In particle physics, supersymmetry is a proposed extension of spacetime symmetry that relates two basic classes of elementary particles: bosons and fermions. A boson is a particle with integer spin (like photons); a fermion is a particle with half-integer spin (like electrons). In addition to the ordinary three spatial dimensions, all known particles also have a spin degree of freedom. Spin gives rise to the Pauli exclusion principle: no two identical fermions can occupy the same quantum state. Consequently, fermions tend to avoid each other, whereas bosons can clump together.
The best-known example of supersymmetry is the Wess–Zumino model, which was originally proposed as a way to simplify and extend the Standard Model of elementary particles. The Wess–Zumino model contains bothbosonicandfermionicfields. The most fundamental difference between these two types of fields is that bosonic fields obey Bose–Einstein statistics while fermionic fields obey Fermi–Dirac statistics. As a result, the Wess–Zumino model has twice as many degrees of freedom as the Standard Model.
In addition to its mathematical beauty, supersymmetry has several attractive features from a physical point of view. First of all, it provides a natural explanation for why there are only three generations of quarks and leptons in the Standard Model: each generation consists of one supermultiplet containing both bosonic and fermionic fields. Secondly, supersymmetry predicts the existence of new particles calledsparticleswhich are partner states to the known particles; these sparticles have not yet been observed experimentally but they are expected to be produced at high energies in accelerators such as the Large Hadron Collider (LHC). Finally, supersymmetry solves several outstanding problems in theoretical physics such asthe hierarchy problemandgauge unification.