As Peter Redl and a colleague prepare to enter the clean room that houses the Cryogenic Dark Matter Search detectors, they take the usual precautions. Blue bunny suits? Check. Caps and booties? Check. Electronic gizmos? Left in the office. Bananas? Nowhere in sight. “If you put a banana next to the detectors, the radioactive decay of the potassium will cause about 2,000 events per minute,” explains Redl, a postdoctoral researcher at Stanford University. “So we’re not allowed to eat bananas.” When you’re searching for the most bashful of cosmic particles, you can’t be too careful.
Although no one has ever seen it, dark matter appears to be roughly five times as abundant in the universe as normal matter, and the leading theory says it consists of subatomic particles. Yet despite some tantalizing hints that have emerged from at least a dozen experiments conducted since 1987, there are still no confirmed detections of such particles.
In early 2014, scientists at the Soudan Underground Laboratory—a center in northeastern Minnesota where more than a dozen universities and three national laboratories in the United States and Europe conduct experiments—thought they had finally seen dark matter: The Minnesota detectors correlated findings with those from an experiment in Italy. It turned out to be a false alarm, but the study and other projects have helped to give scientists a better idea of how to refine their search.
Because dark matter particles almost never interact with the “normal” matter that makes up stars, planets, and everything else we can see or touch, they’re difficult to detect. Moreover, the effect of an encounter is so slight that the collision is easily drowned out by the cacophony of everyday radiation. It’s a bit like trying to detect the buzz of a mosquito at a Metallica concert.
“The name of the game is to try to detect dark matter particles and not fool yourself with other signals,” says Dan Bauer, project scientist for Cryogenic Dark Matter Search (CDMS), one of Soudan’s two dark matter experiments. Bauer works at Fermilab, a Batavia, Illinois research center that directs CDMS. “Dark matter isn’t the only thing coming to us from space: We get cosmic rays, and some of them penetrate down to ground level. That’s a horrible background for dark matter detectors.”
To avoid the jamming effect of cosmic rays, the world’s banana supply, and various other radiation sources, the dark matter detectors are protected by a Faraday cage—an enclosure that prevents electrical current from flowing through the space it surrounds—as well as lead blocks and other shields. Most important, it sits almost half a mile beneath the tree-covered landscape of Minnesota’s Soudan Underground Mine State Park. The Soudan laboratory is an extension of a former iron mine. Overlying layers of Ely greenstone block most of the cosmic rays that bombard Earth’s surface.
The volcanic rock protects more than just the search for dark matter. The Soudan laboratory hosts several experiments that require a radiation-quiet environment. A cabinet that resembles a large commercial oven, for example, houses semiconductor materials; researchers will compare the radiation-shielded semiconductors to identical chips exposed to the relatively radiation-rich environment at high altitudes. Soudan’s largest experiment—6,000 tons of iron plate described by participants as a battleship in a bottle—catches the particles known as neutrinos, which are byproducts of the nuclear reactions in the hearts of stars.
The dark matter experiments are designed to expand the scientific understanding of the birth and evolution of the universe, and to fill in the gaps in physicists’ basic framework of how matter and energy work: the Standard Model.
The Standard Model splits the universe into two kinds of particles: particles of matter, such as the electrons and the building blocks of protons and neutrons, which make up atoms; and particles known as force carriers. Photons—particles of electromagnetic energy—belong to the latter category. The Standard Model has not yet been able to incorporate dark matter. Yet dark matter must exist, because scientists see evidence of it almost everywhere: in the motions of the stars, in the existence of galaxy clusters, in the clumpiness of the early universe.
“We know from all sorts of different observations of the universe that either dark matter exists, or we’re wrong about gravity,” says Fermilab’s Dan Bauer.
At Soudan, the search continues. “We’re looking for things that go beyond the Standard Model,” says John Orrell, co-leader of Soudan’s other dark matter search, the Coherent Germanium Neutrino Technology (CoGeNT) experiment.
Down in the Mines
At 7:30 on a snowy spring morning, staff members and visiting scientists meet Jerry Meier, Soudan’s lab supervisor, at the mineshaft’s headframe, an open steel tower that resembles an old Cape Canaveral gantry. It supports a small elevator car, “the cage,” which originally carried miners deep into the earth.
After a clattering three-minute ride in total darkness, the door opens to reveal an engraved metal sign:
Level No. 27
2341 Feet Below The Surface
689 Feet Below Sea Level