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{{Short description|Composite subatomic particle}}
[[File:Bosons-Hadrons-Fermions-RGB.svg|thumb|upright=1.5|A hadron is a [[Composite particle|composite subatomic particle]]. Every hadron must fall into one of the two fundamental classes of particle, [[boson]]s and [[fermion]]s.]]
{{Standard model of particle physics}}

In [[particle physics]], a '''hadron''' is a [[composite particle|composite subatomic particle]] made of two or more [[quark]]s [[bound state|held together]] by the [[strong interaction|strong nuclear force]]. Pronounced {{IPAc-en|audio=En-us-hadron.ogg|ˈ|h|æ|d|r|ɒ|n}}, the name is derived {{ety|grc|''{{linktext|ἁδρός}}'' (hadrós)|stout, thick}}. They are analogous to [[molecule]]s, which are held together by the [[electromagnetism|electric force]]. Most of the [[mass]] of ordinary [[matter]] comes from two hadrons: the [[proton]] and the [[neutron]], while most of the mass of the protons and neutrons is in turn due to the [[binding energy]] of their constituent quarks, due to the strong force.

Hadrons are categorized into two broad families: [[baryon]]s, made of an odd number of [[quark]]s (usually three) and [[meson]]s, made of an even number of quarks (usually two: one quark and one [[antiparticle|antiquark]]).<ref name=GellMann-1964/> Protons and neutrons (which make the majority of the mass of an [[atom]]) are examples of baryons; [[pion]]s are an example of a meson. A [[tetraquark]] state (an [[exotic meson]]), named the [[Z(4430)]]{{sup|−}}, was discovered in 2007 by the [[Belle experiment|Belle Collaboration]]<ref name=Choi-etal-2008-Belle/> and confirmed as a resonance in 2014 by the [[LHCb]] collaboration.<ref name="LHCb2014">{{Cite journal |last1=Aaij |first1=R. |display-authors=etal |year=2014 |title=Observation of the Resonant Character of the Z(4430)<sup>−</sup> State |journal=Physical Review Letters |volume=112 |issue=22 |pages=222002 |arxiv=1404.1903 |bibcode=2014PhRvL.112v2002A |doi=10.1103/PhysRevLett.112.222002 |pmid=24949760 |s2cid=904429 |collaboration=[[LHCb|LHCb collaboration]]}}</ref> Two [[pentaquark]] states ([[exotic baryon]]s), named {{nowrap|P{{su|p=+|b=c}}(4380)}} and {{nowrap|P{{su|p=+|b=c}}(4450)}}, were discovered in 2015 by the [[LHCb]] collaboration.<ref name=Aaij-etal-2015-LHCb-Jψp/> There are several other [[Exotic hadrons|"Exotic" hadron]] candidates and other colour-singlet quark combinations that may also exist.

Almost all "free" hadrons and antihadrons (meaning, in isolation and not bound within an [[atomic nucleus]]) are believed to be [[particle decay|unstable]] and eventually decay into other particles. The only known possible exception is free protons, which [[Proton decay|appear to be stable]], or at least, take immense amounts of time to decay (order of 10<sup>34+</sup>&nbsp;years). By way of comparison, free neutrons are the [[free neutron decay|longest-lived unstable particle]], and decay with a [[half-life]] of about 611&nbsp;seconds, and have a mean lifetime of 879&nbsp;seconds,{{efn|
The proton and neutrons' respective antiparticles are expected to follow the same pattern, but they are difficult to capture and study, because they immediately annihilate on contact with ordinary matter.
}}<ref name="PDG Live: 2020 Review of Particle Physics">{{cite web |last=Zyla |first=P. A. |date=2020 |title=n MEAN LIFE |url=https://pdglive.lbl.gov/DataBlock.action?node=S017T |access-date=3 February 2022 |website=PDG Live: 2020 Review of Particle Physics |publisher=Particle Data Group}}</ref> see [[free neutron decay]].

