weakly interacting massive particle

Weakly interacting massive particle (WIMP)

A hypothetical elementary particle that might make up most of the matter in the universe, and that is also predicted to exist in supersymmetry theory. Most matter is detected only through its gravitational effects; this “dark matter” has not been observed to emit, absorb, or reflect light of any wavelength. The total amount of dark matter appears to be approximately ten times as great as all the ordinary matter in the universe, and about one hundred times as great as all the visible matter. The nature of the dark matter is not yet known, although many experiments are under way to try to discover it directly or indirectly.

Almost all the currently available data in elementary particle physics can be accounted for by a theory called the standard model, in which matter is made of quarks (the building blocks of protons and neutrons) and leptons (including electrons and neutrinos), while the strong, weak, and electromagnetic forces are transmitted by particles like the photon (the carrier of electromagnetic forces). However, the standard model does not predict the existence of any particle—say, X—that could be the dark matter. Most efforts to go beyond the standard model of particle physics have been based on the idea of supersymmetry, and most versions of supersymmetry predict that there will be a stable weakly interacting massive particle (WIMP) that would be a natural candidate for the X particles. Dark matter made of WIMPs would be “cold” dark matter (CDM), and a version of CDM theory has become the standard theory of structure formation in cosmology. See Elementary particle, Standard model, Supersymmetry

There is now abundant evidence for dark matter around galaxies and clusters of galaxies, and on larger scales in the universe. Gas and satellites at large distances from galaxies have orbital velocities similar to those at smaller distances from the center, which indicates that most of the mass in the galaxy must not be near the center, where most stars are, but in a roughly spherical dark matter halo that extends to perhaps ten times the optical size of the galaxy and has a mass at least ten times that of all the stars. Confirmation of the existence of such dark-matter halos has come from gravitational lensing observations, showing that light from more distant galaxies is bent by the gravity of nearer galaxies.

There is also much evidence for dark matter in clusters of galaxies. The astronomer Fritz Zwicky pointed out in 1933 that the galaxies in one nearby cluster were moving at such high speeds that they would not be held together gravitationally unless there was much more mass than was indicated by the light from their stars. This same was subsequently found to be true of other clusters. Later, similar conclusions were reached from x-ray observations and gravitational lensing observations of clusters.

Supersymmetry is the hypothesis that there is a relationship between the two known classes of particles, bosons and fermions. According to supersymmetry, for every kind of boson in the universe, there must also be a corresponding fermion with the same electric charge and very similar interactions with other particles. Since these hypothetical sypersymmetric partner particles have not been discovered yet, if supersymmetry is right their masses must be too large for them to have been produced at current particle accelerators. Thus far, the evidence for supersymmetry is only indirect, but if the theory is right many supersymmetric partner particles should be produced at accelerators such as the Large Hadron Collider (LHC) being built in Geneva, Switzerland. See Particle accelerator, Quantum statistics

Efforts to detect WIMPs directly are based on detecting their scattering from nuclei. WIMPs can also be detected indirectly, for example by looking for particles coming from their annihilation. WIMPs are also expected to be produced at accelerators such as the LHC from rapid decays of heavier supersymmetric partner particles, and this could be where they are discovered first if they are not seen before that in direct or indirect search experiments. Failure to see supersymmetric particles at LHC energies would mean that current ideas about supersymmetry are wrong.