Fermion
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In particle physics, fermions are particles with half-integer spin. They are named after Enrico Fermi. In the Standard Model there are two types of elementary fermions: quarks and leptons. Since fermion numbers are often approximately conserved, they are sometimes said to be the constituents of matter. Informally speaking, fermions are particles of matter and bosons are particles that transmit forces.
Due to their half-integer spin, as an observer circles a fermion (or a fermion rotates 360°) the wavefunction of the fermion changes sign. This phenomenon is called an antisymmetric wavefunction behavior of a fermion. As a result of the antisymmetric wavefunction, fermions obey Fermi-Dirac statistics, whose consequence is the Pauli exclusion principle—no two fermions can occupy the same quantum mechanical state at the same time. This results in "rigidness" or "stiffness" of fermions and of fermionic matter (atoms, molecules, atomic nuclei, etc).
All observed elementary particles are either fermions or bosons. A composite particle (made up of more fundamental particles) may either be a fermion or a boson, depending only on the number of fermions it contains:
- Composite particles containing an even number of fermions are bosons, such as a meson, or the nucleus of a carbon-12 atom.
- Composite particles containing an odd number of fermions are fermions, such as a baryon, or the nucleus of a carbon-13 atom.
A composite particle may contain any number of bosons with 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) distance. At proximity, where spatial structure begins to be important, a composite particle (or system) behaves according to its constituent makeup. For example, two atoms of helium can not share the same space if it is comparable by size to the size of the inner structure of the helium atom itself (~10−10 m)—despite bosonic properties of the helium atoms. Thus, liquid helium has finite density comparable to the density of ordinary liquid matter.
The known elementary fermions are divided into two groups: quarks and leptons. The elementary particles that make up ordinary matter are fermions, belonging to either the quarks (which form protons and neutrons) or the leptons (such as electrons). The Pauli exclusion principle obeyed by fermions is responsible for the "rigidness" of ordinary matter (it is major contributor to Young modulus), and for the stability of the electron shells of atoms (thus for stability of atomic matter). It also is responsible for complexity of atoms (making it impossible for all atomic electrons to occupy the same energy level) thus making complex chemistry possible. It also results in the pressure exerted by degenerate matter which largely governs the equilibrium state of white dwarfs and neutron stars.
The known fermions of left-handed helicity interact through the weak interaction while the known right-handed fermions do not.
[edit] See also
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| Fermions: Quarks: (Up · Down · Strange · Charm · Bottom · Top) | Leptons: (Electron · Muon · Tau · Neutrinos) | |
| Gauge bosons: Photon | W and Z bosons | Gluons | |
| Not yet observed: Higgs boson | Graviton | Other hypothetical particles | |
