Cryogenic Dark Matter Search
The Cryogenic Dark Matter Search (CDMS) is a series of experiments designed directly to 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, SuperCDMS, is located deep underground in the Soudan Mine in northern Minnesota.
Observations of the large-scale structure of the universe show that matter is aggregated into very large structures that have not had time form 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. 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.
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. 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 WIMP-scattering events to be identified even though they are rare compared to the vast majority of unwanted background interactions.
From Supersymmetry, the probability of a spin-independent interaction between a WIMP and a nucleus would be related to the amount of nucleons in the nucleus. Thus, a WIMP would be more likely to interact with a germanium detector than a silicon detector, since germanium is a much heavier element. Neutrons would be able to interact with both silicon and germanium detectors with similar probability. By comparing rates of interactions between silicon and germanium detectors, CDMS is able to determine the probability of interactions being caused by neutrons.
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 a FET amplifier. CDMS detectors also provide data on the phonon pulse shape which is crucial in rejecting near-surface background events.
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. A new detector with interleaved electrodes, more mass, and even better background rejection is currently taking data at Soudan, under the name of SuperCDMS.
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 25% of the time. Polythene absorbers were fitted to reduce any neutron background.
A 2011 analysis of the data, with lower energy thresholds, looked for evidence for low-mass WIMPs (M < 9 GeV). Their limits rule out hints claimed by a new germanium experiment called CoGeNT and the long-standing DAMA result based on annual modulation.
A further analysis of data in 2013, revealed 3 WIMP detections in a time period where a background reading of 0.7 would be expected. Also, the detections have masses comparable to that expected from WIMPs, including neutralinos. There is a 0.19% chance that these detections are anomalous, and caused by the background noise, giving the result a 99.8% (3 sigma) confidence level. Whilst not conclusive evidence for WIMPs this provides strong weight to the theories. The CDMS collaboration published these results in Physical Review Letters, in May 2013.
Proposed upgrades: SuperCDMS and GEODM
SuperCDMS is the successor to CDMS II. The "super" refers to the larger, improved detectors. There are actually three generations of Super CDMS planned:
- SuperCDMS Soudan, with 9.3 kg of active detector mass made of 15×620 g germanium discs (76.2 mm/3″ diameter × 25.4 mm/1″ thick). This has been operating since March 2012.
- SuperCDMS SNOLAB, with 100–200 kg of active detector mass, made of 1380 g germanium discs (100 mm/3.9″ diameter × 33.3 mm/1.3″ thick). Development is underway, and it is hoped construction will begin in 2014. The deeper SNOLAB site will reduce cosmic ray backgrounds compared to Soudan.
- GEODM (Germanium Observatory for Dark Matter), with more than 1000 kg of detector mass. Preliminary planning hopes to install this in the DUSEL laboratory.
Increasing the detector mass only makes the detector more sensitive if the unwanted background detections do not increase as well, thus each generation must be cleaner and better shielded than the one before. The purpose of building in ten-fold stages like this is to develop the necessary shielding techniques before finalizing the GEODM design.
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- "Latest Results in the Search for Dark Matter Thursday, December 17, 2009"
- CDMS Collaboration (21 Apr 2011). "Results from a Low-Energy Analysis of the CDMS II Germanium Data". arXiv:1011.2482v3 [astro-ph.CO].
- CDMS Collaboration (4 May 2013). "Dark Matter Search Results Using the Silicon Detectors of CDMS II". Physical Review Letters. arXiv:1304.4279.
- Saab, Tarek (2012-08-01), "The SuperCDMS Dark Matter Search", SLAC Summer Institute 2012, SLAC National Accelerator Laboratory, retrieved 2012-11-28