Fermion
Standard model of particle physics 

Large Hadron Collider tunnel at CERN

Constituents

Limitations

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In particle physics, a fermion (a name coined by Paul Dirac^{1} from the surname of Enrico Fermi) is any particle characterized by Fermi–Dirac statistics and following the Pauli exclusion principle; fermions include all quarks and leptons, as well as any composite particle made of an odd number of these, such as all baryons and many atoms and nuclei. Fermions contrast with bosons which obey Bose–Einstein statistics.
A fermion can be an elementary particle, such as the electron; or it can be a composite particle, such as the proton. According to the spinstatistics theorem in any reasonable relativistic quantum field theory, particles with integer spin are bosons, while particles with halfinteger spin are fermions.
Besides this spin characteristic fermions have another specific property: they possess conserved baryon or lepton quantum numbers. Therefore what is usually referred as the spinstatistics relation is in fact a spinstatisticsquantum number relation.^{2}
In contrast to bosons, as a consequence of the Pauli principle only one fermion can occupy a particular quantum state at any given time. If 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; although in the current state of particle physics the distinction between the two concepts is unclear.
Composite fermions, such as protons and neutrons, are key building blocks of matter. Weakly interacting^{clarification needed} fermions can also display bosonic behavior under extreme conditions, such as in superconductivity.
Elementary fermions
The Standard Model recognizes two types of elementary fermions: quarks and leptons. In all, the model distinguishes 24 different fermions: six quarks: the up quark, down quark, strange quark, charmed quark, bottom quark, and top quark; and six leptons (electron, electron neutrino, muon, muon neutrino, tau particle, tau neutrino), each with a corresponding antiparticle.
Mathematically, fermions come in three types  Weyl fermions (massless), Dirac fermions (massive), and Majorana fermions (each its own antiparticle). Most Standard Model fermions are believed to be Dirac fermions, although it is unknown at this time whether the neutrino is a Dirac or a Majorana fermion. Dirac fermions can be treated as a combination^{clarification needed} of two Weyl fermions.
Composite fermions
Composite particles (such as hadrons, nuclei, and atoms) can be bosons or fermions depending on their constituents. More precisely, because of the relation between spin and statistics, a particle containing an odd number of fermions is itself a fermion: it will have halfinteger spin.
Examples include the following:
 A baryon, such as the proton or neutron, contains three fermionic quarks and thus it is a fermion;
 The nucleus of a carbon13 atom contains six protons and seven neutrons and is therefore a fermion;
 The atom helium3 (^{3}He) is made of two protons, one neutron, and two electrons, and therefore it is a fermion.
The number of bosons within a composite particle made up of simple particles bound with a potential 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) distances. At proximity, where spatial structure begins to be important, a composite particle (or system) behaves according to its constituent makeup.
Fermions can exhibit bosonic behavior when they become loosely bound in pairs. This is the origin of superconductivity and the superfluidity of helium3: in superconducting materials, electrons interact through the exchange of phonons, forming Cooper pairs, while in helium3, Cooper pairs are formed via spin fluctuations.
The quasiparticles of the fractional quantum Hall effect are also known as composite fermions, which are electrons with an even number of quantized vortices attached to them.
Skyrmions
In a quantum field theory, there can be field configurations of bosons which are topologically twisted. These are coherent states (or solitons) which behave like a particle, and they can be fermionic even if all the constituent particles are bosons. This was discovered by Tony Skyrme in the early 1960s, so fermions made of bosons are named Skyrmions after him.
Skyrme's original example involved fields which take values on a threedimensional sphere, the original nonlinear sigma model which describes the large distance behavior of pions. In Skyrme's model, reproduced in the large N or string approximation to quantum chromodynamics (QCD), the proton and neutron are fermionic topological solitons of the pion field.^{citation needed}
Whereas Skyrme's example involved pion physics, there is a much more familiar example in quantum electrodynamics with a magnetic monopole. A bosonic monopole with the smallest possible magnetic charge and a bosonic version of the electron will form a fermionic dyon.
The analogy between the Skyrme field and the Higgs field of the electroweak sector has been used ^{3} to postulate that all fermions are skyrmions. This could explain why all known fermions have baryon or lepton quantum numbers and provide a physical mechanism for the Pauli exclusion principle.
See also


Notes
 ^ 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
 ^ Physical Review D volume 87, page 0550003, year 2013, author Weiner, Richard M., title "Spinstatisticsquantum number connection and supersymmetry" arxiv:1302.0969
 ^ Weiner, Richard M. (2010). "The Mysteries of Fermions". International Journal of Theoretical Physics 49 (5): 1174–1180. arXiv:0901.3816. Bibcode:2010IJTP...49.1174W. doi:10.1007/s1077301002927.

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