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This article is from the In-Depth Report The Large Hadron Collider: Countdown

Searching for an Elusive Particle, Physicists Take a Shot in the Dark

Experimenters team up in the search for an invisible, hypothetical particle



XENON Dark Matter Project

Earlier this month, Italian researchers made a claim that, if taken at face value, would earn them a place in scientific history. The DAMA (DArk MAtter) collaboration, based at the University of Rome, announced at a scientific meeting in Venice that it had directly detected dark matter—the invisible, nigh undetectable stuff believed to give added heft to galaxies.

Although other researchers in the field place little confidence in the report, which clashes with their own experimental results, it does highlight the progress and challenges in searching for an elusive quarry that may not actually exist. The efforts of dark-matter hunters are much like the shadowy stuff itself: easily overlooked, but exerting a slow and steady effect.

Physicists know that something has imparted galaxies with more spin than they should have if composed solely of regular "baryonic" matter, the visible stuff of stars and planets. To explain the discrepancy, most look to a hypothetical blizzard of nonbaryonic matter, neither absorbing nor emitting light, swirling around every galaxy. Astronomical measurements imply that this dark matter makes up 23 percent of the universe, compared with visible matter's measly 4 percent share.

One leading candidate for dark matter is dubbed a weakly interacting massive particle (WIMP), because it would weigh more than an atom of gold and only rarely collide with regular matter. If WIMPs exist, a few of them should pass through any given liter-size Coke bottle every second, says dark-matter hunter Tom Shutt, a physicist at Case Western Reserve University in Cleveland.

The standard model of particle physics contains nothing like that, so discovering WIMPs would point researchers to a deeper theory. Looking for WIMPs is "a shot in the dark, but it's a well-motivated shot in the dark," Shutt says.

To be certain of snagging a WIMP—to create an igloo in the dark matter blizzard, in effect—researchers would have to erect a solid block of lead extending from Earth to the nearest star, Shutt says. A more practical solution, he says, is to pile up special crystals or build a giant tank of dense liquid and wait for a WIMP to strike.

In the Cryogenic Dark Matter Search (CDMS), for example, crystals of germanium and silicon act like Jell-O; when struck by the right kind of particle, they jiggle in a detectable way, says physicist Jodi Cooley of Stanford University, a member of the CDMS team. Other detectors use liquefied gases such as xenon for their high density and large nuclei. Now finished, the Xenon10 experiment, housed underground in Italy's Gran Sasso National Laboratory, monitored a tank full of 49 pounds (22 kilograms) of liquid xenon for points of light and electric charges knocked loose by WIMPs. An upgrade called LUX (Large Underground Xenon detector) is in the works.

Typically, researchers bury their WIMP experiments deep underground, chill them to ultracold temperatures, and heavily shield them to block stray particles that might obscure any dark matter signal. The CDMSII experiment sits at the bottom of a mineshaft in Soudan, Minn., and the DAMA/LIBRA experiment resides in the Gran Sasso lab. A new approach works closer to room temperature. Physicist Juan Collar of the University of Chicago reported (and his collaborators have resurrected) a tool from mid-century particle physics experiments—the bubble chamber, a jar of pressurized liquid that boils at a single spot when struck by a passing high-energy particle.

Their experiment, COUPP (Chicagoland Observatory for Underground Particle Physics), based at the Fermi National Accelerator Laboratory in Batavia, Ill., consists of a glass vessel filled with a liter of the fire-extinguishing liquid iodotrifluoromethane (CF3I). Collar says that by adjusting the temperature and pressure of the liquid, the group can tune the chamber to ignore most particles besides WIMP candidates.

Hunting for dark matter is a waiting game. CDMSII has recorded zero WIMP candidates since it began operation in 2006. In 59 days' worth of data collected during 2006 and 2007, Xenon10 registered only 10 hits, none of them WIMP candidates. But the absence of WIMPs conveys information, too. With each passing year, researchers can rule out swaths of potential WIMPs, each having a particular combination of mass and sensitivity to normal matter.

The DAMA researchers claimed to enough WIMP candidates to pinpoint a small seasonal fluctuation in their frequency—reflecting Earth's passage back and forth through the WIMP gale. But researchers say that such a multitude of WIMPs would have been picked up by other detectors by now. Reconciling the discrepancy is challenging because the DAMA approach differs from other experiments. "If you want to prove that type of [seasonal] effect, it's difficult to show that you don't have a systematic effect," Shutt says, such as seasonal temperature changes warping the sensitivity of light-catching photomultiplier tubes used to observe hits in detectors.

Collar says he has endured ribbing from other particle physicists that dark matter experiments do not meet the exacting standards typical for experiments such as those that probe the nature of antimatter or look for the Higgs boson. On the positive side, he notes that DAMA has at least forced researchers to acknowledge that dark matter could be more complicated than they have assumed.

The coming decade may offer the best hope of detecting WIMPs. Later this year, the Large Hadron Collider, the world's next top particle smasher, will fire up near Geneva, Switzerland, and join the hunt by potentially creating WIMPy particles from scratch.

Researchers will only be satisfied they have identified their quarry when multiple experiments confirm one another. "What we're trying to do is so subtle," Collar says, "that we don't believe a single experiment is going to come along and solve dark matter."

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