Rebooting the search for dark matter

Rebooting the search for dark matter

Astrophysicists in Sudbury are hoping to find the missing mass of our universe
July 8, 2009
The 17-metre-wide SNO detector in Vale Inco's Creighton Mine.

Ernest Orlando, Lawrence Berkeley National Laboratory

“This is just a regular day at the office,” says researcher Chris Jillings as a two-kilometre-long cable lowers an elevator down into a maze of sweltering passageways leading to the “clean rooms” at the Sudbury Neutrino Observatory, or SNOLAB. “But every so often, I realize this is all really very cool.” 

You’d think a nickel mine is the last place astrophysicists would look for clues to the nature of the universe. But now, during the International Year of Astronomy, the iconic facility that was built to track neutrinos is being expanded to help lead the worldwide search for dark matter, possibly answering the biggest mystery in the heavens.

As if straight out of a science fiction novel, the 17-metre-wide SNO sphere resembles a giant bug’s eye. The device, which sits deep in Vale Inco’s Creighton Mine, has been drained of the thousand tonnes of heavy water that were used to detect nearly mass-less particles called neutrinos, which can tell us how stars work, how they die and what comprises the universe. After seven years of poking around in the dark, SNOLAB researchers proved that neutrinos do have some mass.

The original SNO detector apparatus will soon be refitted to do next-generation experiments on neutrinos, which will investigate more about the nature of neutrinos. But another attraction in the tunnels these days is DEAP (Dark Matter Experiment with Argon and Pulse-Shape Discrimination), an ambitious project that will attempt to identify dark matter.

Poised atop a three-storey metal catwalk and staircase, Jillings leads the way down into the “cube hall.” By 2011, the 15-by-15-by-18-metre space will house an acrylic container filled with 3,600 kilograms of liquid argon, the centrepiece of the DEAP 3600 experiment.

A graphic of the DEAP-3600 detector.
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A graphic of the DEAP-3600 detector.
SNOLAB

“We’re using argon because to find dark matter, you want a large, clean detector that’s easy and cheap to purify,” says Jillings, standing within the outline that indicates where the gigantic tank will receive particles of the universe’s “missing mass.” The team will be looking for a theoretical kind of dark matter called Weakly Interacting Massive Particles (WIMPs), which, if received, will scatter like billiard balls in the tank, exciting the argon and emitting light. The challenge will be discerning the light emitted by WIMPs from that emitted by neutrons, since they emit the same light as they hurtle into argon.

 “It’s like listening for a whisper at a rock concert back up at the surface,” says SNOLAB’s associate director Fraser Duncan. But underground, there’s much less interference. “Down here, we have 50 million times fewer cosmic rays to get in the way.”

It’s that precious silence from muons (particles similar to electrons, but with more mass) and their associated radiation that makes this latest SNOLAB project world-class. Finding dark matter by eliminating all the background noise would be the science coup of the decade for Canada. “Our Holy Grail is to get rid of all cosmic rays and shield from ground-level radiation,” says Duncan. “The most radioactive thing down here right now is the coffee.”

If all goes well, the DEAP team claims it will be able to identify a few WIMPs interacting in their detector over the course of a year. Finding even one WIMP would make history: no one has ever detected dark matter, and more than a dozen facilities around the world are trying.

“We think that dark-matter particles are out there and that they’re massive,” says Jillings. “But they’re still elusive. If it was easy to just re-create them in particle accelerators, for example, we would have seen them by now.”

The Cube Hall to house DEAP-3600 experiment.
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The Cube Hall to house DEAP-3600 experiment.
SNOLAB

So what if the team actually discovers dark matter? “We'll review our findings to absolutely convince ourselves it isn’t background noise, write a paper and drink a bottle of Veuve Clicquot,” says Jillings. “Then we’ll ask ourselves, ‘What do we want to understand next?’”

Five things you need to know about dark matter

  1. We don’t really know what it is.
  2. Whatever it is, recent measurements in cosmology from satellites and ground-based observations all agree that it comprises 25 percent of all the mass and energy in the universe — more than all the elements we know. Hydrogen, helium and all the other less plentiful elements constitute only 5 percent of the mass of the universe.
  3. There is less dark matter than “dark energy,” which could make up 70 percent of the mass of the universe.
  4. Dark matter could be what dictates the speed and rotation of galaxies and makes galaxies spin flat.
  5. No one has ever measured dark matter.

SNO FACTS

  • Each return elevator ride to the lab and back costs $175.
  • All SNOLAB staff receive the same training for work at Inco as any mining contractor would.
  • There are three kinds of neutrinos: electron, muon and tau.
  • Copper coils wound around the interior walls of the cavity that contain the SNO detector create a magnetic field that counteracts the Earth's magnetic field making the detector more efficient. It is equivalent to adding another 1,000 light sensors to the detector.

Sidebar: “You had me at HALO…”

It sounds like a ”first-person shooter” video game, but HALO (Helium and Lead Observatory), another project at SNOLAB, is an early-detection system for supernovas. Although unlikely in the near future, a supernova just a few dozen light-years away could flood Earth with lethal doses of radiation.

“A burst of neutrinos comes from a star’s core 30 minutes to 10 hours before a supernova explosion,” says HALO project scientist and Laurentian University professor Clarence Virtue. “Our project is networked with other neutrino-sensitive supernova detectors looking for such outbursts. In other words, they corroborate each other’s stories.”

As with the original SNOLAB neutrino experiment, being two kilometres underground allows researchers to get the inside scoop on a number of space phenomena. In the event that HALO were to detect a supernova, the international community would be alerted within 20 minutes.