Hadron

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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. The gluons mediate the color force that binds the quarks together.

Like all subatomic particles, hadrons have quantum numbers corresponding to the representations of the Poincaré group: J^PC(m), where J is the spin, P, the parity, C, the C parity, and m, the mass. In addition they may carry flavour quantum numbers such as isospin (or G parity), strangeness etc. Hadrons can be further divided into two classes:

Baryons are fermions. They always carry an additive conserved quantum number called baryon number (B). B=1 for nucleons (the proton and the neutron), which are part of the atomic nucleus.
Mesons are bosons with B=0.

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. For a baryon these are three quarks and for a meson these are a quark-antiquark pair. Each quark is thus a fermion with B=1/3. Excited baryon or meson states are known as resonances. Each ground state hadron may have many excited states, and hundreds have been observed in particle experiments. Resonances decay extremely quickly (within about 10⁻²⁴ s) via strong interactions.

Mesons which lie outside the quark model classification are called exotic mesons. These include glueballs, hybrid mesons and tetraquarks. 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.

In other phases of QCD matter the hadrons may disappear. 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. This property, which is known as asymptotic freedom, has been experimentally confirmed at the energy scales between a GeV and a TeV.