Hadron physics is studied by colliding hadrons, e.g. protons, with each other or [[high-energy nuclear physics|the nuclei of dense, heavy elements]], such as [[lead]] (Pb) or [[gold]] (Au), and detecting the debris in the produced [[particle shower]]s. A similar process occurs in the natural environment, in the extreme upper-atmosphere, where muons and mesons such as pions are produced by the collisions of [[cosmic ray]]s with rarefied gas particles in the outer atmosphere.<ref>{{cite book |last=Martin |first=B. R. |title=Particle physics |date=2017 |isbn=9781118911907 |edition=Fourth |location=Chichester, West Sussex, UK}}</ref>

==Terminology and etymology==
The term "hadron" is a [[new Greek]] word introduced by [[Lev Okun|L. B. Okun]] in a [[plenary talk]] at the 1962 [[International Conference on High Energy Physics]] at [[CERN]].<ref name=Okun-1962-CERN-plenary/> He opened his talk with the definition of a new category term:
{{blockquote|Notwithstanding 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 {{lang|grc|ἁδρός}} signifies "large", "massive", in contrast to {{lang|grc|λεπτός}} which means "small", "light"). I hope that this terminology will prove to be {{nowrap|convenient. — [[Lev Okun|L. B. Okun]] (1962)<ref name=Okun-1962-CERN-plenary/>}}
}}

==Properties==
[[Image:Hadron colors.svg|right|thumb|upright|All types of hadrons have zero total color charge (three examples shown).|alt=A green and a magenta ("antigreen") arrow canceling out each other out white, representing a meson; a red, a green, and a blue arrow canceling out to white, representing a baryon; a yellow ("antiblue"), a magenta, and a cyan ("antired") arrow canceling out to white, representing an antibaryon.]]

According to the [[quark model]],<ref name=Amsler-etal-2008-PDG/> the properties of hadrons are primarily determined by their so-called ''[[valence quark]]s''. For example, a [[proton]] is composed of two [[up quark]]s (each with [[electric charge]] {{frac|+|2|3}}, for a total of +{{frac|4|3}} together) and one [[down quark]] (with electric charge {{frac|−|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". The simplest ways for this to occur are with a quark of one color and an [[antiparticle|antiquark]] of the corresponding anticolor, or three quarks of different colors. Hadrons with the first arrangement are a type of [[meson]], and those with the second arrangement are a type of [[baryon]].

Massless virtual gluons compose the overwhelming majority of particles inside hadrons, as well as the major constituents of its mass (with the exception of the heavy [[charm quark|charm]] and [[bottom quark]]s; the [[top quark]] vanishes before it has time to bind into a hadron). The strength of the [[Strong interaction|strong-force]] [[gluon]]s which bind the quarks together has sufficient energy ({{mvar|E}}) to have resonances composed of massive ({{mvar|m}}) quarks ([[Mass–energy equivalence|{{mvar|E}} ≥ {{mvar|mc}}<sup>2</sup>]]). One outcome is that short-lived pairs of [[virtual particle|virtual]] quarks and antiquarks are continually forming and vanishing again inside a hadron. Because the virtual quarks are not stable wave packets (quanta), but an irregular and transient phenomenon, it is not meaningful to ask which quark is real and which virtual; only the small excess is apparent from the outside in the form of a hadron. Therefore, when a hadron or anti-hadron is stated to consist of (typically) two or three quarks, this technically refers to the constant excess of quarks versus antiquarks.

Like all [[subatomic particle]]s, hadrons are assigned [[quantum number]]s corresponding to the [[Representation theory|representations]] of the [[Poincaré group]]: {{math|''J''{{sup|PC}} }}({{mvar|m}}), where {{mvar|J}} is the [[Spin (physics)|spin]] quantum number, {{math|P}} the intrinsic parity (or [[Parity (physics)|P-parity]]), {{math|C}} the charge conjugation (or [[C-parity]]), and {{mvar|m}} is the particle's [[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 interaction]]. Hadrons may also carry [[flavour quantum number|flavor quantum numbers]] such as [[isospin]] ([[G-parity]]), and [[strangeness]]. All quarks carry an additive, conserved quantum number called a [[baryon number]] ({{mvar|B}}), which is {{frac|+|1|3}} for quarks and {{frac|−|1|3}} for antiquarks. This means that baryons (composite particles made of three, five or a larger odd number of quarks) have  {{mvar|B}}&nbsp;=&nbsp;1  whereas mesons have {{mvar|B}}&nbsp;=&nbsp;0.

