LUX water tank filled, dark matter search nears

Note the small stream of water at left, filling the LUX water tank on the 4850 Level. The LUX detector itself, in the center, is 61/2 feet tall by 3 feet in diameter. Photomultiplier tubes hang on the sides of the tank.
Credit: 
Matt Kapust

The Large Underground Xenon dark matter detector, nicknamed LUX, has been fully submerged in water in a large stainless steel tank 4,850 feet underground in the Davis Campus.

“This is a major step forward on the road to an operational detector in early 2013,” said Sanford Lab Director Mike Headley said. (Read the Sanford Lab press release here. See our In the News page for media coverage.)

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.”

The LUX detector was moved underground in July. (Click here to see a time-lapse video of the move.)The experiment is in same cavern originally excavated in the mid-1960s for nuclear chemist Ray Davis, who earned a share of the Nobel Prize for Physics for his solar neutrino research. The Davis Cavern has been enlarged and outfitted as a two-story underground laboratory specially designed and outfitted for LUX.

The LUX detector arrived on the top floor of the Davis Cavern on July 12. Then researchers spent the next few weeks slowly lowering the sensitive device from the top floor into its protective water tank on the floor below. In mid-October, LUX scientists, working with Sanford Lab engineers and technicians, began the two-week process of filling the tank with more than 70,000 gallons of ultra-pure, de-ionized water. Now researchers are testing the experiment’s electronics and preparing to fill it with liquid xenon.

The LUX detector will look for dark matter particles, which have been appropriately dubbed “weakly interacting massive particles,” or WIMPs. Although WIMPs have mass and are susceptible to gravity, they are neutral in charge and they don’t emit light. That makes them “weakly interactive” and extremely difficult to detect. Most WIMPs pass right through ordinary matter—including the earth—without interacting at all. Very occasionally, a WIMP will collide with an atom. The LUX detector will look for rare collisions between WIMPs and atoms of liquid xenon in the detector. “Basically the detector works like a turnstile,” said LUX physicist Jeremy Mock said. “It’s designed to detect any particle that moves through it, and we’re looking for WIMPs.” Mock, who is a graduate student at the University of California, Davis, has worked on the experiment for five years, beginning as an undergraduate at Case Western Reserve University.

These rare WIMP-Xenon collisions would be impossible to detect on the surface, where the detector would be bombarded with a shower of particles created by cosmic radiation from space. Nearly a mile of rock overhead will protect LUX from that “noise.” The water tank will protect the experiment from natural radiation in the surrounding rock. The tank also is lined with 20 light-catching devices called photomultiplier tubes (PMTs), which will detect the occasional cosmic-radiation induced particles that penetrate the earth all the way to the experiment. Signals from the PMTs will alert researcher to ignore false signals.

LUX scientists hope to begin filling the detector with xenon by late December or early January. Data collection could begin in February. "We might well uncover something fantastic," Nelson told the Associated Press.

For more information about the LUX experiment, go to luxdarkmatter.org.