FIG. 1. Principle of the CDMS detector. A Ge or Si crystal is cooled to 20 mK and instrumented with charge and bolometric detectors. A WIMP recoils off a nucleus, releasing phonons (heat) and charge.

Despite the feeble couplings and the lack of evidence for the underlying theories, several experiments are now underway with the sensitivity required to detect directly galactic halo dark matter through its interactions with matter and radiation. These experiments divide into two broad classes: searches for weakly interacting massive particles (WIMPs) and searches for axions. There is a very strong theoretical bias for supposing that supersymmetry (SUSY) is a correct description of nature. WIMPs are predicted by this SUSY theory and have the required properties to be dark matter (3). These WIMPs are detected from the signals of an occasional recoil against nucleons. There are efforts around the world to detect these rare recoils. The WIMP part of this overview focuses on the cryogenic dark matter search (CDMS) underway in California. Axions are predicted to arise from a minimal extension of the standard model that explains the absence of the expected large CP violating effects in strong interactions (4). Axions can turn into microwave photons in the presence of a large magnetic field. It is the slight excess of photons above noise that signals the axion. Axion searches are underway in California and Japan. The axion part of this overview focuses on the California effort. Brevity does not allow me to discuss other WIMP and axion searches, likewise there is not space for accelerator and satellite-based searches; these are important topics and I apologize for their omission.

Searching for WIMPs

SUSY, the theory that predicts WIMPs, is really more a class of theories than a specific theory (4). The theory does not do a very good job of constraining what exactly the WIMP is, nor does it do a very good job of constraining the WIMP mass, nor does it do a very good job of constraining the WIMP couplings. This makes the task of detection more difficult. However, WIMPs too light would have been seen in accelerator experiments. WIMPs too heavy are theoretically disfavored. WIMP masses from perhaps 50 to 10,000 TeV are then reasonable masses to suppose. Also, by positing the WIMPs density to account for the missing mass, one can work backwards to the early universe and roughly infer the strength of the WIMP interactions. Experimenters look for by-products of the occasional recoil of a WIMP against a nucleus. These by-products include charge, light, heat, or some or all of the three. The expected signal levels and signal rates are small. The backgrounds of radioactive decays and cosmic rays are serious.

Several new high-sensitivity detectors are beginning to take data around the world. These include CRESST (cryogenic rare

FIG. 2. Sketch of the “icebox” housing the CDMS detectors. The dilution refrigerator is shown to the left. The cold stage temperature is carried within multiple heat shields to the box shown on the right. Removing the refrigerator from the experiment area reduces backgrounds from materials in the refrigerator. The icebox is surrounded by plastic scintillation veto counters, as well as polyethylene neutron absorber.



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement