Dark Matter Experiment Narrows Search for WIMPS

Since November, a physics experiment called the Cryogenic Dark Matter Search (CDMS II) has been looking for components of dark matter, the primary “stuff” of which the universe is made. Conducted from the Soudan Underground Mine in northern Minnesota, the search is for postulated dark matter particles called WIMPS–weakly interacting massive particles. So far, the experiment has found no WIMPs, but neither has it found contamination from stray neutrons. CDMS II member Priscilla Cushman, a physics professor at the University of Minnesota, is delighted.

“It is actually very satisfying to see nothing on our first run because it establishes the degree of background rejection we have been able to achieve,” said Cushman. “We hope the next run will reveal the elusive dark matter particle.”

The results are being presented at a meeting of the American Physical Society on May 3 and 4 in Denver by Harry Nelson and graduate student Joel Sanders, of the University of California-Santa Barbara, and Gensheng Wang and Sharmila Kamat, of Case Western Reserve University.

In a nutshell, the experiment seeks to catch the extremely unsociable WIMPs interacting with matter—specifically, the nuclei of germanium or silicon atoms. The germanium and silicon detectors, which resemble hockey pucks, are stacked inside a detection chamber 2,341 feet below the surface of the Earth. The Earth’s crust filters out cosmic rays and other stray particles. An interaction between a WIMP and a germanium or silicon atom would produce a movement of electric charge and the generation of heat, but so little of both that the detection chamber must be cooled to a tenth of a degree above absolute zero to cut out background noise. But neutrons generated underground could pass through the apparatus and produce a signal similar to that expected for a WIMP. The first data, however, show no neutrons sneaking through, meaning the background is low enough to give the experiment a very good shot at detecting the real mccoy.

University of Minnesota physicists have supplied theoretical underpinnings for CDMS II. Physics professor Keith Olive and his colleagues have applied data from other experiments and observations of the cosmos to predict what the CDMS II detection system will find and how sensitive it must be.

“We’re shaping expectations,” said Olive. “For example, we know about how much dark matter there is—that, combined with accelerator searches, translates to limits on the WIMP mass and the rate of its interactions with ordinary matter.”

Physicists also believe WIMPs could be the as-yet unobserved subatomic particles called neutralinos. These would be evidence for the theory of supersymmetry, which goes beyond physics’ Standard Model of fundamental particles and forces. Supersymmetry predicts that every known particle has a supersymmetric partner with complementary properties, although none of these partners has yet been observed. However, many models of supersymmetry predict that the lightest supersymmetric particle, called the neutralino, has a mass about 100 times that of the proton.

WIMPs, which carry no charge, are a study in contradictions. While physicists expect them to have about 100 times the mass of protons, their ghostly nature allows them to slip through ordinary matter, interacting with it extremely infrequently.

The presence of dark matter in the universe is detected through its gravitational effects on all cosmic scales, from the growth of structure in the early universe to the stability of galaxies today. Most astrophysicists believe that this unseen “dark matter” cannot be made of the ordinary matter forming the stars, planets and other objects in the visible universe. Cosmological observations have determined that dark matter constitutes as much as six times more total mass than ordinary matter. WIMPs produced in the early universe are a major contender for this mysterious component.

“Something out there formed the galaxies and holds them together today, and it neither emits nor absorbs light,” said CDMS II co-spokesman Blas Cabrera of Stanford University. “The mass of the stars in a galaxy is only 10 percent of the mass of the entire galaxy, so the stars are like Christmas tree lights decorating the living room of a large dark house.”

The CDMS II result, described in a paper submitted to the journal Physical Review Letters, shows with 90 percent certainty that the interaction rate of a WIMP whose mass is 60 times that of a proton should be less than one interaction every 25 days per kilogram of germanium. The measurements from the CDMS II detectors are at least four times more sensitive than the best previous measurement by the EDELWEISS experiment, a European collaboration with an underground lab near Grenoble, France.

Many supersymmetric models predict neutralinos with just the right properties to make up the dark matter. So either the dominant mass of our universe will be discovered, or a large range of supersymmetric models will be excluded from possibility. Either way, the CDMS II experiment will play a major role in advancing our understanding of particle physics and of the cosmos.

CDMS II comprises scientists from 13 institutions. It is funded by the U.S. Department of Energy, the National Science Foundation from member institutions. The DOE’s Fermi National Accelerator Laboratory manages the CDMS II project. Working with Cushman are postdoctoral researcher Long Duong and graduate student Angela Reisetter. Olive’s colleagues are Andy Ferstl at Winona State University, John Ellis at CERN in Switzerland and research associates Yudi Santoso and Vassilis Spanos. The CDMS home page is at cdms.berkeley.edu/index.html.