of each neutralino will depend on the details of the mixing (e.g. whether they are more higgsino-like or gaugino-like), but they tend to have masses at the weak scale (100 GeV ~ 1 TeV) and couple to other particles with strengths characteristic of the weak interaction. In this way, except for mass, they are phenomenologically similar to neutrinos, and so are not directly observable in particle detectors at accelerators. In models in which R-parity is conserved and the lightest of the four neutralinos is the LSP, the lightest neutralino is stable and is eventually produced in the decay chain of all other superpartners. In such cases supersymmetric processes at accelerators are characterized by the expectation of a large discrepancy in energy and momentum between the visible initial and final state particles, with this energy being carried off by a neutralino which departs the detector unnoticed. This is an important signature to discriminate supersymmetry from Standard Model backgrounds. Relationship to dark matter As a heavy, stable particle, the lightest neutralino is an excellent candidate to form the universe's cold dark matter. In many models the lightest neutralino can be produced thermally in the hot early universe and leave approximately the right relic abundance to account for the observed dark matter. A lightest neutralino of roughly is the leading weakly interacting massive particle (WIMP) dark matter candidate. Neutralino dark matter could be observed experimentally in nature either indirectly or directly. For indirect observation, gamma ray and neutrino telescopes look for evidence of neutralino annihilation in regions of high dark matter density such as the galactic or solar centre. For direct observation, special purpose experiments such as the Cryogenic Dark Matter Search (CDMS) seek to detect the rare impacts of WIMPs in terrestrial detectors. These experiments have begun to probe interesting supersymmetric parameter space, excluding some models for neutralino dark matter, and upgraded experiments with greater sensitivity are under development. See also List of hypothetical particles Weakly Interacting Slender Particle References Dark matter Fermions Supersymmetric quantum field theory Hypothetical elementary particles