Tower installed in the dilution fridge test facility.