Dark matter hunt deepens at Ont. mine

The hunt for an unknown particle that could explain one of the great mysteries of the universe has ramped up at the deepest underground physics lab in the world.

Researchers from Canada, the U.S. and Europe have recently set up new experiments at SNOLAB, located two kilometres underground near Sudbury, Ont., in hopes of figuring out the composition of dark matter — invisible mass that makes up about a quarter of the universe.

"You know it's there from the way it affects things, by their gravitation attraction to dark matter, but you can't visibly see it," said Nigel Smith, the director of the facility, located in a spotlessly clean offshoot of the Vale Inco Creighton Mine.

University of Chicago researcher Juan Collar and his collaborators at the Chicagoland Observatory for Underground Particle Physics (COUPP) think they may have a way to make dark matter show itself.

"We expect to have possibly even the best sensitivity to dark matter particles in the world," Collar said.

The team installed and turned on the four-kilogram dark matter particle detector at SNOLAB this summer. The researchers have been remotely keeping an eye on the device, known as a bubble chamber, ever since, tweaking the system over time. They expect to reach their maximum sensitivity to dark matter particles within the next three months.

Many physicists hypothesize dark matter is made up of a theoretical type of subatomic particle called a weakly interacting massive particle (WIMP). Unlike the matter we are familiar with, which is made up of charged particles such as protons and electrons, WIMPs don't have any electrical charge and only interact with other particles by gravity. That makes them very difficult to detect, Smith said.

"They would pass right through the Earth without noticing, generally," he said.

On the other hand, physicists believe there are so many of these particles that, occasionally, they should crash into the nucleus of an atom and create a detectable signal. The problem is, the tiny, faint signal would be drowned out by the roaring din of signals generated by cosmic radiation at the surface of the Earth.

Extreme clean

But deep underground in SNOLAB, the level of cosmic rays is reduced 10 millionfold, Smith said. In addition, the lab space, purposely excavated starting 2004, has been painstakingly cleaned of all dust particles and is climate controlled. Researchers have to shower before entering to make sure they don't bring any new dust with them.

That makes Collar optimistic about being able to detect WIMPs passing through the underground lab and into his team's bubble chamber. The old, simple technology has been used to detect subatomic particles for decades.

The steel bubble chamber vessel at SNOLAB is filled with iodotrifluoromethane, or CF3I, which is often used as a fire extinguishing liquid. The liquid is kept at 30 to 40 degrees Celsius — superheated above its boiling point. That means anything that disturbs it could cause boiling and therefore bubble formation.

The system is designed to be disturbed by a collision involving a dark matter particle.

"I like to compare it with the opening of a game of pool," Collar said. "Your cue ball is your WIMP, and when it strikes a material, it produces this tremendous mass of material flying in all directions."

The particle struck by the WIMP hits other particles, generating disorder and heat. That creates a bubble that grows until it is one millimetre in diameter. At that point, the bubble becomes visible to a camera that automatically detects motion and triggers a series of actions. For example, the camera will save a video of the event.

Collar believes the system has a good chance of detecting dark matter because theories suggests WIMPs should interact through one of two methods — one of which is detectable with fluorine and the other with iodine. Both iodine and fluorine are present in the bubble chamber liquid. The researchers expect that a WIMP would leave a single bubble while other kinds of particles would leave multiple bubbles.

The team plans to return in the new year with a detector that at 60 kg is 15 times bigger than the original one and is working on a 500 kg one that will be deployed in 2013.

In the meantime, SNOLAB, which is an expansion of existing facilities constructed for the Sudbury Neutrino Observatory (SNO) solar neutrino experiment, has been busy getting its facilities ready.

Excavation of the new lab space was only completed in 2007.

"We've just really finished making it clean and installing the new infrastructure there," Smith said. "This is an extremely busy and extremely exciting time for the facility."

The last section of lab space is expected to open in the spring of 2011.