Fermions form one of the two fundamental classes ofsubatomic particle, the other beingbosons. All subatomic particles must be one or the other. A composite particle (hadron) may fall into either class depending on its composition.
In addition to the spin characteristic, fermions have another specific property: they possess conserved baryon or leptonquantum numbers. Therefore, what is usually referred to as the spin-statistics relation is, in fact, a spin statistics-quantum number relation.[1]
As a consequence of the Pauli exclusion principle, only one fermion can occupy a particularquantum state at a given time. Suppose multiple fermions have the same spatialprobability distribution, then, at least one property of each fermion, such as its spin, must be different. Fermions are usually associated withmatter, whereas bosons are generallyforce 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 showsuperfluidity for uncharged particles andsuperconductivity for charged particles.
Composite fermions, such as protons andneutrons, are the key building blocks ofeveryday matter.
English theoretical physicistPaul Dirac coined the name fermion from the surname of Italian physicistEnrico Fermi.[2]
Most Standard Model fermions are believed to be Dirac fermions, although it is unknown at this time whether theneutrinos are Dirac or Majorana fermions (or both). Dirac fermions can be treated as a combination of two Weyl fermions.[3]: 106 In July 2015, Weyl fermions have been experimentally realized inWeyl semimetals.
Composite particles (such ashadrons, 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 acarbon-13 atom contains six protons and seven neutrons.
The atomhelium-3 (3He) consists of two protons, one neutron, and two electrons. Thedeuterium 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 thesuperfluidity of helium-3: in superconducting materials, electrons interact through the exchange ofphonons, formingCooper pairs, while in helium-3, Cooper pairs are formed via spin fluctuations.
^Notes on Dirac's lectureDevelopments 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
