Baryon
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'''Baryons''' are the family of [[subatomic particle]]s with a baryon number of 1. The term baryon is usually used to refer to triquarks — baryons made of three [[quark]]s. "Exotic" baryons made of four quarks and one antiquark, are known as the [[pentaquark]]s, but their existence is not generally accepted. Each baryon has a corresponding antiparticle (anti-baryon) where quarks are replaced by their corresponding antiquarks and their corresponding antiquarks replaced by quarks. Amongst the baryons are the [[proton]]s and [[neutron]]s, which make up [[atomic nucleus|atomic nuclei]], but many other unstable baryons exist as well. The term "baryon" is derived from the [[Greek language|Greek]] ''{{polytonic|βαρύς}}'' (''barys''), meaning "heavy," because at the time of their naming it was believed that baryons were characterized by having greater mass than other particles.
Baryons are a subset of the [[hadrons]] (which are the particles made of quarks) and participate in the [[strong interaction]].
==Background==
[[Image:Quark structure proton.svg|thumb|120px|Quark structure proton: 2 up quarks and 1 down quark.]]
Baryons are strongly interacting [[fermion]]s — that is, they experience the [[strong nuclear force]] and are described by [[Fermi-Dirac statistics]], which apply to all particles obeying the [[Pauli exclusion principle]]. This is in contrast to the [[boson]]s, which do not obey the exclusion principle.
Baryons, along with [[meson]]s, are [[hadron]]s, meaning they are particles composed of [[quark]]s. Quarks have baryon numbers of B = {{frac|1|3}} and antiquarks have baryon number of B = −{{frac|1|3|}}. The term "baryon" usually refer to ''triquarks''—baryons mades of three quarks (B = {{frac|1|3}} + {{frac|1|3}} + {{frac|1|3}} = 1), but there are other "exotic" baryons, such as pentaquarks — baryons made of four quarks and one antiquark (B = {{frac|1|3}} + {{frac|1|3}} + {{frac|1|3}} + {{frac|1|3}} − {{frac|1|3}} = 1), but their existence is not generally accepted. Theorically, heptaquarks (5 quarks, 2 antiquarks), nonaquarks (6 quarks, 3 antiquarks), etc. could also exist.
==Classification==
{{main|Isospin}}
[[Image:Baryon decuplet.svg|thumb|Combinations of three u, d or s-quarks with a total spin of 3/2 form the so-called '''baryon decuplet'''.]]
[[Image:Octeto bariônico.png|thumb|right|The '''octet''' of light spin-1/2 baryons.]]
Baryons are classified into groups according to their [[isospin]] values and [[quark]] content. There are six groups of triquarks — [[nucleon]] ({{SubatomicParticle|Nucleon}}), [[Delta baryon|Delta]] ({{SubatomicParticle|Delta}}), [[Lambda baryon|Lambda]] ({{SubatomicParticle|Lambda}}), [[Sigma baryon|Sigma]] ({{SubatomicParticle|Sigma}}), [[Xi baryon|Xi]] ({{SubatomicParticle|Xi}}), and [[Omega baryon|Omega]] ({{SubatomicParticle|Omega}}). The rules for classification are defined by the [[Particle Data Group]]. These rules consider the {{SubatomicParticle|link=yes|Up quark}}, {{SubatomicParticle|link=yes|Down quark}} and {{SubatomicParticle|link=yes|Strange quark}} quarks to be ''light'' and the {{SubatomicParticle|link=yes|Charm quark}}, {{SubatomicParticle|link=yes|Bottom quark}}, and {{SubatomicParticle|link=yes|Top quark}} to be ''heavy''. The rules cover all the particles that can be made from three of each of the six quarks ([[up quark|up]], [[down quark|down]], [[strange quark|strange]], [[charm quark|charm]], [[bottom quark|bottom]], [[top quark|top]]) — even though baryons made of top quarks are not expected to exist because of the top quark's short lifetime—but not pentaquarks.<ref name=PDGBaryonsymbols>{{cite journal |author=W.-M. Yao et al. |publisher=Particle Data Group |title=Naming scheme for hadrons |journal= Journal of Physics G |volume=33 |issue=1 |year=2006 |url=http://pdg.lbl.gov/2007/reviews/namingrpp.pdf |accessdate= 2008-04-20 |quote= }}</ref>:
* Baryons with three {{SubatomicParticle|link=yes|Up quark}} and/or {{SubatomicParticle|link=yes|Down quark}} quarks are {{SubatomicParticle|link=yes|Nucleon}}'s ([[isospin]] {{frac|1|2}}) or {{SubatomicParticle|link=yes|Delta}}'s (isospin {{frac|3|2}}).
* Baryons with two {{SubatomicParticle|link=yes|Up quark}} and/or {{SubatomicParticle|link=yes|Down quark}} quarks are {{SubatomicParticle|link=yes|Lambda}}'s (isospin 0) or {{SubatomicParticle|link=yes|Sigma}}'s (isospin 1). If the third quark is heavy, its identity is given by a subscript.
