In particle physics, supersymmetry (SUSY) is a proposed extension of spacetime symmetry that relates two basic classes of elementary particles: bosons, which have an integer-valued spin, and fermions, which have a half-integer spin. Each particle from one group is associated with a particle from the other, called its superpartner, whose spin differs by a half-integer. In a theory with unbroken supersymmetry each pair of superpartners shares the same mass and internal quantum numbers besides spin, but since no superpartners have been observed yet, supersymmetry must be a spontaneously broken symmetry.[citation needed] The failure of the Large Hadron Collider to find evidence for supersymmetry has led some physicists to suggest that the theory should be abandoned. Experiments with the Large Hadron Collider also yielded an extremely rare particle decay event which casts doubt on supersymmetry. A major weakness of SUSY is that it is not falsifiable, because its breaking mechanism and the minimum mass above which it is restored are unknown.[citation needed] This minimum mass can be pushed upwards to arbitrarily large values, without disproving the symmetry.Supersymmetry differs notably from currently known symmetries in that it establishes a symmetry between classical and quantum physics, which up to now has not been observed in any other domain. While any number of bosons can occupy the same quantum state, for fermions this is not possible because of the exclusion principle, which allows only one fermion in a given state. But when the occupation numbers become large, quantum physics approaches the classical limit. This means that while bosons also exist in classical physics, fermions do not. That makes it difficult to expect that bosons, if at all, possess the same quantum numbers as fermions.There is only indirect evidence for the existence of supersymmetry, primarily in the form of evidence for gauge coupling unification. However this refers only to electroweak and strong interactions and does not provide the ultimate unification of all interactions, since it leaves gravitation untouched.Supersymmetry is also motivated by solutions to several theoretical problems, for generally providing many desirable mathematical properties, and for ensuring sensible behavior at high energies. Supersymmetric quantum field theory is often much easier to analyze, as many more problems become exactly solvable. When supersymmetry is imposed as a local symmetry, Einstein's theory of general relativity is included automatically, and the result is said to be a theory of supergravity. It is also a feature of a candidate of a theory of everything, superstring theory.A central motivation for supersymmetry close to the TeV energy scale is the resolution of the hierarchy problem of the Standard Model. Without the extra supersymmetric particles, the Higgs boson mass is subject to quantum corrections which are so large as to naturally drive it close to the Planck mass barring its fine tuning to an extraordinarily tiny value. In the supersymmetric theory, on the other hand, these quantum corrections are canceled by those from the corresponding superpartners above the supersymmetry breaking scale, which becomes the new characteristic natural scale for the Higgs mass. Other attractive features of TeV-scale supersymmetry are the fact that it often provides a candidate dark matter particle at a mass scale consistent with thermal relic abundance calculations, provides a natural mechanism for electroweak symmetry breaking and allows for the precise high-energy unification of the weak, the strong and electromagnetic interactions. Therefore, scenarios where supersymmetric partners appear with masses not much greater than 1 TeV are considered the most well-motivated by theorists. These scenarios would imply that experimental traces of the superpartners should begin to emerge in high-energy collisions at the LHC relatively soon. As of September 2011, no meaningful signs of the superpartners have been observed, which is beginning to significantly constrain the most popular incarnations of supersymmetry. However, the total parameter space of consistent supersymmetric extensions of the Standard Model is extremely diverse and can not be definitively ruled out at the LHC.Another theoretically appealing property of supersymmetry is that it offers the only "loophole" to the Coleman–Mandula theorem, which prohibits spacetime and internal symmetries from being combined in any nontrivial way, for quantum field theories like the Standard Model under very general assumptions. The Haag-Lopuszanski-Sohnius theorem demonstrates that supersymmetry is the only way spacetime and internal symmetries can be consistently combined.The Minimal Supersymmetric Standard Model is one of the best studied candidates for physics beyond the Standard Model.
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