Weakly Interacting Massive Particles - Experimental Detection

Experimental Detection

Because WIMPs may only interact through gravitational and weak forces, they are extremely difficult to detect. However, there are many experiments underway to attempt to detect WIMPs both directly and indirectly. Although predicted scattering rates for WIMPs from nuclei are significant for large detector target masses, prediction that halo WIMPs may, as they pass through the Sun, interact with solar protons and helium nuclei. Such an interaction would cause a WIMP to lose energy and become "captured" by the Sun. As more and more WIMPs thermalize inside the Sun, they begin to annihilate with each other, forming a variety of particles including high-energy neutrinos. These neutrinos may then travel to the Earth to be detected in one of the many neutrino telescopes, such as the Super-Kamiokande detector in Japan. The number of neutrino events detected per day at these detectors depends upon the properties of the WIMP, as well as on the mass of the Higgs boson. Similar experiments are underway to detect neutrinos from WIMP annihilations within the Earth and from within the galactic center.

While most WIMP models indicate that a large enough number of WIMPs would be captured in large celestial bodies for these experiments to succeed, it remains possible that these models are either incorrect or only explain part of the dark matter phenomenon. Thus, even with the multiple experiments dedicated to providing indirect evidence for the existence of cold dark matter, direct detection measurements are also necessary to solidify the theory of WIMPs.

Although most WIMPs encountering the Sun or the Earth are expected to pass through without any effect, it is hoped that a large number of dark matter WIMPs crossing a sufficiently large detector will interact often enough to be seen—at least a few events per year. The general strategy of current attempts to detect WIMPs is to find very sensitive systems that can be scaled up to large volumes. This follows the lessons learned from the history of the discovery and (by now) routine detection of the neutrino.

A technique used by the Cryogenic Dark Matter Search (CDMS) detector at the Soudan Mine relies on multiple very cold germanium and silicon crystals. The crystals (each about the size of a hockey puck) are cooled to about 50 mK. A layer of metal (aluminium and tungsten) at the surfaces is used to detect a WIMP passing through the crystal. This design hopes to detect vibrations in the crystal matrix generated by an atom being "kicked" by a WIMP. The tungsten metal sensors are held at the critical temperature so they are in the superconducting state. Large crystal vibrations will generate heat in the metal and are detectable because of a change in resistance.

In February 2010, researchers at the Soudan Mine CDMS II experiment announced that they had observed two events that may have been caused by WIMP-nucleus collisions. CoGeNT, a smaller detector using a single germanium puck, designed to sense WIMPs with smaller masses, reported hundreds of detection events in 56 days. Juan Collar, who presented the results to a conference at the University of California, was quoted: "If it's real, we're looking at a very beautiful dark-matter signal" (Other explanations, such as an unexplained radioactive decay process in the electronics, might cause a spurious signal) The experiment estimated the WIMP masses at 7-11 GeV (approximately 10× the mass of a proton), which is at lower limit of detection of the CDMSII experiment.

The Directional Recoil Identification From Tracks (DRIFT) collaboration is attempting to utilize the predicted directionality of the WIMP signal in order to prove the existence of WIMPs. DRIFT detectors use a 1m3 volume of low pressure carbon disulfide gas as a target material. The use of a low pressure gas means that a WIMP colliding with an atom in the target will cause it to recoil several millimetres leaving a track of charged particles in the gas. This charged track is drifted to an MWPC readout plane that allows it to be reconstructed in three dimensions, which can then be used to determine the direction the WIMP came from.

Another way of detecting atoms "knocked about" by a WIMP is to use scintillating material, so that light pulses are generated by the moving atom. Experiments such as DEAP at SNOLAB or WARP at the LNGS plan to instrument a very large target mass of liquid argon for sensitive WIMP searches. Another example of this technique is the DAMA/NaI and DAMA/LIBRA detector in Italy. It uses multiple materials to identify false signals from other light-creating processes. This experiment observed an annual change in the rate of signals in the detector. This annual modulation is one of the predicted signatures of a WIMP signal, and on this basis the DAMA collaboration has claimed a positive detection. Other groups, however, have not confirmed this result. The CDMS and EDELWEISS experiments would be expected to observe a significant number of WIMP-nucleus scatters if the DAMA signal were in fact caused by WIMPs. Since the other experiments do not see these events, the interpretation of the DAMA result as a WIMP detection can be excluded for most WIMP models. It is possible to devise models that reconcile a positive DAMA result with the other negative results, but as the sensitivity of other experiments improves, this becomes more difficult. The CDMS data taken in the Soudan Mine and made public in May 2004 exclude the entire DAMA signal region given certain standard assumptions about the properties of the WIMPs and the dark matter halo.

The PICASSO (Project in Canada to Search for Supersymmetric Objects) experiment is a direct dark matter search experiment that is located at SNOLAB in Canada. It uses bubble detectors with Freon as the active mass. PICASSO is predominantly sensitive to spin-dependent interactions of WIMPs with the fluorine atoms in the Freon.

A bubble detector is a radiation sensitive device that uses small droplets of superheated liquid that are suspended in a gel matrix. It uses the principle of a bubble chamber but since only the small droplets can undergo a phase transition at a time the detector can stay active for much longer periods than a classic bubble chamber. When enough energy is deposited in a droplet by ionizing radiation the superheated droplet undergoes a phase transition and becomes a gas bubble. The PICASSO detectors contain Freon droplets with an average diameter of 200 µm. The bubble development in the detector is accompanied by an acoustic shock wave that is picked up by piezo-electric sensors. The main advantage of the bubble detector technique is that the detector is almost insensitive to background radiation. The detector sensitivity can be adjusted by changing the temperature of the droplets. Freon-loaded detectors are typically operated at temperatures between 15°C and 55°C. There is another similar experiment using this technique in Europe called SIMPLE.

PICASSO reports results (November 2009) for spin-dependent WIMP interactions on 19F. No dark matter signal has been found, but for WIMP masses of 24 Gev/c2 new stringent limits have been obtained on the spin-dependent cross section for WIMP scattering on 19F of 13.9 pb (90% CL). This result has been converted into a cross section limit for WIMP interactions on protons of 0.16 pb (90% CL). The obtained limits restrict recent interpretations of the DAMA/LIBRA annual modulation effect in terms of spin dependent interactions.