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In particle physics, a fermion is a subatomic particle that follows Fermi–Dirac statistics. Fermions have a half-integer spin (spin , spin , etc.) and obey the Pauli exclusion principle. These particles include all and and all composite particles made of an odd number of these, such as all and many and atomic nucleus. Fermions differ from , which obey Bose–Einstein statistics.
Some fermions are elementary particles (such as ), and some are composite particles (such as ). For example, according to the spin-statistics theorem in relativistic quantum field theory, particles with integer spin are . In contrast, particles with half-integer spin are fermions.
In addition to the spin characteristic, fermions have another specific property: they possess conserved baryon or lepton . Therefore, what is usually referred to as the spin-statistics relation is, in fact, a spin statistics-quantum number relation.
As a consequence of the Pauli exclusion principle, only one fermion can occupy a particular quantum state at a given time. Suppose multiple fermions have the same spatial probability distribution, then, at least one property of each fermion, such as its spin, must be different. Fermions are usually associated with matter, whereas bosons are generally force carrier particles. However, in the current state of particle physics, the distinction between the two concepts is unclear. Weakly interacting fermions can also display bosonic behavior under extreme conditions. For example, at low temperatures, fermions show superfluidity for uncharged particles and superconductivity for charged particles.
Composite fermions, such as protons and , are the key building blocks of baryonic matter.
English theoretical physicist Paul Dirac coined the name fermion from the surname of Italian physicist Enrico Fermi.Notes on Dirac's lecture Developments in Atomic Theory at Le Palais de la Découverte, 6 December 1945, UKNATARCHI Dirac Papers BW83/2/257889. See note 64 on page 331 in "The Strangest Man: The Hidden Life of Paul Dirac, Mystic of the Atom" by Graham Farmelo
Mathematically, there are many varieties of fermions, with the three most common types being:
Most Standard Model fermions are believed to be Dirac fermions, although it is unknown at this time whether the are Dirac or Majorana fermions (or both). Dirac fermions can be treated as a combination of two Weyl fermions. In July 2015, Weyl fermions have been experimentally realized in .
Examples include the following:
The fermionic or bosonic behaviour of a composite particle is only observed when the constituent particles remain far apart. When they are close together, the spatial structure becomes important and the composite particles behave according to their constituent makeup.
Fermions can exhibit bosonic behavior when they become bound in pairs. This is the origin of superconductivity and the of helium-3. In superconducting materials, electrons interact through the exchange of , forming , while in helium-3, the atoms interact and pair via spin fluctuations.
The quasiparticles of the fractional quantum Hall effect are also known as composite fermions; they consist of electrons with an even number of quantized vortices attached to them.
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