A baryon is a class of composite subatomic particle built from an odd number of valence quarks, most commonly three. Familiar examples are the proton and neutron, which together form atomic nuclei and account for nearly all ordinary visible matter. Baryons carry baryon number +1, are color-neutral combinations of quarks bound by the strong force, and are members of the broader family of hadrons. For a general introduction to composite particles see composite particle resources.

Basic properties

Each quark has intrinsic spin 1/2, so the combined spin of three quarks gives baryons total spin values such as 1/2 or 3/2 when quark spins align differently. Baryons are fermions and therefore obey Fermi-Dirac statistics and the Pauli exclusion principle; this underlies the structure of atoms and the stability of matter. Quark interactions inside baryons are mediated by gluons, and the binding energy and quantum numbers determine families of baryons and their excited states, often observed experimentally as resonances. Excited states may decay electromagnetically or by the strong and weak interactions, emitting photons or other particles in the process; see descriptions of radiative transitions for examples.

Classification and examples

Traditional baryons are grouped by flavor symmetries into multiplets such as the baryon octet and decuplet introduced in the quark model. Stable and long-lived baryons include the proton; the free neutron is unstable and undergoes beta decay, converting to a proton plus leptons. Other examples include strange, charm and bottom baryons (for instance the Λ family) that contain heavier quarks. Mesons, by contrast, are two-quark hadrons such as pions and kaons; for a contrast with leptons and mesons, see leptons and mesons.

History and theoretical context

The term baryon derives from the Greek word "barys," meaning heavy, reflecting early perceptions of these particles relative to lighter mesons. The quark model and symmetry schemes introduced by Gell-Mann and others organized baryons within flavor SU(3) multiplets and predicted new states that were later observed. Quantum chromodynamics (QCD) provides the modern framework: quarks carry color charge, interact by gluon exchange, and cannot be isolated due to confinement.

Importance and notable facts

  • Baryons constitute the atomic nuclei of ordinary matter and thus determine most macroscopic matter properties; see material on visible matter.
  • Conservation of baryon number is a key principle in low-energy physics; its violation would have major consequences in cosmology and particle physics (baryogenesis).
  • Beyond three-quark states, experiments have reported exotic combinations such as pentaquarks—states with five quark constituents—expanding the concept of what can be a baryon.

For technical introductions and experimental data consult overview articles and reviews: spin and angular momentum in particles, quantum statistics, and discussions of differences between fermions and bosons at boson descriptions. Introductory summaries and experimental catalogs are available in many physics references and databases (fermion basics, composite particle reviews).