Hadron: Difference between revisions
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In [[particle physics]], a '''hadron''' ({{Pron-en|ˈhædrɒn||En-us-hadron.ogg}} is a particle composed of two or three [[quark]]s. |
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In [[particle physics]], a '''hadron''' ({{Pron-en|ˈhædrɒn||En-us-hadron.ogg}}, from the {{lang-el|ἁδρός}}, ''hadrós'', "stout, thick") is a particle made of [[quark]]s [[bound state|held together]] by the [[strong force]] (similarly to how [[molecule]]s are held together by the [[electromagnetic force]]). There are two known types of hadrons: [[meson]]s (made of one quark and one [[antiparticle|antiquark]]) and [[baryon]]s (made of three quarks). Other combinations may exist, such as [[tetraquark]]s ([[exotic meson]]s) and [[pentaquark]]s ([[exotic baryon]]s), but no evidence conclusively suggests their existence {{as of|2009|lc=on}}. |
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The quarks are [[bound state|held together]] by the [[strong force]]. (Compare the concept of [[molecule]]s being held together by the [[electromagnetic force]]). |
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Indeed note that quarks cannot exist in isolation, but only as hadrons. |
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There are two known types of hadrons: |
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[[Meson]]s are one quark and one [[antiparticle|antiquark]]. |
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[[Baryon]]s are three quarks. |
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Other combinations may exist, such as [[tetraquark]]s (which would be [[exotic meson]]s) and [[pentaquark]]s (which would be [[exotic baryon]]s), but no evidence conclusively suggests their existence {{as of|2009|lc=on}}. |
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The best-known mesons are [[pion]]s and [[kaon]]s, while the best-known baryons are [[proton]]s and [[neutron]]s. |
The best-known mesons are [[pion]]s and [[kaon]]s, while the best-known baryons are [[proton]]s and [[neutron]]s. |
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"Hadron" comes from the {{lang-el|ἁδρός}}, ''hadrós'', meaning stout or thick. |
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==Introduction== |
==Introduction== |
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Revision as of 13:05, 13 January 2010
In particle physics, a hadron (Template:Pron-en is a particle composed of two or three quarks.
The quarks are held together by the strong force. (Compare the concept of molecules being held together by the electromagnetic force).
Indeed note that quarks cannot exist in isolation, but only as hadrons.
There are two known types of hadrons:
Mesons are one quark and one antiquark.
Baryons are three quarks.
Other combinations may exist, such as tetraquarks (which would be exotic mesons) and pentaquarks (which would be exotic baryons), but no evidence conclusively suggests their existence as of 2009[update].
The best-known mesons are pions and kaons, while the best-known baryons are protons and neutrons.
"Hadron" comes from the Template:Lang-el, hadrós, meaning stout or thick.
Introduction
According to the quark model,[1] 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 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 +1⁄3 for quarks and −1⁄3 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.[2]
All free hadrons except the proton are unstable.
Baryons
All known baryons are made of three valence quarks, so in terms of their behavior, they are fermions. As quarks posses baryon number B = 1⁄3, baryons have baryon number B = 1. The two most well-known types of baryon are the proton and the neutron.
In principle, some baryons could be composed of further quark–antiquark pairs in addition to their three quarks. These hypothetical baryons containing an additional quark–antiquark pair are called pentaquarks.[3] Several pentaquarks candidates were found in the early 2000s, but upon further review these states have now been established as non-existent.[4] (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.
Each type of baryon has a corresponding antiparticle (antibaryon) in which quarks are replaced by their corresponding antiquarks. For example: just as a proton is made of two up-quarks and one down-quark, its corresponding antiparticle, the antiproton, is made of two up-antiquarks and one down-antiquark.
Mesons
Mesons are hadrons composed of a quark–antiquark pair. In terms of their behavior, they are bosons. 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 in holding atomic nuclei together via the residual strong force.
In principle, mesons with more than one quark–antiquark pair may exist; a hypothetical 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 other hypothetical "exotic" mesons lie outside the quark model of classification. These include glueballs and hybrid mesons (mesons bound by excited gluons).
See also
References
- ^ C. Amsler et al. (Particle Data Group) (2008). "Review of Particle Physics – Quark Model" (PDF). Physics Letters B. 667: 1. doi:10.1016/j.physletb.2008.07.018.
- ^ S. Bethke (2007). "Experimental tests of asymptotic freedom". Progress in Particle and Nuclear Physics. 58: 351. doi:10.1016/j.ppnp.2006.06.001. arXiv:hep-ex/0606035.
- ^
S. Kabana (2005). "Review of the experimental evidence on pentaquarks and critical discussion". arXiv:hep-ex/0503020.
{{cite arXiv}}:|class=ignored (help) - ^ C. Amsler et al. (Particle Data Group) (2008). "Review of Particle Physics – Pentaquarks" (PDF). Physics Letters B. 667: 1. doi:10.1016/j.physletb.2008.07.018.
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