Dark-matter search plunges to new depths

U. CHICAGO (US)—Researchers are expecting a bubble chamber more than a mile underground will reveal dark matter’s secret identity.

Scientists are deploying a 4-kilogram bubble chamber at SNOLab, part of the Sudbury Neutrino Observatory in Ontario, Canada, in anticipation that dark matter particles will leave bubbles in their tracks when passing through the liquid in one of the chambers.

A second 60-kilogram chamber will follow later this year.

Dark matter accounts for nearly 90 percent of all matter in the universe. Although invisible to telescopes, scientists can observe the gravitational influence that dark matter exerts over galaxies.

“There is a lot more mass than literally meets the eye,” says Juan Collar, associate professor of physics at the University of Chicago. “When you look at the matter budget of the universe, we have a big void there that we can’t explain.”

Likely suspects for what constitutes dark matter include WIMPS (Weakly Interacting Massive Particles) and axions.

Theorists originally proposed the existence of both groups of subatomic particles to address issues unrelated to dark matter.

“These seem to be perfect to explain all of these observations that give us this evidence for dark matter, and that makes them very appealing,” Collar says.

SNOLab will be the most ambitious in a series of underground locations, Collar says. In 2004, he and colleagues established the Chicagoland Observatory for Underground Particle Physics (COUPP) at Fermi National Accelerator Laboratory.

“We started with a detector the size of a test tube and now have increased the mass by a factor of more than a thousand,” says Andrew Sonnenschein, a physicist at Fermilab.

“It’s exciting to see the first bubble chamber being sent off to SNOLab, because the low level of interference we can expect from the cosmic rays there will make our search for dark matter enormously more sensitive.”

COUPP extends to the city of Chicago’s flood-control infrastructure, called the Deep Tunnel.

The city has granted COUPP scientists access to the tunnel, 330 feet underground, to test prototypes of their instruments. The collaboration also tested instruments in a chamber 350 feet below Fermilab, and in a sub-basement of the Laboratory for Astrophysics and Space Research on the UChicago campus.

The troublesome underground radiation sources consist of charged particles that lose energy as they traverse through a mile or more of rock. But rock has no impact on particles that interact weakly with matter, such as WIMPS, thus the move to Sudbury.

“SNOLab is a very special, spectacular place, because the infrastructure that the Canadians have developed down there is nothing short of amazing,” Collar says.

“Even though SNOLab sits atop a working nickel mine, conditions there are pristinely antiseptic.

Collar also is a member of the Coherent Germanium Neutrino Technology (CoGeNT) collaboration, which operates a detector that sits nearly half a mile deep at the Soudan Underground Mine State Park in northern Minnesota.

The 60–kilogram detector that Collar and colleagues will install at SNOLab later this year, meanwhile, undergoes testing in a tunnel 350 feet beneath Fermilab.

“Most of us have been concentrating on intermediate-mass WIMPS for decades,” Collar explains.

“In the last few years the theoreticians have been telling us more and more, look, under these other sets of assumptions, it could be a lighter WIMP. This device is actually the first of its kind in the sense that it’s targeted specifically for light WIMPS. We’re seeing interesting things with it that we don’t fully understand yet.”

Collar estimates that it’ll take a decade or more for physicists to become completely convinced that they’ve seen dark-matter particles.

“It’s going to take a lot of information from very many different points of view and entirely independent techniques. One day we’ll figure it out.”

Researchers from Indiana University contributed to the study.

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