Dark Matter Research

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WIMPs (Weakly Interacting Massive Particles)

WIMPs, by definition, interact only very weakly and are expected to have a mass of between 10 and 1000 GeV. The only detectable interaction that WIMPs may undergo with `normal' matter is scattering and this is what dark matter detectors try to observe via different processes.

WIMPs come from the theory of supersymmetry and there are several reasonable possibilities. It is understood that the dark matter WIMP must be the Lightest Supersymmetric Particle (LSP), which is stable. The four main LSP contenders are the sneutrino, the gravitino, the axino and the neutralino.

The sneutrino is the SUSY boson partner of the neutrino and is expected to have a mass in the range 55.0 GeV - 2.3 TeV. Unfortunately the sneutrino is ruled out as the dominant dark matter component by direct galactic dark matter searches, which have probed the sneutrino-nucleon cross-section region and have not observed these particles.

The gravitino is the spin-3/2 fermion SUSY partner to the, as yet undiscovered, graviton. Its weak gravitational coupling to matter, however, means it is almost impossible to directly detect, and so is a less favoured WIMP candidate at present.

The axino is the superpartner of the axion and is expected to have similar properties to the gravitino. Axinos are not currently, however, one of the most popular dark matter candidates.

There are four neutralinos, which are combinations of the fermion superpartners of the neutral B, Z and Higgs bosons. The lightest neutralino is the favourite of the four to be a dark matter candidate and is expected to have a mass in the range 37 GeV - 500 GeV, with the lower limit coming from collider experiments and the upper limit from WMAP's measurement of the mass-energy density of matter in the Universe. The lightest neutralino is currently the
favourite dark matter candidate worldwide.

SUSY WIMPs, specifically the neutralino, are currently the most favoured candidate for dark matter, since no extra restrictions need be forced on the SUSY theory for them to account for dark matter, it is simply a natural result of SUSY particles decoupling after the Big Bang. The majority of dark matter experiments, therefore, search for these particles.

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