Homestake experiment seeks to detect dark matter

By Bill Harlan, Journal staff
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A small cavern deep underground in the Homestake gold mine at Lead could be a site where physicists once again win a Nobel Prize.

That's a bold prediction, but physicist Bob Svoboda of the University of California at Davis says, "The odds are not that bad at all."

The first Nobel Prize for an experiment at Homestake went to the late Ray Davis in 2002, for a "neutrino" detector that shed new light on how the sun works.

The new experiment, which could begin as soon as late 2008, will try to be the first ever to detect "dark matter," a mysterious substance that could answer the question, why doesn't the universe and everything in it simply explode?

Nobel Prize or not, the dark matter experiment will be a milestone for the Homestake project. Scientists from around the nation - joined by local and state officials - have been urging the National Science Foundation to convert the mine into an underground laboratory for eight years.

Last July the NSF finally chose Homestake as the site for a proposed national laboratory 7,400 feet underground. Meanwhile, the South Dakota Science and Technology Authority is opening an interim, state-owned laboratory 4,850 feet underground, where the "Davis cavern" is.

The Davis experiment shut down shortly before gold mining ended in 2001.

Davis had designed a detector around a 110,000-gallon tank of dry-cleaning fluid, which he constructed in a mined-out cavern 50 feet long by 30 feet wide by 32 feet high.

When a neutrino collided with a chlorine atom in the tank, it created an argon atom. Davis figured out how to count the individual argon atoms, one by one, to deduce the "neutrino flux."

Brown University physicist Richard Gaitskell, who is working with Svoboda and physicist Tom Shutt of Case Western Reserve University on the dark matter experiment, remembers standing in the "Davis cavern" during a tour of the underground mine in 2000.

"I was recorded on video asking the question, why the hell haven't they given Davis a Nobel Prize?"

Gaitskell and the rest of the dark-matter team hope to get equally revolutionary results, by being the first to detect the building blocks of dark matter. Those building blocks are called WIMPS, for "weakly interacting massive particles."

Dark matter was proposed in the 1970s to explain an apparent lack of gravity. There doesn't appear to be enough mass in the universe to hold everything together.

To get the right amount of gravity, scientists theorize there's a lot of dark matter - about six times more of it in the universe than plain-old regular matter.

The matter familiar to us includes everything we can see, hear, touch, taste, smell or that we can detect (so far) with scientific instruments.

However, all that stuff, which is made of electrons, protons, neutrons and neutrinos, accounts for only about 4 percent of the mass in the universe. The rest, the theory goes, is dark matter (23 percent) and dark energy (73 percent).

"We're not even the main stuff the universe is made up of," Svoboda says.

Experiments already have observed dark matter indirectly. Scientists think it bends the light from stars and alters the spin of galaxies.

But so far, WIMPs themselves remain hidden.

"I've been looking for dark matter for 19 years now," Gaitskell said in a telephone interview from Brown University.

"Once or twice the hairs on the back of my neck have started going up." So far, the WIMP alert has always been a false alarm.

LUX shed light?

Svoboda, Gaitskell and their colleagues hope to install in the Davis cavern a $3 million device called the "Large Underground Xenon detector," or LUX.

LUX's central tank will hold 300 kilograms of icy cold liquid xenon. The cylindrical xenon tank will be suspended inside a larger spherical tank, 20 feet in diameter, that is filled with water to further shield the experiment from stray radiation.

Most important, like the Davis experiment, LUX will be shielded by 4,850 feet of hard Homestake rock.

The dark-matter team thinks central 100 kilograms of xenon in the tank will be so well shielded it won't register a single hit of background radiation in a year. "In the middle of the detector it should be very quiet, Gaitskell said.

In fact, he says LUX is 100 times more sensitive than previous dark-matter detectors - most notably in Gran Sasso, Italy, and in the Soudan Underground Laboratory in northern Minnesota.

WIMPs are tiny, and they have relatively low energy, making them even more elusive than the wily neutrino. (Trillions upon trillions of WIMPs and neutrinos zip through our bodies every second, unnoticed.)

The LUX team will use devices called photomultipliers to monitor the xenon tank for the tell-tale energy burst that will signal a collision between a WIMP and a xenon nucleus. It will be an event like nothing ever seen before.

"We'll finally have solved a riddle that has lasted for 80 years."

Future experiments

Actually, one WIMP won't do it. The team will have to detect several WIMPs to verify the result. (Gaitskell said dark-matter hunters call this verification the "GPW," for "gold-plated WIMP.")

Later, the dark-matter team hopes to build an even bigger WIMP detector at the deeper national lab 7,400 feet underground at Homestake.

"We don't know how small WIMPs are," Shutt says. "That's why we'll have to keep building bigger detectors for a while."

For now, Shutt and Svoboda and other researchers are designing LUX equipment that can fit into the historic Davis cavern.

Svoboda hopes to start installation as early as next summer, if the NSF gives final funding approval.

"We're very optimistic about that," Gaitskell said.

Once installed, LUX will occupy only one end of the Davis cavern, so other researchers, including physicists from South Dakota universities, hope to install a "low-background counting facility."

It will measure radiation to calibrate other experiments, like LUX. "I'm already working with one of the chaps from the University of South Dakota," Gaitskell said in his crisp London accent.

Then, when LUX cranks up, maybe late next year or in early 2009, scientists will patiently watch for evidence of the particle they hope will explain gravity. (Or at least why there's so much of it.)

Svoboda puts the odds of detecting a WIMP at Homestake at 30 to 50 percent.

Shutt says, "If we discover dark matter, it will be the ultimate Copernican revolution."

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