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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CoGeNT sits in a truck-size cabinet at the back of the lab’s first cavern. A grid of instruments around the cabinet records and identifies strikes by neutrons and other heavy non-WIMP particles, allowing scientists to disregard those hits in the germanium detector. Inside that grid, an aluminum shell and a wall of lead bricks block much of the terrestrial radiation. Finally, to reduce vibrations from the germanium atoms themselves, the detector is placed in a thermos bottle of liquid nitrogen, which chills it to –321 degrees Fahrenheit.

In these conditions, the detector records minute electrical pulses caused by collisions between the germanium nuclei and any particles that have passed through the shielding, including WIMPs.

But there’s no flashing light or clanging alarm to alert a bored scientist that she’s just hit the scientific jackpot. The detectors record thousands of interactions between the detector material and subatomic particles. Those interactions all closely resemble one another, so researchers must conduct a long and painstakingly detailed analysis to winnow out the WIMP collisions from those of more mundane particles.

When CoGeNT detected a few signals in early 2014 that had also been seen by an Italian dark matter experiment, the scientists published a paper that reported the possible detection of WIMPs. A few months later, however, the scientists weren’t so sure. “We don’t know if what we’re seeing is real or not,” says Collar, using “real” to mean “of cosmological origin.” “There are a number of possibilities that aren’t related to dark matter.”

John Orrell of Pacific Northwest National Laboratory says the detections were at the limit of their experiment’s capabilities. When he compared his results to those of CDMS, which is only a few yards from CoGeNT, he concluded that the observations probably weren’t seeing WIMPs at all. “Our goal was to say, ‘Yes, we see an excess number of events’ or ‘Yes, all the events are due to the background radiation and systematic effects in the experiment,’ ” Orrell says. The analysis was inconclusive. “There was a lot of effort to make it one way or the other, but nature didn’t work out that way for us. It’s a very frustrating outcome.”

The Two Detectors
CDMS is like CoGeNT’s bigger, touchier, chillier cousin. It’s housed in a two-story, high-tech cocoon near the front of the cavern, surrounded by similar shielding but with a far more complicated refrigeration system—remember, “C” is for “cryogenic”—which uses liquid nitrogen and liquid helium to cool the detectors to a fraction of a degree above absolute zero (about –460 degrees Fahrenheit).

The colder detectors are designed to register not just the electrical pulses from dark matter impacts but also a small rise in temperature caused by the vibrations of a germanium nucleus recoiling after it’s smacked. This one-two combination provides a far more sensitive test for WIMP collisions than the more limited CoGeNT experiment. And as the cryogenic detector team reported in early 2014, it has yielded little evidence of dark matter.

“I wish we could say we’ve found dark matter, but I can’t,” says Bauer. “We had three events that looked like they might have the characteristics of a dark matter signal, but you need really convincing proof to claim a discovery, and we didn’t have it. And we’ve taken more data since then and we’ve shown that it wasn’t dark matter.”

Yet both of the Soudan experiments, along with the others being conducted around the world, have helped physicists narrow the range of WIMP masses. Particles at the heavy end of the range should have revealed themselves by now because they would produce a big thunk with a correspondingly large recoil by the detector material. The lack of confirmed detections suggests that if WIMPs exist, they are likely at the smaller end of the scale.

“The simplest theory isn’t working, which puts theory in tension with the experimental evidence,” says Cushman.

To try to ease that tension, both CoGeNT and CDMS researchers are planning upgrades to their experiments’ sensitivities. In 2015, the CoGeNT team will replace the current germanium detectors with two new ones that have a combined mass 10 times greater than the current one. “The best scenario is we turn on the new experiment and we instantly rule out more possibilities,” says Collar. “And if we see the same sort of anomaly, we can study it in more detail.”

The Cryogenic Dark Matter Search team, in the meantime, will receive funding from the Department of Energy and the National Science Foundation to build a much larger and more sensitive experiment to go in a much deeper mine in Ontario, Canada. (Two other dark matter experiments also received a go-ahead: one to search for WIMPs and another to look for other dark matter candidates known as axions.)

Despite the possible expansions, though, the hunters still aren’t sure whether these or any other detectors will ever find dark matter particles.

“There’s always an evil scenario—that WIMPs interact only gravitationally,” Cushman says. “If that’s so, then we’ll never detect them.”

Orrell, however, remains optimistic. “I’m very bullish on WIMPs. I’m not entirely convinced the vanilla models are correct, so the dark matter particle may be more complicated than we think…. But when dark matter is discovered, it’ll be really eye-opening for the particle physics community. We’ll need to redevelop our theories to go beyond the Standard Model.”

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