Dark Matter Detectives

The hunt for the most elusive particles in the universe is half a mile underground.

The octagonal 26-foot-tall MINOS detector is one of several experiments housed in a former iron mine in Minnesota; two others seek cosmic dark matter. (Courtesy Fermilab)
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The CoGeNT “office” looks like a cross between an industrial warehouse and the Batcave. Off to the left, carts like the ones from Indiana Jones and the Temple of Doom wait to take tourists into the dark tunnel that once led to the mine. But turn right, follow a curving hallway, and pass through a swinging door, and it’s a scene from a different movie: Two caverns excavated for the laboratory are filled with offices, racks of electronic equipment, stacks of aluminum tubes, machine shops, a soft drink machine, and the kind of bric-a-brac you’d find in any warehouse. But the dewars of liquid helium and nitrogen, the rows of computer monitors, and the flashing red lights warning pacemaker wearers to STAY BACK might have you thinking you’ve stumbled into a James Bond villain’s secret lair.

“It’s a unique place to work,” says Meier, who joined the lab 25 years ago. “Since we’re underground, there are some challenges. Everything you bring down has to go back up. There’s no potable water—we have to bring down bottles. We train for mine rescue operations, we even train people on what to do if a bat lands on you.” (Screaming isn’t on the list.)

The hunt for precious and elusive material was made possible by the prospectors of 130 years earlier, who discovered gold in the hills of the Vermilion Range, about 200 miles north of Minneapolis–St. Paul. But it was scarce and difficult to extract, and the gold rush fizzled.

Miners discovered rich veins of iron, however, excavating the first ore from the site in 1882. U.S. Steel took over the mine in the early 1900s, eventually sinking 17 shafts and honeycombing the hills with more than 50 miles of tunnels, or “drifts.” The mine shut down in 1962 when it became more profitable to strip-mine lower-quality ore in other parts of the state than to excavate it from deep below the surface. U.S. Steel then donated the mine to the state of Minnesota.

University of Minnesota physicist Marvin L. Marshak toured the mine while on vacation in the mid-1970s and remembered the place when he began pondering a new physics problem a few years later.As part of their efforts to unite all the basic forces of nature into a “theory of everything,” some physicists had predicted that a proton—the positively charged component of an atom’s nucleus—would eventually fall apart.

To study the problem, Marshak needed a deep hole in the ground, protected from cosmic rays, the high-energy protons and electrons constantly raining into Earth’s upper atmosphere from exploding stars and eruptions in distant galaxies. As one of these particles strikes atoms of oxygen, nitrogen, and other elements in the air, the impact starts a chain reaction that generates cascades of particles and energy that can flood electronic detectors, making it impossible to find a signal emitted by a decaying proton. That’s why physicists looking for rare-particle events place their experiments below the surface.

“I had been looking at mines in Colorado but I didn’t find anything, so I was getting depressed,” Marshak says, pulling a copy of his original proposal off a shelf in his office at the University of Minnesota in Minneapolis. “And my wife was upset that I was running around the U.S. looking at mines.” Marshak and his wife, Anita, had visited the Soudan site together. “She said, ‘Why don’t you go use that?’ ”

Marshak, who still directs the Soudan lab, called the park superintendent, who agreed to lease him space in an abandoned drift. The original proton decay experiment began in 1981 and ran for six years. When it ended, Marshak and colleagues built a bigger detector, Soudan 2, which operated until 2001 but didn’t see a single proton decay. Nor has any other detector in the years since. (More recent research has concluded that the lifespan of a proton must be longer than was believed when the proton decay experiments of the 1980s and ’90s were being designed, Marshak explains.)

By the time Soudan 2 concluded, however, physicists were beginning to search for the hypothetical particles known as dark matter.

Dark Matters
Evidence of dark matter appeared as early as the 1930s, when Swiss astronomer Fritz Zwicky, working at the California Institute of Technology, found that the thousands of galaxies in the Coma Cluster weren’t massive enough to hold the cluster together. He postulated that the galaxies were bound by a missing mass, for which he coined a term: “dark matter.” Various observations have demonstrated that this matter can’t be black holes, faint stars, hidden planets, or other large objects. The leading theory says it consists of WIMPs—Weakly Interacting Massive Particles.

According to the theory, these subatomic particles were created in the Big Bang. They’re much heavier than particles of normal matter, with estimates ranging from a few to a few thousand times the mass of a proton. They exert a gravitational pull on the visible matter around them, making it easy to see the effects of large aggregations of WIMPs, like the ones astronomers observe (via their gravitational effects)in galaxies and galaxy clusters.

Individual particles are difficult to detect, however, because they interact with normal matter only through the “weak” force, one of the fundamental forces of nature. (The other three are gravity, electromagnetism, and the strong force, which holds the nuclei of atoms together.) Although it is responsible for radioactive decay and other phenomena, the weak force is so puny it requires a WIMP to score a direct hit on an atomic nucleus—like a cue ball bumping into a clump of billiard balls—to produce any reaction at all. WIMP detectors all look for this tiny reaction, which could take the form of a slight increase in temperature, a change in electrical charge, or a spark of light.

Both of the Soudan dark matter experiments try to catch WIMPs with “hockey pucks” of ultra-pure germanium, which provide a big, heavy target for the dark matter particles. CoGeNT uses a single puck, whereas CDMS has 15 (an upgrade from the original experiment installed at Soudan in 2003). Instruments are etched into the detectors’ surfaces like transistors on a computer chip, allowing scientists to record any impacts within the crystal structures.

“If a dark matter particle comes in and hits the nucleus of a germanium atom, it causes the nucleus to recoil, heats the crystal, and produces a signal,” says University of Minnesota physicist Priscilla Cushman, a member of the CDMS team. “But the germanium nucleus is like a grape inside the Metrodome, while the electrons around it are the walls of the Metrodome. Everything else is empty space, so these encounters shouldn’t happen very often.”

Clean Air Act
Just because the detectors are deep below the surface doesn’t mean they’re completely isolated from radiation. “There are neutrons from the rock, radon in the air—all kinds of sources of radiation,” says University of Chicago physicist Juan Collar, founder of the CoGeNT experiment. “But we have a big bag of tricks to clean up the spectrum from known sources.”

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