Physics Gifts

- Art Gallery -

In particle physics, a fermion is a particle that follows Fermi–Dirac statistics and generally has half odd integer spin 1/2, 3/2 etc. These particles obey the Pauli exclusion principle. Fermions include all quarks and leptons, as well as all composite particles made of an odd number of these, such as all baryons and many atoms and nuclei. Fermions differ from bosons, which obey Bose–Einstein statistics.

Some fermions are elementary particles, such as the electrons, and some are composite particles, such as the protons. According to the spin-statistics theorem in relativistic quantum field theory, particles with integer spin are bosons, while 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 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 particular quantum state at a 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. Weakly interacting fermions can also display bosonic behavior under extreme conditions. At low temperature fermions show superfluidity for uncharged particles and superconductivity for charged particles.

Composite fermions, such as protons and neutrons, are the key building blocks of everyday matter.

The name fermion was coined by English theoretical physicist Paul Dirac from the surname of Italian physicist Enrico Fermi.[2]

Elementary fermions

The Standard Model recognizes two types of elementary fermions: quarks and leptons. In all, the model distinguishes 24 different fermions. There are six quarks (up, down, strange, charm, bottom and top), and six leptons (electron, electron neutrino, muon, muon neutrino, tauon and tauon neutrino), along with the corresponding antiparticle of each of these.

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 neutrinos 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 in Weyl semimetals.
Composite fermions
See also: List of particles § Composite particles

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 half-integer 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 carbon-13 atom contains six protons and seven neutrons and is therefore a fermion.
The atom helium-3 (3He) is made of two protons, one neutron, and two electrons, and therefore it is a fermion; Also the deuterium atom is made of one proton, one neutron, and one electron, and therefore it is a fermion as well.

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 helium-3: in superconducting materials, electrons interact through the exchange of phonons, forming Cooper pairs, 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, which are electrons with an even number of quantized vortices attached to them.

Main article: Skyrmion

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 three-dimensional 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.

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[4] 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

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
Boson

Notes

Weiner, Richard M. (4 March 2013). "Spin-statistics-quantum number connection and supersymmetry". Physical Review D. 87 (5): 055003–05.arXiv:1302.0969. Bibcode:2013PhRvD..87e5003W. doi:10.1103/physrevd.87.055003. ISSN 1550-7998. S2CID 118571314.
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
T. Morii; C. S. Lim; S. N. Mukherjee (1 January 2004). The Physics of the Standard Model and Beyond. World Scientific. ISBN 978-981-279-560-1.

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/s10773-010-0292-7. S2CID 118515608.

vte

Particles in physics
Elementary
Fermions
Quarks

Up (quark antiquark) Down (quark antiquark) Charm (quark antiquark) Strange (quark antiquark) Top (quark antiquark) Bottom (quark antiquark)

Leptons

Electron Positron Muon Antimuon Tau Antitau Electron neutrino Electron antineutrino Muon neutrino Muon antineutrino Tau neutrino Tau antineutrino

Bosons
Gauge

Photon Gluon W and Z bosons

Scalar

Higgs boson

Ghost fields

Faddeev–Popov ghosts

Hypothetical
Superpartners
Gauginos

Gluino Gravitino Photino

Others

Axino Chargino Higgsino Neutralino Sfermion (Stop squark)

Others

Axion Curvaton Dilaton Dual graviton Graviphoton Graviton Inflaton Leptoquark Magnetic monopole Majoron Majorana fermion Dark photon Planck particle Preon Sterile neutrino Tachyon W′ and Z′ bosons X and Y bosons

Composite
Hadrons
Baryons

Nucleon
Proton Antiproton Neutron Antineutron Delta baryon Lambda baryon Sigma baryon Xi baryon Omega baryon

Mesons

Pion Rho meson Eta and eta prime mesons Phi meson J/psi meson Omega meson Upsilon meson Kaon B meson D meson Quarkonium

Exotic hadrons

Tetraquark Pentaquark

Others

Atomic nuclei Atoms Exotic atoms
Positronium Muonium Tauonium Onia Pionium Superatoms Molecules

Hypothetical
Baryons

Hexaquark Heptaquark Skyrmion

Mesons

Glueball Theta meson T meson

Others

Mesonic molecule Pomeron Diquark R-hadron

Quasiparticles

Anyon Davydov soliton Dropleton Exciton Hole Magnon Phonon Plasmaron Plasmon Polariton Polaron Roton Trion

Lists

Baryons Mesons Particles Quasiparticles Timeline of particle discoveries

Related

History of subatomic physics
timeline Standard Model
mathematical formulation Subatomic particles Particles Antiparticles Nuclear physics Eightfold way
Quark model Exotic matter Massless particle Relativistic particle Virtual particle Wave–particle duality Particle chauvinism

Wikipedia books

Hadronic Matter Particles of the Standard Model Leptons Quarks

vte

Standard Model
Background

Particle physics
Fermions Gauge boson Higgs boson Quantum field theory Gauge theory Strong interaction
Color charge Quantum chromodynamics Quark model Electroweak interaction
Weak interaction Quantum electrodynamics Fermi's interaction Weak hypercharge Weak isospin


Constituents

CKM matrix Spontaneous symmetry breaking Higgs mechanism Mathematical formulation of the Standard Model

Beyond the
Standard Model
Evidence

Hierarchy problem Dark matter Cosmological constant problem Strong CP problem Neutrino oscillation

Theories

Technicolor Kaluza–Klein theory Grand Unified Theory Theory of everything

Supersymmetry

MSSM Superstring theory Supergravity

Quantum gravity

String theory Loop quantum gravity Causal dynamical triangulation Canonical quantum gravity Superfluid vacuum theory Twistor theory

Experiments

Gran Sasso INO LHC SNO Super-K Tevatron

Physics Encyclopedia

World

Index

Hellenica World - Scientific Library

Retrieved from "http://en.wikipedia.org/"
All text is available under the terms of the GNU Free Documentation License