Hadrons have [[excited state]]s known as [[resonance (particle physics)|resonances]]. Each [[ground state]] hadron may have several excited states; several hundred different resonances have been observed in experiments. Resonances decay extremely quickly (within about 10{{sup|−24}}&nbsp;[[second]]s) via the strong nuclear force.

In other [[phase (matter)|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 [[gluon]]s will no longer be confined within hadrons, "because the [[coupling constant|strength]] of the strong interaction [[coupling constant#Running coupling|diminishes with energy]]". This property, which is known as [[asymptotic freedom]], has been experimentally confirmed in the energy range between 1&nbsp;[[GeV]] (gigaelectronvolt) and 1&nbsp;[[TeV]] (teraelectronvolt).<ref name=Bethke-2007/> All [[free particle|free]] hadrons [[proton decay|except (''possibly'') the proton and antiproton]] are [[Exponential decay|unstable]].

==Baryons==
{{Main|Baryon|Exotic baryon}}

[[Baryon]]s are hadrons containing an odd number of valence quarks (at least 3).<ref name=GellMann-1964/> Most well-known baryons such as the [[proton]] and [[neutron]] have three valence quarks, but [[pentaquark]]s with five quarks—three quarks of different colors, and also one extra quark-antiquark pair—have also been proven to exist. Because baryons have an odd number of quarks, they are also all [[fermion]]s, ''i.e.'', they have half-integer [[Spin (physics)|spin]]. As quarks possess [[baryon number]] ''B''&nbsp;=&nbsp;{{frac|1|3}}, baryons have baryon number ''B''&nbsp;=&nbsp;1. Pentaquarks ''also'' have ''B''&nbsp;=&nbsp;1, since the extra quark's and antiquark's baryon numbers cancel.

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.

As of August 2015, there are two known pentaquarks, {{nowrap|P{{su|p=+|b=c}}(4380)}} and {{nowrap|P{{su|p=+|b=c}}(4450)}}, both discovered in 2015 by the [[LHCb]] collaboration.<ref name=Aaij-etal-2015-LHCb-Jψp/>

==Mesons==
{{Main|Meson|Exotic meson}}
[[Meson]]s are hadrons containing an even number of valence quarks (at least two).<ref name=GellMann-1964/> Most well known mesons are composed of a quark-antiquark pair, but possible [[tetraquark]]s (four quarks) and [[hexaquark]]s (six quarks, comprising either a dibaryon or three quark-antiquark pairs) may have been discovered and are being investigated to confirm their nature.<ref name=Mann-2013-06-Wired/> Several other hypothetical types of [[exotic meson]] may exist which do not fall within the quark model of classification. These include [[glueball]]s and [[hybrid meson]]s (mesons bound by excited [[gluon]]s).

Because mesons have an even number of quarks, they are also all [[boson]]s, with integer [[Spin (physics)|spin]], ''i.e.'', 0, +1, or −1. They have baryon number {{nobr|{{math| ''B'' {{=}} {{sfrac|1|3}} − {{sfrac|1|3}} {{=}} 0 }}.}} Examples of mesons commonly produced in particle physics experiments include [[pion]]s and [[kaon]]s. Pions also play a role in holding [[atomic nuclei]] together via the [[residual strong force]].

