Type of subatomic particle
In
particle physics
, a
fermion
is a particle that follows
Fermi?Dirac statistics
. Fermions have a half-odd-integer spin (
spin
1
/
2
,
spin
3
/
2
, etc.) and obey the
Pauli exclusion principle
. These particles include all
quarks
and
leptons
and 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
electrons
), and some are
composite particles
(such as
protons
). For example, according to the
spin-statistics theorem
in
relativistic
quantum field theory
, particles with
integer
spin
are
bosons
. 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 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. 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
neutrons
, are the key building blocks of
everyday matter
.
English theoretical physicist
Paul Dirac
coined the name fermion from the surname of Italian physicist
Enrico Fermi
.
[2]
Elementary fermions
[
edit
]
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, there are many varieties of fermions, with the three most common types being:
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
[
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]
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.
- The nucleus of a
carbon-13
atom contains six protons and seven neutrons.
- The atom
helium-3
(
3
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
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
; they consist of electrons with an even number of quantized vortices attached to them.
See also
[
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]
Notes
[
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]
External links
[
edit
]
This audio file
was created from a revision of this article dated 11 July 2021
(
2021-07-11
)
, and does not reflect subsequent edits.