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In particle physics, a hadron ( , from the , hadrós, "stout, thick") is a particle made of quarks held together by the strong force (similar to how molecules are held together by the electromagnetic force). Hadrons are either mesons (made of one quark and one antiquark) or baryons (made of three quarks). Other combinations, such as tetraquarks (an "exotic" meson) and pentaquarks (an "exotic" baryon), may be possible but no evidence conclusively suggests their existence . The best known mesons are pions and kaons, while the best known baryons are protons and neutrons.

Introduction

All types of hadrons have zero total color charge.
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 + ) and one down quark (with electric charge − ). Adding these together yields the proton charge of +1. Although quarks also carry color charge, hadrons must have zero total color charge because of a phenomenon called color confinement. That is, hadrons must be "colorless" or "white". There are two ways to accomplish this: three quarks of different colors, or a quark of one color and an antiquark 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 parity (or P-parity), and C, the charge conjugation (or C-parity), and the particle's mass, m. 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 interaction. 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 + for quarks and − for antiquarks. This means that baryons (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; several 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 no longer be confined within hadrons because the strength of the strong interaction diminishes with energy. This property, which is known as asymptotic freedom, has been experimentally confirmed at the energy scales between a GeV and a TeV.

All free hadrons except the proton are unstable.

Baryons

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 (or antibaryons) could be composed of further quark–antiquark pairs in addition to their three quarks (or antiquarks). Baryons containing a single additional quark–antiquark pair are called pentaquarks. Several pentaquarks candidates were found in the early 2000s, but upon further review these states have now been established as non-existent. (This does not rule against pentaquarks in general, only the candidates put forward). No evidence of baryon states with even more quark–antiquark pairs has been found.

Mesons

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. Several tetraquark candidates were found in the 2000s, but their status in under debate.

Several hypothetical "exotic" mesons lie outside the quark model classification. These include glueballs and hybrid mesons (mesons bound by excited gluons).

See also



References




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