Physics:Quantum fermion
A fermion is a subatomic particle that follows Fermi–Dirac statistics. Fermions have half-integer spin and obey the Pauli exclusion principle. These particles include all quarks, leptons, and composite particles made from an odd number of fermions, such as protons, neutrons, many nuclei, and many atoms.
Fermions differ from bosons, which obey Bose–Einstein statistics. In relativistic quantum field theory, particles with integer spin behave as bosons, while particles with half-integer spin behave as fermions.
Description
Fermions possess conserved baryon or lepton quantum numbers in addition to their spin properties. Because of the Pauli exclusion principle, no two identical fermions can occupy exactly the same quantum state at the same time.[1]
If several fermions occupy the same spatial region, at least one quantum property, such as spin orientation, must differ between them. This exclusion principle is responsible for much of the structure and stability of ordinary matter, including electron shells in atoms.
Fermions are generally associated with matter, while bosons are usually associated with force mediation. However, under special conditions fermions can collectively display bosonic behaviour. Examples include superconductivity and superfluidity.
Composite fermions such as protons and neutrons are the primary building blocks of ordinary baryonic matter.
The term fermion was introduced by English physicist Paul Dirac in honour of Italian physicist Enrico Fermi.[2]
Elementary fermions
Template:Standard model of particle physics
The Standard Model recognizes two families of elementary fermions:
In total there are 24 elementary fermions when antiparticles are included.
Quarks
The six known quarks are:
Quarks carry color charge and participate in the strong interaction. They combine to form hadrons such as protons and neutrons.
Leptons
The six leptons are:
Leptons do not participate in the strong interaction. Neutrinos interact only weakly and gravitationally.
Types of fermions
Mathematically, several forms of fermions are known:
- Weyl fermions (massless)
- Dirac fermions (massive)
- Majorana fermions (their own antiparticles)
Most Standard Model fermions are believed to behave as Dirac fermions, although the exact nature of neutrinos remains uncertain.[3]Template:Rp
In 2015, Weyl fermions were experimentally realized in Weyl semimetals.
Composite fermions
Composite particles can behave as fermions or bosons depending on the number of constituent fermions.
Examples include:
- a proton, containing three quarks
- a neutron, containing three quarks
- the nucleus of carbon-13
- helium-3 atoms
- deuterium atoms
A composite particle containing an odd number of fermions behaves overall as a fermion and has half-integer spin.
The fermionic or bosonic behaviour of composite systems is most evident when their constituents remain spatially separated. At shorter distances, the internal structure becomes important.
Fermion pairing
Under certain conditions, fermions can pair together and collectively behave like bosons.
Superconductivity
In superconductors, electrons form Cooper pairs through interactions mediated by phonons. These paired electrons can move collectively without electrical resistance.
Superfluidity
In helium-3, fermionic helium atoms pair through spin interactions and form a superfluid state at extremely low temperatures.
= Composite fermions
Quasiparticles observed in the fractional quantum Hall effect are called composite fermions. They consist of electrons bound to quantized vortices.
Physical interpretation
Fermions are responsible for the structure of matter because the Pauli exclusion principle prevents identical fermions from collapsing into the same state. This principle explains:
- atomic shell structure
- chemical behaviour
- stability of white dwarfs and neutron stars
- degeneracy pressure
- the organization of matter at microscopic scales
Properties
- half-integer spin
- obey Fermi–Dirac statistics
- follow the Pauli exclusion principle
- include quarks and leptons
- form ordinary matter
- can combine into composite fermions
- may form paired bosonic states in superconductors and superfluids
See also
Table of contents (72 articles)
Index
Full contents
Notes
- ↑ Weiner, Richard M. (4 March 2013). "Spin-statistics-quantum number connection and supersymmetry". Physical Review D 87 (5): 055003–05. doi:10.1103/physrevd.87.055003. ISSN 1550-7998. Bibcode: 2013PhRvD..87e5003W. https://journals.aps.org/prd/abstract/10.1103/PhysRevD.87.055003. Retrieved 28 March 2022.
- ↑ 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
- ↑ Morii, T.; Lim, C. S.; Mukherjee, S. N. (1 January 2004). The Physics of the Standard Model and Beyond. World Scientific. ISBN 978-981-279-560-1.
References
Source attribution: Fermion