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Title: Underground Astronomy
URL: http://sciencematters.berkeley.edu/archives/volume5/issue37/story1.php
Date: 05/29/2008
Publication: Science Matters @ Berkeley
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Underground Astronomy

by Kathleen M. Wong

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Professor of physics Bernard Sadoulet has designed a sophisticated set of equipment to search for particles of dark matter. Image credit: CDMS

Most scientists who study the cosmos keep their eyes fastened firmly on the sky. Not so Bernard Sadoulet. A Berkeley professor of physics, Sadoulet is stalking dark matter, the elusive material that forms the scaffolding of the universe. And the place he\'s laid his traps is just as shadowy-a former iron mine more than 2,300 feet underground.

Sadoulet admits that the bowels of the earth make for an unconventional place to study the universe. But the particles that are the leading candidates for dark matter are most easily detected far from the cosmic rays that rain down from space.

"We have good reasons to believe that only about five percent of the energy in the universe is made of ordinary matter. So 95 percent is unknown. And we are trying to determine experimentally what that is," Sadoulet says.

Roughly 75 percent of the mystery material is known as dark energy-a force that behaves like anti-gravity, pushing galaxies apart. The remainder is called dark matter, and cosmologists believe it provides the gravitational mass that holds the universe together.

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An elaborate cooling and shielding system surrounds the dark matter detectors, preventing interference from heat and other types of radiation.

The jury is still out on what dark matter is. About all scientists know is that it has mass, clumps under the influence of gravity, and can\'t be seen because light passes right through it.

"It is interesting to observe that the most inert component in the universe is ultimately responsible for the formation of structure and, ultimately, stars, planets, and life," Sadoulet says.

Speculations about dark matter\'s identity range from the side effects of additional dimensions to ultralight particles known as neutrinos. But several lines of thinking have converged on heavy particles known as WIMPs (weakly interacting massive particles).

Despite their acronym, WIMPs are atomic behemoths, approximately 100 times the mass of a proton. Their great mass would explain why they have not yet been seen in laboratories; previous generations of particle accelerators weren\'t powerful enough to produce them. In nature, these giant particles would have formed in the early universe in the burst of energy released by the Big Bang.

"If these particles are the dark matter, they form a dark halo around the galaxy. We are in this halo, and there are billions of these particles going through us all the time," Sadoulet says.

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One of the detectors used to identify candidate particles of dark matter known as WIMPS. Silicon and germanium atoms coating the detector\'s surface will emit a distinctive pattern of vibrations and electrons if hit by a WIMP.

Sadoulet leads an experiment to find these particles within Minnesota\'s Soudan Mine. His Cryogenic Dark Matter Search employs detectors made of silicon or germanium crystals cooled to nearly absolute zero. Any WIMPS or other particles crashing into the crystals will trigger a distinctive set of vibrations called phonons and extract electrons. The ratio of the electrons and phonons generated distinguishes a bona-fide WIMP from other forms of radiation such as gamma-rays. Sadoulet is already working on the next generation of this experiment, which will involve caching more sensitive and efficient detectors in a Canadian physics laboratory nearly three times deeper than Soudan.

Other scientists are tackling the problem of dark matter with a satellite capable of detecting the gamma rays emitted by colliding WIMPS, and a powerful new particle accelerator called the Large Hadron Collider that will attempt to manufacture WIMPs from scratch. Sadoulet\'s detectors will be essential for confirming whether any of these artificial particles match the profile of a WIMP.

"Within five years, three totally different approaches to catching WIMPS should be in operation, and we may be at the brink of a discovery" says Sadoulet. "It\'s an interesting time to be searching for dark matter."

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