The Cryogenic Dark Matter Search (CDMS) is a series of experiments designed to directly detect particle dark matter in the form of WIMPs. Using an array of semiconductor detectors at millikelvin temperatures, CDMS has set the most sensitive limits to date on the interactions of WIMP dark matter with terrestrial materials. The first experiment, CDMSI, was run in a tunnel under the Stanford University campus. The current experiment, CDMSII, is located deep underground in the Soudan Mine in Minnesota.
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Observations of the large-scale structure of the universe show that matter is aggregated into very large structures that would not have time to have formed under the force of their own self-gravitation. It is generally believed that some form of missing mass is responsible for increasing the gravitational force at these scales, although this mass has not been directly observed. This is a problem; normal matter in space will heat up until it gives off light, so if this missing mass exists, it is generally assumed to be in a form that is not commonly observed on earth.
A number of proposed candidates for the missing mass have been put forward over time. Early candidates included heavy baryons that would have had to be created in the big bang, but more recent work on nucleosynthesis seems to have ruled most of these out.[1] Another candidate are new types of particles known as weakly interacting massive particles, or "WIMP"s. As the name implies, WIMPs interact weakly with normal matter, which explains why they are not easily visible.[1]
Detecting WIMPs thus presents a problem; if the WIMPs are very weakly interacting, detecting them will be extremely difficult. Detectors like CDMS and similar experiments measure huge numbers of interactions within their detector volume in order to find the extremely rare WIMP events.
The CDMS detectors measure the ionization and phonons produced by every particle interaction in their germanium and silicon crystal substrates.[1] These two measurements determine the energy deposited in the crystal in each interaction, but also give information about what kind of particle caused the event. The ratio of ionization signal to phonon signal differs for particle interactions with atomic electrons ("electron recoils") and atomic nuclei ("nuclear recoils"). The vast majority of background particle interactions are electron recoils, while WIMPs (and neutrons) are expected to produce nuclear recoils. This allows the vast majority of the unwanted background interactions to be rejected, so that any WIMP-scattering events can be identified even if they are very rare.
CDMS detectors are disks of germanium or silicon, cooled to millikelvin temperatures by a dilution refrigerator. The extremely low temperatures are needed to limit thermal noise which would otherwise obscure the phonon signals of particle interactions. Phonon detection is accomplished with superconduction transition edge sensors (TESs) read out by SQUID amplifiers, while ionization signals are read out using an FET amplifier. CDMS detectors also provide data on the phonon pulse shape which is crucial in rejecting near-surface background events.
Simultaneous detection of ionization and heat with semiconductors at low temperature was an original idea by Lawrence M. Krauss, Mark Srednicki and Frank Wilczek.[2]
CDMS collected WIMP search data in a shallow underground site at Stanford University through 2002, and has operated (with collaboration from the University of Minnesota) in the Soudan Mine since 2003. Its results so far are negative, but CDMS is able to provide the most sensitive limits on WIMP dark matter in many interesting models. CDMS is currently/when taking data at Soudan with its full complement of detectors, aiming to increase the sensitivity of WIMP searches by a further factor of 10. The current/when sensitivity is nearing the levels where theoretically the detector should start to detect dark matter.
On December 17, 2009, the collaboration announced the possible detection of two candidate WIMPs, one on August 8, 2007 and the other on October 27, 2007. However, due to this low number of events, the team could not claim that the detections were true WIMPs; they may have been false positives from background noise such as neutron collisions. It is estimated that such noise would produce two or more events in one quarter of the time.[3] Polythene absorbers were fitted to reduce any neutron background.[4]
A 2011 analysis found little evidence of WIMPs < 9GeV (in conflict with other DM expts).[5]
SuperCDMS, a proposed successor to CDMS II, may have the sensitivity to explore and detect neutralinos.