For the particle collider, see Large Hadron Collider.In particle physics, a hadron (, from the Greek, Modern (1453-): ἁδρός, hadrós, "stout, thick") is a bound state of quarks. Hadrons are held together by the strong force, similarly to how molecules are held together by the electromagnetic force. There are two subsets of hadrons: baryons and mesons; the most well known baryons are protons and neutrons.
According to the quark model, the properties of hadrons are primarily determined by their so-called valence quarks. For example, a proton is composed of two up quarks (each with electric charge +2/3) and one down quark (with electric charge -1/3). Adding these together yields the proton charge of +1. Although the constituent quarks also carry color charge (nothing to do with visual color), a property of the strong nuclear force called color confinement requires that any composite state carry no residual color charge. That is, hadrons must be colorless. There are two ways to accomplish this: three quarks of different colors, or a quark of one color and an anti-quark carrying the corresponding anticolor. Hadrons based on the former are called baryons, and those based on the latter are called mesons.
Like all subatomic particles, hadrons are assigned quantum numbers corresponding to the representations of the Poincaré group: JPC(m), where J is the spin quantum number, P, the intrinsic (or P) parity, and C, the charge conjugation, or C parity, and the particle four-momentum, m, (i.e., its mass). Note that the mass of a hadron has very little to do with the mass of its valence quarks; rather, due to mass–energy equivalence, most of the mass comes from the large amount of energy associated with the strong nuclear force. Hadrons may also carry flavor quantum numbers such as isospin (or G parity), and strangeness. All quarks carry an additive, conserved quantum number called a baryon number (B), which is +1/3 for quarks and -1/3 for antiquarks. This means that baryons --which are groups of three quarks-- have B=1 while mesons have B=0.
Hadrons have excited states known as resonances. Each ground-state hadron may have several excited states; hundreds of resonances have been observed in particle physics experiments. Resonances decay extremely quickly (within about 10-24 seconds) via the strong nuclear force.
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, the theory of quantum chromodynamics (QCD) predicts that quarks and gluons will interact weakly and will no longer be confined within hadrons. This property, which is known as asymptotic freedom, has been experimentally confirmed at the energy scales between a GeV and a TeV.
All hadrons except the proton are unstable.
See main article: Baryon. All known baryons are made of three valence quarks, and are therefore fermions. They have baryon number B=1, while antibaryons (composed of three antiquarks) have B=-1. In principle, some baryons could be composed of further quark-antiquark pairs in addition to the three quarks (or antiquarks) that make up basic baryons. Baryons containing a single additional quark-antiquark pair are called pentaquarks. Evidence for these states was claimed by several experiments in the early 2000s, though this has since been refuted . No evidence of baryon states with even more quark-antiquark pairs has been found.
See main article: Meson. Mesons are bosons composed of a quark-antiquark pair. They have baryon number B=0. Examples of mesons commonly produced in particle physics experiments include pions and kaons. The former also play a role holding atomic nuclei together via the residual strong force. Hypothetical mesons have more than one quark-antiquark pair; a meson composed of two of these pairs is called a tetraquark. Currently there is no evidence of their existence. Mesons that lie outside the quark model classification are called exotic mesons. These include glueballs and hybrid mesons (mesons bound by excited gluons).