A detective hunt to solve two of the universe's biggest mysteries is taking a team of physicists, including ones from Case Western Reserve University, a half-mile below the Earth's surface.

Equipped with detectors chilled to near absolute zero, scientists of the Cryogenic Dark Matter Search (CDMS) II team, managed by the U.S. Department of Energy's Fermi National Accelerator Laboratory, last month began their quest to find WIMPs, or weakly interacting massive particles suspected as a major component of dark matter. They also are searching for evidence of supersymmetric particles called neutralinos that support the physicists' theory of supersymmetry.

"This arrow from particle physics and this arrow from cosmology seem to be pointing to the same place," said Dan Akerib, deputy project manager of CDMS and Case associate professor of physics. "Detection of a neutralino would be very big for cosmology, and it would also be very big for particle physics."

The scientists use a detector located deep underground in the historic Soudan Iron Mine in northeastern Minnesota. The experimenters seek signals of WIMPs, particles much more massive than a proton but interacting so weakly with other particles that thousands would pass through a human body each second without leaving a trace.

The launching of the detector is a new milestone in the project, according to Akerib, and within the year, CDMS researchers plan to begin reporting information from their analyses of project data.

By watching how galaxies spin-how gravity affects their contingent stars-astronomers have known for 70 years that the matter seen cannot constitute all the matter in the universe. If it did, galaxies would fly apart. Recent calculations indicate that ordinary matter containing atoms makes up only 4 percent of the energy-matter content of the universe. "Dark energy" makes up 73 percent, and an unknown form of dark matter makes up the last 23 percent.

Remarkably, in the kind of convergence that gets physicists' attention, the characteristics of this cosmic missing matter particle now appear to match those of the supersymmetric neutralino.

Measurements of the cosmic microwave background, residual radiation left over from the Big Bang, have recently placed severe constraints on the nature and amount of dark matter. The lightweight neutrino can account for only a few percent of the missing mass. If neutrinos constituted the main component of dark matter, they would act on the cosmic microwave background of the universe in ways that the recent Wilkinson Microwave Anisotropy Probe should have observed-but did not.

Meanwhile, particle physicists have kept a lookout for particles that will extend the Standard Model, the theory of fundamental particles and forces. Supersymmetry, a theory that takes a big step toward the unification of all of the forces of nature, predicts that every matter particle has a massive supersymmetric counterpart. No one has yet seen one of these supersymmetric "superpartners." Theory specifies the neutralino as the lightest neutral superpartner, and the most stable, a necessary attribute for dark matter. The neutralino's predicted density and rate of interaction also make it a likely dark matter candidate.

Only occasionally would a WIMP hit the nucleus of a terrestrial atom, and the constant background "noise" from more mundane particle events-such as the common cosmic rays constantly showering the earth-would normally drown out these rare interactions.

Installing the CDMS II detector beneath 740 meters of earth screens out most particle noise from cosmic rays. Chilling the detector to 50 thousandths of a degree above absolute zero reduces background thermal energy to allow detection of individual particle collisions.

Project managers at Fermilab estimate that with sufficiently low backgrounds, CDMS needs only a few interactions to make a strong claim for detection of WIMPs.

The CDMS II experiment is a collaboration of scientists from 12 institutions with support from department of energy's Office of Science and the National Science Foundation.

Other Case researchers involved with CDMS are Richard Schnee, visiting assistant professor; Michael Dragowky, senior research associate in physics, and graduate students Donald Driscoll, Sharmila Kamat and Gensheng Wang.

Over the past several years, the Case team has been part of the development and testing of the detector in labs at Rockefeller Hall.

In addition to Case and the Fermi National Accelerator Laboratory, CDMS II collaborators include Brown University, Lawrence Berkeley National Accelerator Laboratory, National Institute of Standards and Technology, Princeton University, Santa Clara University, Stanford University, University of California at Berkeley, University of California at Santa Barbara, University of Colorado at Denver and University of Minnesota.

Funding for the CDMS II experiment comes from the Office of Science of the U.S. Department of Energy and the Astronomy and Physics Division of the National Science Foundation.

Return to the online edition of the 12-11-03 Campus News.