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A Cosmic Quest for Dark Matter

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Scientists are hunting one of the biggest prizes in physics: tiny particles called wimps that could unlock some of the universe’s oldest secrets

Gautam Naik writes: A mile under Italy’s Gran Sasso mountain, scientists are seeking one of the smallest objects in the universe—and one of the biggest prizes in physics: a wimp.
A wimp—a weakly interacting massive particle—is thought to be the stuff of dark matter, an invisible substance that makes up about a quarter of the universe but has never been seen by humans. Gravity is the force that holds things together, and the vast majority of it emanates from dark matter. Ever since the big bang, this mystery material has been the universe’s prime architect, giving it shape and structure. Without dark matter, there would be no galaxies, no stars, no planets. Solving its mystery is crucial to understanding what the universe is made of. _65714169_dark_matter_624 “If we don’t assume that 85% of the matter in the universe is this unknown material, the laws of relativity and gravity would have to be modified. That would be significant,” says physicist Giuliana Fiorillo, a member of the 150-strong team searching for the particles at the Gran Sasso National Laboratory, 80 miles east of Rome. The quest for dark matter has intensified since the discovery of the Higgs boson particle two years ago, which helped to narrow the field in which wimps might be hiding. Today, more than 20 different teams of researchers are hunting for the elusive stuff, using some of the most elaborate and delicate experiments ever devised. Dark-matter detectors have been installed on the sea bed nearly 8,200 feet beneath the surface. Others operate deep inside mines. There is one on the International Space Station. China’s new dark-matter experiment sits 1.5 miles beneath a marble mountain. When it restarts later this year, the Large Hadron Collider will look for wimps, too, by smashing together subatomic particles. Scientists estimate that visible matter makes up just 4% of the universe, while dark matter makes up 23%. The remaining 73% is an even bigger puzzle, a repulsive force known as “dark energy.” Dark matter neither emits nor absorbs light. We know it is out there, because scientists can measure the immense gravitational force it exerts on stars, galaxies and other cosmic bodies. The best candidate for what dark matter consists of is the wimp: an ethereal being that barely interacts with normal matter. Every second, billions of wimps flow through the Earth without hitting anything.

So how do you catch one of these elusive ghosts? Researchers hope to detect a wimp on the vanishingly rare occasion when one happens to smack into the nucleus of an atom, leaving behind a telltale signal.

But the profusion of other forms of radiation can easily mimic the wimp signal. That is why dark matter traps are often set in mines or deep beneath mountains. Those places receive only one-millionth of the cosmic radiation that routinely hits the Earth’s surface. By burrowing down, scientists can eliminate a lot of the background noise that might be mistaken for a true wimp signal…. (read more)

WSJ

Write to Gautam Naik at gautam.naik@wsj.com

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