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

LEAD, S.D. — The scientists don hard hats, jumpsuits and steel-toed boots to pile into a metal cage for a rumbling 11-minute descent into an abandoned South Dakota gold mine. They step over old mine-cart rails, through rough-walled tunnels and into a bright white room. There, they cast off their dusty garb and enter a lab hidden nearly a mile beneath the Earth.

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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

Inside, Patrick Phelps peers at valves connected to half a million dollars' worth of some of the purest xenon in the world.

"Is everyone ready?" the Case Western Reserve University graduate student calls out over growling machinery filling the cavernous space.

Ice piles on a nearby tank, digital displays glow green, and bundles of wires curl in every direction.

"Let's do it," says Attila Dobi, a University of Maryland graduate student.

Here, with a two-story state-of-the-art detector sheltered in what was once North America's deepest gold mine, the scientists are panning the cosmos for a flash of something far more elusive than gold: dark matter.

Dark matter outnumbers normal matter in the universe 5 to 1, yet remains one of physics' ultimate mysteries. It can't be seen or felt, and passes through Earth like a phantom. Scientists think it might be made of hypothetical "weakly interacting massive particles" - WIMPs - though they have yet to find one.

But astronomers know dark matter exists: Without its gravity, spiral galaxies' pinwheel arms would be ripped from their bodies and flung into space.

On the rarest of occasions, a dark matter particle might just bump into a normal-matter atom. The trick is to catch that signal amid the hail of cosmic particles bombarding Earth so thickly that hundreds pass through your body each second.

Scientists strain their ears for that dark matter whisper against the background roar. They dig deeper and deeper into the Earth, to mute the cosmic noise. They fill tanks with liquid xenon that's dense as rock, armor it with a 70,000-gallon shield of water and wait for particles to hit it.

When a particle strikes the xenon, they listen for the right rhythm. A normal particle can tap out a staccato beat. Dark matter will strike just once.

And they listen for the faintest notes.

"The dark matter will sound different from the background, so you can actually tell the difference," Yale University physicist Dan McKinsey says, raising his voice over the cavern's buzz of machinery.

The Large Underground Xenon experiment, or LUX, is estimated to be 10 times more sensitive than all other such detectors put together.

Now, for the dark-matter hunters deep in the Homestake gold mine, half a decade of preparation may finally be over.

Before "turning on" the detector for the first time this day in early February, several researchers spend hours going through their checklist, closing dozens of valves to control the flow of xenon. It's a painstaking process, and they toil in pairs to check each other's work. Phelps calls out the valve numbers: 15, 16, 30, 31.

"Long list," Dobi says finally, as they close another.

"It is," Phelps replies evenly, moving on to the next valve. "Seven?"

Xenon is costly, but the scientists say it's worth the roughly $1,000-per-kilogram price tag. Like a field full of brawny defensive linemen standing shoulder to shoulder, the massive atoms in the dense liquid block out unwanted particles - and raise the odds of trapping a slippery dark matter particle trying to squeeze by.

These players are also self-disciplined. Xenon is so chemically inert that electron signals from any collisions can pass freely through the liquid. These crucial signals let scientists eliminate phony dark matter candidates.

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