In particle physics, a collective term for all composite particles which experience strong interactions. All baryons and mesons are hadrons (eg protons and pions).
In particle physics, a hadron is a subatomic particle which experiences the strong nuclear force. These are not fundamental particles but are composed of fermions, called quarks and antiquarks, and of bosons, called gluons.
Like all subatomic particles, hadrons have quantum numbers corresponding to the representations of the Poincaré group: JPC(m), where J is the spin, P, the parity, C, the C parity, and m, the mass.
Most hadrons can be classified by the quark model which posits that all the quantum numbers of baryons are derived from those of the valence quarks. Each ground state hadron may have many excited states, and hundreds have been observed in particle experiments.
Mesons which lie outside the quark model classification are called exotic mesons. The only baryons which lie outside the quark model at present are the pentaquarks, but the evidence for their existence is unclear as of 2005.
All hadrons are single particle excitations of the basic theory of strong interactions, called quantum chromodynamics. Due to a property called confinement that this theory enjoys at energies below the QCD scale, these excitations are not quarks and gluons, which are the basic fields, but the hadrons which are composite, and carry no color charge. For example, at very low temperature and low pressure, unless there are sufficiently many very massive flavors of quarks, QCD predicts that quarks and gluons will interact weakly and in particular no longer be confined.
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