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Large Underground Xenon Experiment to Look for Dark Matter is Now Underwater

An experiment to look for one of nature's most elusive subatomic particles is finally underwater, in a stainless steel tank nearly a mile underground beneath the Black Hills of South Dakota. And among the dozens of scientists involved in the research collaboration is the University of Rochester's Professor Frank Wolfs.

Wolfs explained that the Large Underground Xenon experiment, LUX, will be the most sensitive device yet to look for dark matter. It will use digital signal processing electronics that were first developed by Wojtek Skulski, now a visiting senior scientist at the University, when he was a senior research associate working with Wolfs. Skulski has since started his own company, SkuTek Instrumentation, specializing in the sort of electronics needed for experiments like this. The firmware for the electronics was developed by Eryk Druszkiewicz, a Ph.D. student from Rochester's Department of Electrical and Computer Engineering.

Thought to comprise more than 80 percent of the mass of the universe, dark matter has so far eluded direct detection. The LUX detector, under construction for more than three years at the Sanford Lab in South Dakota, was installed underground in a protective tank in July. The tank was filled with water last week, and all systems are functioning well. The researchers from 17 research universities and national laboratories in the U.S. and Europe expect to have the detector operational in early 2013.

Dark matter particles are neutral and they don't emit light, which makes them hard to detect. So LUX scientists will look for evidence of collisions between dark matter particles—called weakly interacting massive particles, or WIMPs—and atoms of xenon inside the LUX detector.

One of the components of the detector is the electronic trigger mechanism, which is what Wolfs and two graduate students from the departments of physics and electrical and computing engineer have been working on. "The trigger makes a decision on whether any signal seen in the detector is something worth analyzing," Wolfs explained. "It selects only those events that fit a certain criteria, so we don't have to use resources recording and analyzing too much unnecessary information, which becomes a problem if you're trying to run these experiments for a very long time."

Physicist Harry Nelson of the University of California, Santa Barbara, who helped design, build, and fill the sophisticated water tank that now holds the experiment, says LUX could help solve a vexing mystery. "The nature of the dark matter is one of the top three open questions in particle physics," Nelson said. "We know that matter like us—electrons, protons, and neutrons—makes up only one sixth of the known matter in the universe. The evidence that the other five sixths is present out there in galaxies is overwhelming. Detecting dark matter in a laboratory like Sanford, here on Earth, would be a huge step forward in nailing down what the stuff really is."

LUX, however, requires a very quiet environment. In July, the experiment was installed 4,850 feet underground in the Sanford Lab, where it is protected from the cosmic radiation that constantly bombards the surface of the earth. LUX also must be protected from the small amounts of natural radiation from the surrounding rock. That's why the detector, which is about the size of a telephone booth, was lowered into a very large stainless steel tank—20 feet tall by 25 feet in diameter. The tank has now been filled with more than 70,000 gallons of ultra-pure de-ionized water that will shield the detector from gamma radiation and stray neutrons.

The water tank also is lined with 20 devices called "photomultiplier tubes," or PMTs, each capable of detecting a single photon of light. Very occasionally, a high-energy particle caused by cosmic radiation will penetrate the earth all the way down to the LUX experiment. If that happens, the resulting tiny flash of light in the water will alert researchers that a corresponding signal in the detector was not caused by dark matter.

Now that LUX is underwater, researchers are testing the experiment's complex electronics—a process that will take weeks. "We have tested the electronics on the surface and it's all working, but we know want to learn how the detector works in the conditions it will actually be operating," Wolfs said.

The detector itself is a double-walled titanium cylinder about 6½ feet tall by 3 feet in diameter. The cylinder is a vacuum thermos—or "cryostat"—that holds about a third of a ton of xenon, cooled to a liquid state at minus 160 degrees Fahrenheit. Inside the cryostat, 122 smaller PMTs will detect when a WIMP bumps into a xenon atom. The collision will produce two flashes of light—one at the point of impact and a second flash in a thin layer of xenon gas at the top of the detector. The second, stronger flash will be caused by electrons released during the collision and drawn upwards by a strong electrical field inside the detector. The trigger electronics will compare data from the two flashes and will only keep data that is consistent. Researchers will then analyze this to determine whether dark matter has been discovered.

Why search for dark matter? Nelson pointed out that practical applications of fundamental research are sometimes not immediately apparent. However, research into the nature of electricity in the 19th century and into the structure of the atom in the early 20th century led to technological advances now essential to manufacturing, communications, medicine and high-speed computing. "Unforeseen consequences are one of the most exciting aspects of scientific exploration and discovery," Nelson said.

Today, dark matter's invisibility both "confounds and motivates" researchers, Nelson said, and scientists are looking for it in a number of experiments around the world. "The Sanford Lab provides us with a crucial facility to forge ahead toward the goal of directly detecting dark matter here on Earth," he said. "The most popular theories in particle physics indicate that success will come soon, either at the Sanford Lab or a little later at labs in Canada, Italy or China."


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