The direct detection of putative dark matter particles, as opposed to measuring their collective gravitational effects, remains a significant challenge. A number of experiments are actively searching for WIMPs (= Weakly Interacting Massive Particles) as the currently favored candidates for dark matter. Particle physics models with supersymmetric extensions to the Standard Model suggest that the most abundant particle of dark matter would have a mass significantly greater than the proton. The mass is expected to lie somewhere in the range of under 10 times the mass of a proton to possibly as much as 10,000 times the mass of a proton (around 10 to 10,000 GeV/c^2 where GeV is a billion electron volts of energy and we divide by the square of the speed of light to convert to mass). The WIMP name reflects that these particles would only interact with other matter via the weak nuclear force and via gravity. They do not react via either the strong nuclear force or electromagnetism.
It is believed that WIMPs are produced in the Big Bang as a decay mode from the massive release of energy during the inflation phase. The currently most favored candidate WIMP is the proposed least massive supersymmetric particle (LSP), which is expected to be stable. Supersymmetric particles are considered to have large masses and would have the same quantum numbers (properties) as corresponding Standard Model particles, except for their spins, that would differ by 1/2 from their partners. The local density of dark matter is estimated to be about .3 GeV / cc (GeV per cubic centimeter). If the WIMP mass is 100 GeV/ c^2 there would be about 3 particles per liter.
Two major techniques are being employed to search for cosmic WIMPs. One of these is to detect the direct impact of WIMPs with atomic nuclei (via elastic scattering) in underground laboratories here on Earth. These would be very rare events, so large detectors are required and experiments must gather data for a long time. Such an impact leaves products from the interaction and it is these products that are actually detected in an experiment. A second technique is to look for gamma rays, which are produced in the galactic halo of the Milky Way or also the Sun’s interior, when dark matter (WIMP) collisions with ordinary matter occur at those locations. The gamma rays produced in this way can in principle be detected with satellites in Earth orbit.
Beyond these two general techniques to detect WIMPs there is the hope of actually creating these dark matter particles via high energy collisions at the Large Hadron Collider.
One recent set of results is from the XENON collaboration, which is funded by the US government and 6 European nations. The XENON100 experiment is located underground in Italy, in the Gran Sasso National Laboratory. The heart of the detector consists of cooled Xenon of quantity 65 kilograms. The target is in both the liquid and gas phases. When a WIMP strikes a Xenon atom directly, electrons are either knocked out of the Xenon atom or boosted to higher energy orbital levels in the atom. Both scintillation light, due to subsequent decay of the electron orbital, and ionization electrons, are thus generated. The 100 days of exposure of XENON100 analyzed to date have yielded 3 events, but one expects 2 events from background neutrons producing similar signatures, so there is statistically no detection. This result does allow the placement of upper limits on the WIMP cross-section for interaction as a function of mass.
The result appears to be in conflict with another experiment, also located at the same Gran Sasso laboratory, run by the DAMA team. The DAMA/Libra experiment claims a statistically significant detection of an annually modulated “WIMP wind” which reflects the variation in the Earth’s orbital direction with respect to the diffuse background of WIMP particles. The intensity is well above XENON100 limits for certain possible mass ranges of the WIMP major constituent particle.
The race is on to secure the direct detection of dark matter particles, beyond their extensive apparent gravitational effects. Rapid progress in enhancing the sensitivity of detection methods, typically including the use of larger detectors, will increase the probability of better WIMP detection and mass determination in the future.
M. Drees, G. Gerbier and the Particle Data Group, 2010. “Dark Matter” Journal of Physics G37(7A) pp 255-260
J. Feng, 2010. Ann. Rev. Astron. Astrophys. 48: 495, “Dark Matter Candidates from Particle Physics and Methods of Detection” (also available at: arxiv.org/pdf/1003.0904)
S. Perrenod, 2011. Dark Matter, Dark Energy, Dark Gravity, chapter 4, BookBrewer Publishing