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= Fermion =
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Introduction
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In particle physics, a fermion is a particle that follows Fermi-Dirac
statistics. 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.
A fermion can be an elementary particle, such as the electron, or it
can be a composite particle, such as the proton. According to the
spin-statistics theorem in any reasonable 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.
As a consequence of the Pauli exclusion principle, only one fermion
can occupy a particular quantum state at any 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.
Elementary fermions
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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
quarks), and six leptons (electron, electron neutrino, muon, muon
neutrino, tau particle and tau 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. 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 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.
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.
Skyrmions
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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 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
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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
* Boson
License
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Original Article:
http://en.wikipedia.org/wiki/Fermion
