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## FermionIn particle physics, - 12 leptons - 6 particles (e · μ · τ · ν
_{e}· ν_{μ}· ν_{τ}) with 6 corresponding antiparticles, of which 3 are neutrinos and 3 are antineutrinos.
In contrast to bosons, only one fermion can occupy a quantum state at a given time (they obey the Pauli Exclusion Principle). Thus, if more than one fermion occupies the same place in space, the properties of each fermion (e.g. its spin) must be different from the rest. Therefore fermions are usually related with matter while bosons are related with radiation, though the separation between the two is not clear in quantum physics. ## Additional recommended knowledge
## Basic propertiesDue to their half-integer spin, as an observer circles a fermion (or as the fermion rotates 360° about its axis) the wavefunction of the fermion changes sign. A related phenomenon is called an The Pauli exclusion principle obeyed by fermions is responsible for the "rigidness" of ordinary matter (it is a major contributor to Young modulus), and for the stability of the electron shells of atoms (thus for stability of atomic matter). It also is responsible for the complexity of atoms (making it impossible for all atomic electrons to occupy the same energy level), thus making complex chemistry possible. It is also responsible for the pressure within degenerate matter which largely governs the equilibrium state of white dwarfs and neutron stars. In large systems, the difference between bosonic and fermionic statistics is only apparent at large densities when their wave functions overlap. At low densities, both types of statistics are well approximated by Maxwell-Boltzmann statistics, which is described by classical mechanics. ## Elementary fermionsAll observed elementary particles are either fermions or bosons. The known elementary fermions are divided into two groups: quarks and leptons. The quarks make up protons and neutrons, which are composite fermions. Leptons include the electron and similar, heavier particles (muon and tauon) and neutrino. The known fermions of left-handed helicity interact through the weak interaction while the known right-handed fermions do not. ## Composite fermionsIn addition to elementary fermions and bosons, composite particles (made up of more fundamental particles) are also either fermions or bosons, depending only on the number of fermions they contain: - A composite particle containing an even number of fermions is a
*boson*. Examples: - A composite particle containing an odd number of fermions is a
*fermion*. Examples:
The number of bosons within a composite particle has no effect on whether it is a boson or a fermion. Fermionic or bosonic behavior of a composite particle (or system) is only seen at large (compared to size of the system) distance. At proximity, where spatial structure begins to be important, a composite particle (or system) behaves according to its constituent makeup. For example, two atoms of helium can not share the same space if it is comparable by size to the size of the inner structure of the helium atom itself (~10 ## See also- List of particles
- Fermionic field
- Identical particles
- Parastatistics
Categories: Subatomic particles | Fermions |
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This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Fermion". A list of authors is available in Wikipedia. |