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Supersymmetry requires that each particle have a supersymmetric partner. These partners are called sparticles. Generally the name of a particle's supersymmetric partner is formed by adding an "s" to that particle's name. The supersymmetric partner of the electron is called the selectron, that of the quark the squark. The supersymmetric partner of a particle is of the opposite type along the boson/fermion axis. No supersymmetric particles have been found as of yet. It is theorized that, if they exist, they are too heavy to be observed at the energies available in current particle accelerators, though it is hoped that the Large Hadron Collider, when it comes online in 2007, will provide a glimpse of supersymmetry. The original string theory was a 26-dimensional model of the bosonic string. Remember that bosons are particles that transmit forces, and that all rigid matter is composed of fermions, therefore a universe composed of only bosons is not very realistic, and certainly not ours. Adding fermions to the model induces states of negative probablity that are called ghosts, and considered to be unphysical. It was discovered that the only way to get rid of the ghost states was to require that all particles have a supersymmetric partner. The "super" in supersymmetry comes from Lie supergroups and superalgebras, which are symmetric in even dimensions and antisymmetric in odd, hence supersymmetric.
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