Fermion

In particle physics, fermions, named after Enrico Fermi, are particles which form totally-antisymmetric composite quantum states. As a result, they are subject to the Pauli exclusion principle and obey Fermi-Dirac statistics. According to the spin-statistics theorem of quantum field theory, fermions have half-integer spin. One important aspect of half-integer spin is that particles with such spin have to be rotated by two 360-degree rotations to return them to their initial state.

All elementary particles are either fermions or bosons. Composite particles composed of fermions may be either fermions or bosons, depending on the number of fermionic constituents: Particles composed of an even number of fermions are themselves bosons (such as mesons); those composed of an odd number of fermions are themselves fermions (such as baryons).

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 of fermions is responsible for the stability of the electron shells of atoms, making complex chemistry possible. It also allows the stability of degenerate matter under extreme pressures.

See also

• List of particles
• Fermionic field
• Identical particles
• Parastatistics