April 10, 2013

Scientists listening for dark matter, deep in the Earth

The trick is to capture a dark matter particle among the hail of other bombarding cosmic stuff.

Amina Khan / Los Angeles Times

(Continued from page 1)

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In a photo provided by Sanford Lab, researchers work on the top floor of the Large Underground Xenon experiment at a shuttered gold mine in Lead, S.D.

The Associated Press

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Scientists suspect that dark matter may explain why the arms of this sprial galaxy don’t fly out into space. But, so far, none have been able to prove its existence.

The Associated Press

Once the researchers release the xenon to fill the detector, they'll have to carefully control the flow. If it gets too warm, the pressure will surge as the liquid evaporates. If it gets too cold, ice can form and wreck sensors inside.

"We're just going to have to really fine-tune it," Dobi says. "There's going to be a lot of panicking in the beginning."

If the pressure does spiral out of control, burst discs will explode and allow precious xenon gas to pour into a giant black balloon. The deflated bag stretches across the cavern, a worst-case scenario hanging over their heads.

"I'll feel a lot better once we've actually started. Because until we've started, I'm not 100 percent sure it's going to work," says Simon Fiorucci, a Brown University physicist who's been wrangling the detector's operations on-site. He's 98 percent certain, he adds. "But we've been burned by 98 before."

The lead scientists may rule the project, but the grad students own it. They've spent long, intimate stretches with the detector, and it shows. When a device needs ventilation, they drill a lopsided smiley face into it. When they need to cool down some xenon, they pour liquid nitrogen into a blue Igloo picnic cooler.

"It's efficient for holding," Dobi explains. Then he pauses. "If you want to play with liquid nitrogen, we could get some for you. Feel free to freeze your lunch and then shatter it into little pieces."

This offer alarms McKinsey. "That would probably freak out the lab," the Yale professor says.

Dobi shrugs. "Not in this lab."

A rainbow-maned unicorn-on-a-stick stands sentinel on a worktable. Dobi rides it around the lab on particularly stressful nights.

The machine they have built is rather like a bigger, better version of the dark matter detector at the Gran Sasso particle physics laboratory in Italy.

The resemblance is no accident. Many of LUX's lead scientists have seceded from the European collaboration, whose future experiments would be cramped for space. The physicists faced a choice: help build the next experiment in Italy, or try for a larger detector that would take longer to build but whose greater volume would offer far more sensitive results.

It was a gamble, LUX scientist Richard Gaitskell says - one that depended on whether scientists thought a discovery was just around the corner or further out on the horizon.

Now, ready to finally fill the detector with super-purified xenon and bring the experiment to life, the graduate students are ecstatic. They open the bottles with ceremony, smiling at the valve's first hissing pop of air as if it's from a bottle of champagne.

Then an alarm goes off. A sensor isn't working, and they can't see whether the xenon level is rising as it should.

Fiorucci, sitting cross-legged among the detector's machinery as he diagnoses the problem, isn't too concerned.

The detector is working - the team just doesn't know what exactly is happening in it right now. And he can wait. He's not even expecting to find dark matter.

"That's not around the corner, I don't think," he says. But, he adds, even if they find nothing, "we will have not found dark matter better than everyone else."

Within three days, the detector will fill up with more than 800 pounds of xenon cooled to minus 148 degrees Fahrenheit. In the weeks ahead, they'll start circulating the xenon and pulling out impurities, preparing LUX to start taking dark matter measurements.

For now, many of the grad students are ready for a well-earned break.

Paper unicorns pop up in surprising nooks around the lab, bearing Technicolor testament to the demanding hours the scientists have spent underground. "At nights, in our dark moments, we just color and drink beer," Dobi says. Asked to explain the logic, he jokes: "It's magical. It just kind of happened."

Then he shrugs. "Well, we're looking for magical particles, anyway."


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