Fermion
From Free net encyclopedia
In particle physics, fermions are particles with half-integer spin; the particle type is named after Enrico Fermi.
Due to their half integer spin as an observer circles a fermion (or a fermion rotates 360 by the observer) the wave function of the fermion changes sign to opposite. So, fermions have an antisymmetric wavefunction. 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.
All elementary particles are either fermions or bosons (except the hypothetical plekton). A composite particle (made up of elementary particles) may either be a fermion or a boson, depending only on the number of fermions inside:
- Composite particles containing an even number of fermions become bosons, such as a meson, or the nucleus of a Carbon 12 atom.
- Composite particles containing odd number of fermions remain 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.
Of course, fermionic or bosonic behavior of composite particle (or system) is only seen at large (compared to size of the system) distance. At close proximity at which spatial structure begins to be important, composite particle (or system) behaves according to its constituent makeup. For example, two atoms of He can not share the same space if it is comparable by size to the size of inner structure of He atom itself (~10^-10 m) - despite bosonic properties of He atoms. Thus, liquid He 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 "rigidness" of ordinary matter and for the stability of the electron shells of atoms, making complex chemistry possible. It also results in the pressure exerted by degenerate matter.
See also
| edit | |
| Fermions: Quarks | Leptons | |
| 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 | |
ca:Fermió cs:Fermion de:Fermion et:Fermion el:Φερμιόνιο es:Fermión fa:فرمیون fr:Fermion ko:페르미온 hr:Fermion id:Fermion is:Fermíeind it:Fermione he:פרמיון la:Fermion hu:Fermion nl:Fermion ja:フェルミ粒子 no:Fermion pl:Fermion pt:Férmion ru:Фермион sl:Fermion fi:Fermioni sv:Fermion tr:Fermiyon zh:费米子