In astrophysics, WIMPs, or weakly interacting massive particles, are hypothetical particles serving as one possible solution to the dark matter problem. These particles interact through the weak nuclear force and gravity, and possibly through other interactions no stronger than the weak force. Because they do not interact with electromagnetism they cannot be seen directly, and because they do not interact with the strong nuclear force they do not react strongly with atomic nuclei. Spiral Galaxy ESO 269-57 // Astrophysics is the branch of astronomy that deals with the physics of the universe, including the physical properties (luminosity, density, temperature and chemical composition) of astronomical objects such as stars, galaxies, and the interstellar medium, as well as their interactions. ... In cosmology, dark matter refers to matter particles, of unknown composition, that do not emit or reflect enough electromagnetic radiation to be detected directly, but whose presence can be inferred from gravitational effects on visible matter such as stars and galaxies. ... The weak nuclear force or weak interaction is one of the four fundamental forces of nature. ... Gravity is a force of attraction that acts between bodies that have mass. ... The nuclear force (or nucleon-nucleon interaction or residual strong force) is the force between two or more nucleons. ...

Theoretical arguments

Although the existence of WIMPs in nature is hypothetical at this point, it would resolve a number of astrophysical and cosmological problems related to dark matter. The main theoretical characteristics of a WIMP particle are:

• Interaction only through the weak nuclear force and gravity, or at least with interaction cross-sections no higher than the weak scale;
• Large mass compared to standard particles.

Because of their lack of interaction with normal matter, they would be dark and invisible through normal electromagnetic observations. Because of their large mass, they would be relatively slow moving and therefore cold. As result they would tend to remain clumpy. Simulations of a universe full of cold dark matter produce galaxy distributions that are roughly similar to that which is observed. WIMPs are considered one of the main candidates for "cold dark matter", the other being MACHOs (Massive Compact Halo Object). (These names were deliberately chosen for contrast, with MACHOs named later than WIMPs). The weak nuclear force or weak interaction is one of the four fundamental forces of nature. ... Gravity is a force of attraction that acts between bodies that have mass. ... Darkness is the absence of light. ... Cold dark matter (or CDM) is a refinement of the big bang theory that contains the additional assumption that most of the matter in the Universe consists of material which cannot be observed by its electromagnetic radiation and hence is dark while at the same time the particles making up... Look up Macho in Wiktionary, the free dictionary This is a disambiguation page: a list of articles associated with the same title. ...

Also, in contrast to MACHOs, there are no known particles within the standard model of particle physics that have all the properties of WIMPs. The particles that have little interaction with normal matter, such as neutrinos, are all very light, and hence would be fast moving or hot. Hot dark matter would smear out large scale structure of galaxies and is not considered a viable cosmological model. WIMP-like particles are predicted by supersymmetry, a popular type of extension to the standard model, although none of the large numbers of new particles in supersymmetry have been observed. The Standard Model of Fundamental Particles and Interactions The Standard Model of particle physics is a theory which describes the strong, weak, and electromagnetic fundamental forces, as well as the fundamental particles that make up all matter. ... The neutrino is an elementary particle. ... Look up hot in Wiktionary, the free dictionary. ... Hot dark matter is a form of dark matter, which consists of particles that travel with relativistic velocities. ... This article or section is in need of attention from an expert on the subject. ...

Experimental detection

Because WIMPs may only interact via the 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.

Indirect detection efforts rest upon the theoretical 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 (see Solar WIMP capture). As more and more WIMPs thermalize inside the Sun, they begin to annihilate with each other, forming a variety of particles including 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. The Sun is the star at the center of our solar system. ... Annihilation occurs when a particle collides with an antiparticle. ... The neutrino is an elementary particle. ... Super-Kamiokande, or Super-K for short, is a neutrino observatory in Japan. ... The Higgs boson is a hypothetical massive scalar elementary particle predicted to exist by the Standard Model of particle physics. ... Earth (often referred to as The Earth) is the third planet in the solar system in terms of distance from the Sun, and the fifth in order of size. ...

It is important to note that, 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.

CDMS parameter space excluded as of 2004. DAMA result is located in green area and is disallowed.

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 millikelvins. A layer of metal (aluminum 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. Image File history File links Download high resolution version (1027x683, 163 KB) Summary CDMS (cryogenic dark matter search) parameter space exclusion of DAMA result represented in green parameter space. ... Image File history File links Download high resolution version (1027x683, 163 KB) Summary CDMS (cryogenic dark matter search) parameter space exclusion of DAMA result represented in green parameter space. ... The Cryogenic Dark Matter Search (CDMS) is an experiment designed to directly detect particle dark matter in the form of WIMPs. ... The Soudan Underground Mine is described as Minnesotas oldest, deepest, and richest iron mine. ... General Name, Symbol, Number germanium, Ge, 32 Chemical series metalloids Group, Period, Block 14, 4, p Appearance grayish white Atomic mass 72. ... General Name, Symbol, Number silicon, Si, 14 Chemical series metalloids Group, Period, Block 14, 3, p Appearance dark gray, bluish tinge Atomic mass 28. ... It has been suggested that crystallization processes be merged into this article or section. ... The kelvin (symbol: K) is the SI unit of temperature, and is one of the seven SI base units. ... Superconductivity is a phenomenon occurring in certain materials at low temperatures, characterised by the complete absence of electrical resistance and the damping of the interior magnetic field (the Meissner effect. ... Electrical resistance is a measure of the degree to which an electrical component opposes the passage of current. ...

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. An example of this technique is the DAMA/NaI 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 of 2004 exclude the entire DAMA signal region given certain standard assumptions about the properties of the WIMPs and the dark matter halo. A scintillator is a device or substance that absorbs high energy (ionizing) electromagnetic or charged particle radiation then, in response, fluoresces photons at a characteristic Stokes-shifted (longer) wavelength, releasing the previously absorbed energy. ... The DAMA/NaI experiment [1] was designed to detect dark matter using the direct detection technique. ... A year is the time between two recurrences of an event related to the orbit of the Earth around the Sun. ... The Soudan Underground Mine is described as Minnesotas oldest, deepest, and richest iron mine. ...

External links

• Particle Data Group review article on WIMP search

Experiments

• CDMS web site
• COUPP web site
• CRESST web site
• DAMA web site
• EDELWEISS web site
• HDMS web site
• XENON web site
• ZEPLIN web site