A hadron (pronounced /hɑːdɹɒn/, from Greek ἁδρός, hadros, thick), in particle physics, is any strongly interacting composite subatomic particle. All hadrons are composed of quarks. Hadrons are divided into two classes:
- Baryons, strongly interacting fermions such as a neutron or a proton, made up of three quarks.
- Mesons, strongly interacting bosons consisting of a quark and an antiquark.
Notice that mesons are composite bosons, but they are not composed of bosons (quarks are fermions).
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. In addition they may carry flavour quantum numbers such as isospin (or G parity), strangeness etc. Moreover,
- Baryons 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 have B=0.
Most hadrons can be classified by the quark model which posits that all the quantum numbers are derived from those of the valence quarks (the quarks which form the hadron). For instance, since each quark has B=1/3, each baryon, composed of three quarks, has B=1.
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 evidence for their existence is unclear as of 2006.
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 high temperature and high pressure, unless there are sufficiently many 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.
- Large Hadron Collider (LHC)
- Subatomic particles: list of particles, leptons
- Quantum chromodynamics, quark model, confinement
- Quark star
- QCD matter
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