hadron
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had·ron
(hăd'rŏn') 
n.
Any of a class of subatomic particles that are composed of quarks and take part in the strong interaction.
n.
Any of a class of subatomic particles that are composed of quarks and take part in the strong interaction.
[Greek hadros, thick + -ON1.]
hadronic had·ron'ic adj.Wiley Book of Astronomy:
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hadron
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Any of the subatomic particles that are built from quarks and thus interact via the strong force. The hadrons fall into two groups: mesons and baryons. Except for protons and neutrons, which are bound in nuclei, all hadrons have short lives and are produced in high-energy collision of subatomic particles. All hadrons are subject to gravitation; charged hadrons are subject to electromagnetic forces. Some hadrons break up by way of the weak force (as in radioactive decay); others decay via the strong and electromagnetic forces.
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The generic name of a class of particles which interact strongly with one another. Examples of hadrons are protons, neutrons, the π, K, and D mesons, and their antiparticles. Protons and neutrons, which are the constituents of ordinary nuclei, are members of a hadronic subclass called baryons, as are strange and charmed baryons. Baryons have half-integral spin, obey Fermi-Dirac statistics, and are known as fermions. Mesons, the other subclass of hadrons, have zero or integral spin, obey Bose-Einstein statistics, and are known as bosons. The electric charges of baryons and mesons are either zero or ±1 times the charge on the electron. Masses of the known mesons and baryons cover a wide range, extending from the pi meson, with a mass approximately one-seventh that of the proton, to values of the order of 10times the proton mass. The spectrum of meson and baryon masses is not understood. See also Baryon; Bose-Einstein statistics; Fermi-Dirac statistics; Meson; Neutron; Proton.
Based on an enormous body of data, hadrons are now thought of as consisting of elementary fermion constituents known as quarks which have electric charges of +
|e| and 
|e|, where |e| is the absolute value of the electron charge. For example, a quark-antiquark pair makes up a meson, while three quarks constitute a baryon. See also Elementary particle; Quarks.
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categories related to 'hadron'
- Nuclear and Particle Physics - hadron: any heavy elementary particle that takes part in the strong interaction
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Hadron
In particle physics, a hadron 
i/ˈhædrɒn/ (Greek: ἁδρός, hadrós, "stout, thick") is a composite particle made of quarks held together by the strong force (as atoms and molecules are held together by the electromagnetic force). Hadrons are categorized into two families: baryons (made of three quarks) and mesons (made of one quark and one antiquark).
The best-known hadrons are protons and neutrons (both baryons), which are components of atomic nuclei. All hadrons except protons are unstable and undergo particle decay–however neutrons are stable inside atomic nuclei. The best-known mesons are the pion and the kaon, which were discovered during cosmic ray experiments in the late 1940s and early 1950s. However, these are not the only hadrons; a great number of them have been discovered and continue to be discovered (see list of baryons and list of mesons).
Other types of hadron may exist, such as tetraquarks (or, more generally, exotic mesons) and pentaquarks (exotic baryons), but no current evidence conclusively suggests their existence.[1][2]
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Etymology
The term hadron was introduced by Lev B. Okun in a plenary talk at the 1962 International Conference on High Energy Physics.[3] In this talk he said:
Not withstanding the fact that this report deals with weak interactions, we shall frequently have to speak of strongly interacting particles. These particles pose not only numerous scientific problems, but also a terminological problem. The point is that "strongly interacting particles" is a very clumsy term which does not yield itself to the formation of an adjective. For this reason, to take but one instance, decays into strongly interacting particles are called non-leptonic. This definition is not exact because "non-leptonic" may also signify "photonic". In this report I shall call strongly interacting particles "hadrons", and the corresponding decays "hadronic" (the Greek ἁδρός signifies "large", "massive", in contrast to λεπτός which means "small", "light"). I hope that this terminology will prove to be convenient.
Properties
According to the quark model,[4] 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, for a total of +4⁄3 together) 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". These are the simplest of the two ways: three quarks of different colors, or a quark of one color and an antiquark carrying the corresponding anticolor. Hadrons with the first arrangement are called baryons, and those with the second arrangement are 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 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 in the energy range between 1 GeV (gigaelectronvolt) and 1 TeV (teraelectronvolt).[5]
All free hadrons except the proton (and antiproton) are unstable.
Baryons
Main article: Baryon
All known baryons are made of three valence quarks, so they are fermions (i.e. they have odd half-integral spin because they have an odd number of quarks). As quarks possess baryon number B = 1⁄3, baryons have baryon number B = 1. The best-known baryons are the proton and the neutron.
One can hypothesise baryons with further quark–antiquark pairs in addition to their three quarks. Hypothetical baryons with one extra quark–antiquark pair (5 quarks in all) are called pentaquarks.[6] Several pentaquark candidates were found in the early 2000s, but upon further review these states have now been established as non-existent.[7] (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 either.
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
Main article: Meson
Mesons are hadrons composed of a quark–antiquark pair. They are bosons (integral spin - i.e. an even multiple of ½ - as they have an even number of quarks). They have baryon number B = 0. Examples of mesons commonly produced in particle physics experiments include pions and kaons. Pions 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 with two pairs is called a tetraquark. Several tetraquark candidates were found in the 2000s, but their status is 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
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Look up hadron in Wiktionary, the free dictionary. |
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Book: Hadronic Matter |
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- Hadronization
- Large Hadron Collider (LHC)
- Leptons
- List of particles
- Standard model
- Subatomic particles
References
- ^ W.-M. Yao et al. (2006): Particle listings – Θ+
- ^ C. Amsler et al. (2008): Pentaquarks
- ^ L.B. Okun (1962). "The Theory of Weak Interaction". Proceedings of 1962 International Conference on High-Energy Physics at CERN. Geneva. p. 845. Bibcode 1962hep..conf..845O.
- ^ C. Amsler et al. (Particle Data Group) (2008). "Review of Particle Physics – Quark Model". Physics Letters B 667: 1. Bibcode 2008PhLB..667....1P. doi:10.1016/j.physletb.2008.07.018. http://pdg.lbl.gov/2008/reviews/quarkmodrpp.pdf.
- ^ S. Bethke (2007). "Experimental tests of asymptotic freedom". Progress in Particle and Nuclear Physics 58 (2): 351. arXiv:hep-ex/0606035. Bibcode 2007PrPNP..58..351B. doi:10.1016/j.ppnp.2006.06.001.
- ^ S. Kabana (2005). "AIP Conference Proceedings". arXiv:hep-ex/0503020 [hep-ex]. doi:10.1063/1.1920947.
- ^ C. Amsler et al. (Particle Data Group) (2008). "Review of Particle Physics – Pentaquarks". Physics Letters B 667: 1. Bibcode 2008PhLB..667....1P. doi:10.1016/j.physletb.2008.07.018. http://pdg.lbl.gov/2008/reviews/pentaquarks_b801.pdf.
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