Editing Fermion

{{Short description|Type of subatomic particle}}
{{distinguish|fermium}}
{{Use dmy dates|date=March 2022}}
[[File:Bosons-Hadrons-Fermions-RGB.svg|thumb|upright=1.6|Fermions form one of the two fundamental classes of [[subatomic particle]], the other being [[boson]]s. All subatomic particles must be one or the other. A composite particle ([[hadron]]) may fall into either class depending on its composition.]]
In [[particle physics]], a '''fermion''' is a [[subatomic particle]] that follows [[Fermi–Dirac statistics]]. Fermions have a [[half-integer]] spin ([[spin 1/2|spin {{sfrac|1|2}}]], [[Spin (physics)#Higher spins|spin {{sfrac|3|2}}]], etc.) and obey the [[Pauli exclusion principle]]. These particles include all [[quark]]s and [[lepton]]s and all [[composite particle]]s made of an [[even and odd|odd number]] of these, such as all [[baryon]]s and many [[atom]]s and [[atomic nucleus|nuclei]]. Fermions differ from [[boson]]s, which obey [[Bose–Einstein statistics]].

Some fermions are [[elementary particle]]s (such as [[electron]]s), and some are [[composite particle]]s (such as [[proton]]s). For example, according to the [[spin-statistics theorem]] in [[Theory of relativity|relativistic]] [[quantum field theory]], particles with [[integer]] [[Spin (physics)|spin]] are [[boson]]s. 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 [[quantum number]]s. Therefore, what is usually referred to as the spin-statistics relation is, in fact, a spin statistics-quantum number relation.<ref>{{cite journal |last=Weiner |first=Richard M. |date=4 March 2013 |title=Spin-statistics-quantum number connection and supersymmetry |url=https://journals.aps.org/prd/abstract/10.1103/PhysRevD.87.055003 |journal=Physical Review D |volume=87 |issue=5 |pages=055003–05 |arxiv=1302.0969 |bibcode=2013PhRvD..87e5003W |doi=10.1103/physrevd.87.055003 |issn=1550-7998 |access-date=28 March 2022 |s2cid=118571314}}</ref>

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 [[neutron]]s, are the key building blocks of [[baryonic matter|everyday matter]].

English theoretical physicist [[Paul Dirac]] coined the name fermion from the surname of Italian physicist [[Enrico Fermi]].<ref>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</ref>

==Elementary fermions{{anchor|elementary_fermion_anchor}}==
{{Standard model of particle physics|cTopic=[[Elementary particle]]s}}
The [[Standard Model]] recognizes two types of elementary fermions: [[quark]]s and [[lepton]]s. In all, the model distinguishes 24&nbsp;different fermions. There are six quarks ([[up quark|up]], [[down quark|down]], [[strange quark|strange]], [[charm quark|charm]], [[bottom quark|bottom]] and [[top quark|top]]), and six leptons ([[electron]], [[electron neutrino]], [[muon]], [[muon neutrino]], [[tauon]] and [[tauon neutrino]]), along with the corresponding [[antiparticle]] of each of these.

Mathematically, there are many varieties of fermions, with the three most common types being:
* [[Weyl fermion]]s (massless),
* [[Dirac fermion]]s (massive), and
* [[Majorana fermion]]s (each its own antiparticle).
Most Standard Model fermions are believed to be Dirac fermions, although it is unknown at this time whether the [[neutrino]]s are Dirac or Majorana fermions (or both). Dirac fermions can be treated as a combination of two Weyl fermions.<ref name="MoriiLim2004">{{cite book |last1=Morii |first1=T. |title=The Physics of the Standard Model and Beyond |last2=Lim |first2=C. S. |last3=Mukherjee |first3=S. N. |date=1 January 2004 |publisher=[[World Scientific]] |isbn=978-981-279-560-1}}</ref>{{rp|106}} In July 2015, Weyl fermions have been experimentally realized in [[Weyl semimetal]]s.

==Composite fermions==
{{see also|List of particles#Composite particles}}

Composite particles (such as [[hadron]]s, 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 half-integer spin.

Examples include the following:
*A baryon, such as the proton or neutron, contains three fermionic quarks.
*The nucleus of a [[carbon-13]] atom contains six protons and seven neutrons.
*The atom [[helium-3]] (<sup>3</sup>He) consists of two protons, one neutron, and two electrons. The [[deuterium]] atom consists of one proton, one neutron, and one electron.

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 [[superfluid]]ity of helium-3: in superconducting materials, electrons interact through the exchange of [[phonon]]s, forming [[Cooper pair]]s, while in helium-3, Cooper pairs are formed 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.

==See also==
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* [[Anyon]], 2D quasiparticles
* [[Chirality (physics)]], left-handed and right-handed
* [[Fermionic condensate]]
* [[Weyl semimetal]]
* [[Fermionic field]]
* [[Identical particles]]
* [[Kogut–Susskind fermion]], a type of lattice fermion
* [[Majorana fermion]], each its own antiparticle
* [[Parastatistics]]
* [[Skyrmion]], a hypothetical particle
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==Notes==
{{reflist}}

==External links==
{{Spoken Wikipedia|date=2021-07-11|En-Fermion-article.ogg}}

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{{Standard model of physics}}

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[[Category:Fermions| ]]
[[Category:Quantum field theory]]
[[Category:Enrico Fermi]]