fermion

A fermion is a particle with half-integer spin that obeys Fermi-Dirac statistics. Fundamental fermions include quarks and leptons, while many composite particles such as protons and neutrons are also fermions due to their overall half-integer spin. In contrast, particles with integer spin are bosons and follow Bose-Einstein statistics.

A central property of fermions is the Pauli exclusion principle: no two fermions can occupy the same quantum state simultaneously. This antisymmetry of the multi-fermion wavefunction is fundamental to the electronic structure of atoms, the arrangement of electrons in shells, and the chemistry that follows.

The spin-statistics theorem, a result of relativistic quantum field theory, links half-integer spin to fermionic behavior and integer spin to bosonic behavior. This has wide-reaching implications across physics, from atomic structure to high-energy processes.

Categories and examples: fundamental fermions include the six quark flavors (up, down, charm, strange, top, bottom) and the charged leptons (electron, muon, tau) along with their neutrinos. Composite fermions include protons and neutrons, as well as other baryons, which arise from bound states of an odd number of quarks. Atoms can be fermionic or bosonic depending on the total number of constituent fermions.

Fermions play a key role in many areas of science. Their statistics govern the behavior of electrons in solids, giving rise to phenomena such as electronic conduction and the Fermi surface, and they provide the degeneracy pressure that supports white dwarfs and neutron stars. In condensed matter, fermion pairing can lead to superconductivity.