==See also==
{{div col |colwidth=15em |content=

* [[Exotic hadron]]
* Hadron therapy, a.k.a. [[particle therapy]]
* [[Hadronization]], the formation of hadrons out of quarks and gluons
* [[Large Hadron Collider]] (LHC)
* [[List of particles]]
* [[List of baryons]]
* [[List of mesons]]
* [[Standard model]]
* [[Subatomic particle]]

}}

==Footnotes==
{{notelist}}

==References==
{{reflist|25em|refs=

<ref name="Aaij-etal-2015-LHCb-Jψp">
{{cite journal |last=Aaij |first=R. |display-authors=etal |year=2015 |title=Observation of J/ψp resonances consistent with pentaquark states in Λ{{su|p=0|b=b}}&nbsp;→&nbsp;J/ψK{{sup|−}}p decays |journal=[[Physical Review Letters]] |volume=115 |issue=7 |pages=072001 |arxiv=1507.03414 |bibcode=2015PhRvL.115g2001A |doi=10.1103/PhysRevLett.115.072001 |pmid=26317714 |s2cid=119204136 |collaboration=[[LHCb|LHCb collaboration]]}}
</ref>

<ref name="Amsler-etal-2008-PDG">
{{cite journal |last=Amsler |first=C. |display-authors=etal |year=2008 |title=Quark Model |url=http://pdg.lbl.gov/2008/reviews/quarkmodrpp.pdf |journal=[[Physics Letters B]] |series=Review of Particle Physics |volume=667 |issue=1 |pages=1–6 |bibcode=2008PhLB..667....1A |doi=10.1016/j.physletb.2008.07.018 |hdl-access=free |collaboration=[[Particle Data Group]] |hdl=1854/LU-685594}}
</ref>

<ref name="Bethke-2007">
{{Cite journal |last=Bethke |first=S. |year=2007 |title=Experimental tests of asymptotic freedom |journal=[[Progress in Particle and Nuclear Physics]] |volume=58 |issue=2 |pages=351–386 |arxiv=hep-ex/0606035 |bibcode=2007PrPNP..58..351B |doi=10.1016/j.ppnp.2006.06.001 |s2cid=14915298}}
</ref>

<ref name="Choi-etal-2008-Belle">
{{cite journal |last=Choi |first=S.-K. |display-authors=etal |year=2008 |title=Observation of a resonance-like structure in the {{Subatomic particle|Pion+-}}Ψ′ mass distribution in exclusive B→K{{Subatomic particle|Pion+-}}Ψ′ decays |journal=Physical Review Letters |volume=100 |issue=14 |pages=142001 |arxiv=0708.1790 |bibcode=2008PhRvL.100n2001C |doi=10.1103/PhysRevLett.100.142001 |pmid=18518023 |s2cid=119138620 |collaboration=[[Belle experiment|Belle Collaboration]]}}
</ref>

<ref name="GellMann-1964">
{{cite journal |last=Gell-Mann |first=M. |year=1964 |title=A schematic model of baryons and mesons |journal=Physics Letters |volume=8 |issue=3 |pages=214–215 |bibcode=1964PhL.....8..214G |doi=10.1016/S0031-9163(64)92001-3}}
</ref>

<ref name="Mann-2013-06-Wired">
{{cite news |last=Mann |first=Adam |date=2013-06-17 |title=Mysterious subatomic particle may represent exotic new form of matter |url=https://www.wired.com/wiredscience/2013/06/four-quark-particle |access-date=2021-08-27 |magazine=[[Wired (magazine)|Wired]] |department=Science}} — News story about {{math|Z}}(3900) particle discovery.
</ref>

<ref name="Okun-1962-CERN-plenary">
{{cite conference |last=Okun |first=L. B. |author-link=Lev Okun |year=1962 |title=The theory of weak interaction |conference=International Conference on High-Energy Physics |type=plenary talk |page=845 |bibcode=1962hep..conf..845O |place=CERN, Geneva, CH |book-title=Proceedings of 1962 International Conference on High-Energy Physics at CERN}}
</ref>

}} <!-- end "refs=" -->

== External links ==
* {{Wiktionary-inline|hadron}}

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