Closing In on The Mysterious Dark Matter?

Initial data from the Cryogenic Dark Matter Search (CDMS II) was reported at the APS April meeting in Denver. This underground observatory in Northern Minnesota has provided unprecedented sensitivity into the search for so-called Weakly Interacting Massive Particles (WIMPs). Should evidence of WIMPs be observed, it could answer the dual mystery of both the dark matter problem and supersymmetry.

The CDMS II team practices "underground astronomy" with particle detectors located nearly half a mile below Earth's surface in a former iron mine. Earth's crust blocks cosmic rays and the background particles they produce. Made of germanium and silicon crystals, the detectors are chilled to within one-tenth of a degree of absolute zero. They are capable of measuring both the charge and vibration produced by particle interactions within the crystals.

The detectors are now able to look for signals just one-fourth as intense as any seen before, and the team expects to improve sensitivity by a factor of 20 over the next few years. WIMPs will signal their presence by releasing less charge than most background particles produce for the same amount of vibration.

A WIMP, which carries no charge, is expected to have roughly one hundred times the mass of a proton. Yet WIMPs are able to slip through ordinary matter while barely leaving a trace. The presence of dark matter in the universe is detected through its gravitational effects, from the growth of structure in the early universe to the stability of galaxies today. Dark matter cannot be made of the ordinary matter forming objects in the visible universe, and constitutes as much as seven times more total mass than ordinary matter. WIMPs are a strong contender for dark matter.

The nature of dark matter is fundamental to our understanding of the formation of the universe. With the CDMS II collaboration, either the dominant mass of the universe will be discovered, or a large number of supersymmetric models will be excluded as possibilities.

WIMPs might be the as-yet-unobserved subatomic particles called neutralinos. That would provide strong evidence for supersymmetry, which predicts that every known particle has a supersymmetric partner with complementary properties, although no such partners have been observed to date. Many supersymmetry models predict that the lightest such particle, called the neutralino, has a mass of about 100 times that of the proton.