* Baryons with one {{SubatomicParticle|link=yes|Up quark}} or {{SubatomicParticle|link=yes|Down quark}} quark are {{SubatomicParticle|link=yes|Xi}}'s (isospin {{frac|1|2}}). One or two subscripts are used if one or both of the remaining quarks are heavy.
* Baryons with no {{SubatomicParticle|link=yes|Up quark}} or {{SubatomicParticle|link=yes|Down quark}} quarks are {{SubatomicParticle|link=yes|Omega}}'s (isospin 0), and subscripts indicate any heavy quark content.
* Baryons that decay strongly have their masses as part of their names. For example, Sigmas ({{SubatomicParticle|Sigma}}) and Omegas ({{SubatomicParticle|Omega}}) do not decay strongly, but Deltas ({{nowrap|{{SubatomicParticle|Delta}}(1232)}}), and charmed Xis ({{nowrap|{{SubatomicParticle|Charmed Xi+}}(2645)}}) do.
Quarks carry charge, so knowing the charge of a particle indirectly gives the quark content. For example, the rules above say that the {{SubatomicParticle|Bottom sigma}} contains a bottom and some combination of two up and/or down quarks. A {{SubatomicParticle|Bottom sigma0}} must be one up quark (Q={{frac|2|3}}), one down quark (Q=−{{frac|1|3}}), and one bottom quark (Q=−{{frac|1|3}}) to have the correct charge (Q=0).
The number of baryons within one group (excluding resonances) is given by the number of isospin projections possible (2 × isospin + 1). For example there are four {{SubatomicParticle|Delta}}'s, corresponding to the four isospin projections of the isospin value I = {{frac|3|2}}: {{SubatomicParticle|Delta++}} (I<sub>z</sub> = {{frac|3|2}}), {{SubatomicParticle|Delta+}}(I<sub>z</sub> = {{frac|1|2}}), {{SubatomicParticle|Delta0}}(I<sub>z</sub> = −{{frac|1|2}}), and {{SubatomicParticle|Delta-}}(I<sub>z</sub> = −{{frac|3|2}}). Another example would be the three {{SubatomicParticle|Bottom sigma}}'s, corresponding to the three isospin projections of the isospin value I = 1: {{SubatomicParticle|Bottom sigma+}} (I<sub>z</sub> = 1), {{SubatomicParticle|Bottom sigma0}}(I<sub>z</sub> = 0), and {{SubatomicParticle|Bottom sigma-}}(I<sub>z</sub> = −1).
==Baryonic matter==
'''Baryonic [[matter]]''' is matter composed mostly of baryons (by mass), which includes [[atom]]s of any sort (and thus includes nearly all matter that we may encounter or [[experience]] in everyday life, including our bodies). '''Non-baryonic matter''', as implied by the name, is any sort of matter that is not primarily composed of baryons. This might include such ordinary matter as [[neutrino]]s or free [[electron]]s; however, it may also include exotic species of non-baryonic [[dark matter]], such as [[supersymmetry|supersymmetric particles]], [[axion]]s or [[black hole]]s. The distinction between baryonic and non-baryonic matter is important in [[physical cosmology|cosmology]], because [[Big Bang nucleosynthesis]] models set tight constraints on the amount of baryonic matter present in the early [[universe]].
The very existence of baryons is also a significant issue in cosmology because we have assumed that the Big Bang produced a state with equal amounts of baryons and anti-baryons. The process by which baryons come to outnumber their antiparticles is called [[baryogenesis]] (in contrast to a process by which [[lepton]]s account for the predominance of matter over antimatter, [[leptogenesis (physics)|leptogenesis]]).
==Baryogenesis==
Experiments are consistent with the number of quarks in the universe being a constant and, more specifically, the number of [[baryon]]s being a constant; in technical language, the total [[baryon number]] appears to be ''[[conservation law|conserved]].'' Within the prevailing [[Standard Model]] of particle physics, the number of baryons may change in multiples of three due to the action of [[sphaleron]]s, although this is rare and has not been observed experimentally. Some [[grand unified theory|grand unified theories]] of particle physics also predict that a single [[proton]] can decay, changing the baryon number by one; however, this has not yet been observed experimentally. The [[baryogenesis|excess of baryons over antibaryons]] in the present universe is thought to be due to non-conservation of baryon number in the very early universe, though this is not well understood.
==See also==
* [[List of baryons]]
* [[Baryon number]]
* [[Particle physics]]
* [[Pentaquark]]
* [[List of particles]]
* [[Proton decay]]
* [[Charmed baryons]]
==Notes==
<references/>
==References and further reading==
{{particles}}
[[Category:Baryons]]
[[Category:Nuclear physics]]
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