View source for Physics:Hadron

{{Short description|Composite subatomic particle}}
[[File:Bosons-Hadrons-Fermions-RGB-png2.png|thumb|338x338px|A hadron is a composite subatomic particle. Every hadron must fall into one of the two fundamental classes of particle, bosons and fermions]]
In [[Physics:Particle physics|particle physics]], a '''hadron''' ({{IPAc-en|audio=En-us-hadron.ogg|ˈ|h|æ|d|r|ɒ|n}}; {{lang-grc|ἁδρός|hadrós}}; "stout, thick") is a composite subatomic particle made of two or more [[Physics:Quark|quark]]s [[Physics:Bound state|held together]] by the [[Physics:Strong interaction|strong interaction]]. They are analogous to [[Physics:Molecule|molecule]]s that are held together by [[Physics:Electromagnetism|the electric force]]. Most of the mass of ordinary matter comes from two hadrons: the [[Physics:Proton|proton]] and the [[Physics:Neutron|neutron]], while most of the mass of the [[Physics:Nucleon|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: [[Physics:Baryon|baryon]]s, made of an odd number of [[Physics:Quark|quark]]s (usually three quarks) and [[Physics:Meson|meson]]s, made of an even number of quarks (usually two quarks: one quark and one [[Physics:Antiparticle|antiquark]]).<ref name=GellMann-1964/> [[Physics:Proton|Proton]]s and [[Physics:Neutron|neutron]]s (which make the majority of the mass of an [[Physics:Atom|atom]]) are examples of baryons; [[Physics:Pion|pion]]s are an example of a meson. "Exotic" hadrons, containing more than three valence quarks, have been discovered in recent years. A [[Physics:Tetraquark|tetraquark]] state (an [[Physics:Exotic meson|exotic meson]]), named the [[Physics:Z(4430)|Z(4430)]]{{sup|−}}, was discovered in 2007 by the [[Physics: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 |arxiv = 1404.1903|doi = 10.1103/PhysRevLett.112.222002|pmid = 24949760|title = Observation of the Resonant Character of the Z(4430)<sup>−</sup> State|journal = Physical Review Letters|volume = 112|issue = 22|pages = 222002|year = 2014|last1 = Aaij|first1 = R.|bibcode = 2014PhRvL.112v2002A|s2cid = 904429|display-authors = etal |collaboration=LHCb collaboration}}</ref> Two [[Physics:Pentaquark|pentaquark]] states ([[Physics:Exotic baryon|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 more 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 [[Physics:Atomic nucleus|atomic nucleus]]) are believed to be [[Physics:Particle decay|unstable]] and eventually decay into other particles. The only known possible exception is free protons, which [[Physics: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 [[Physics:Free neutron decay|longest-lived unstable particle]], and decay with a [[Physics:Half-life|half-life]] of about 879&nbsp;seconds.{{efn|
The [[Physics:Nucleon|proton and neutrons']] respective [[Physics:Antiparticle|antiparticle]]s 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 |last1=Zyla |first1=P. A. |title=n MEAN LIFE |url=https://pdglive.lbl.gov/DataBlock.action?node=S017T |website=PDG Live: 2020 Review of Particle Physics |publisher=Particle Data Group |access-date=3 February 2022 |date=2020}}</ref>

Hadron physics is studied by colliding hadrons, e.g. protons, with each other or [[Physics:High-energy nuclear physics|the nuclei of dense, heavy elements]], such as lead (Pb) or gold (Au), and detecting the debris in the produced [[Physics:Particle shower|particle shower]]s. A similar process occurs in the natural environment, in the extreme upper-atmosphere, where [[Physics:Muon|muon]]s and mesons such as [[Physics:Pion|pion]]s are produced by the collisions of [[Astronomy:Cosmic ray|cosmic ray]]s with rarefied gas particles in the outer atmosphere.<ref>{{cite book |last1=Martin |first1=B. R. |title=Particle physics |date=2017 |location=Chichester, West Sussex, United Kingdom |isbn=9781118911907 |edition=Fourth}}</ref>

==Terminology and etymology==
The term "hadron" is a new Greek word introduced by L.B. Okun and in a plenary talk at the 1962 International Conference on High Energy Physics at [[Organization:CERN|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. — 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 [[Physics:Quark model|quark model]],<ref name=Amsler-etal-2008-PDG/> the properties of hadrons are primarily determined by their so-called ''valence quarks''. For example, a [[Physics:Proton|proton]] is composed of two [[Physics:Up quark|up quark]]s (each with [[Physics:Electric charge|electric charge]] {{frac|+|2|3}}, for a total of +{{frac|4|3}} together) and one [[Physics:Down quark|down quark]] (with electric charge {{frac|−|1|3}}). Adding these together yields the proton charge of +1. Although quarks also carry [[Physics:Color charge|color charge]], hadrons must have zero total color charge because of a phenomenon called [[Physics:Color confinement|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 [[Physics:Antiparticle|antiquark]] of the corresponding anticolor, or three quarks of different colors. Hadrons with the first arrangement are a type of [[Physics:Meson|meson]], and those with the second arrangement are a type of [[Physics:Baryon|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 [[Physics:Charm quark|charm]] and [[Physics:Bottom quark|bottom quark]]s; the [[Physics:Top quark|top quark]] vanishes before it has time to bind into a hadron). The strength of the [[Physics:Strong force|strong force]] [[Physics:Gluon|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 [[Physics: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) 2 or 3&nbsp;quarks, this technically refers to the constant excess of quarks vs. antiquarks.

Like all [[Physics:Subatomic particle|subatomic particle]]s, hadrons are assigned [[Physics:Quantum number|quantum number]]s corresponding to the [[Representation theory|representations]] of the [[Physics:Poincaré group|Poincaré group]]: {{math|''J''{{sup|PC}} }}({{mvar|m}}), where {{mvar|J}} is the [[Physics:Spin|spin]] quantum number, {{math|P}} the intrinsic parity (or [[Physics:Parity|P-parity]]), {{math|C}} the charge conjugation (or C-parity), and {{mvar|m}} is the particle's [[Physics:Mass|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|mass–energy equivalence]], most of the mass comes from the large amount of energy associated with the [[Physics:Strong interaction|strong interaction]]. Hadrons may also carry flavor quantum numbers such as [[Physics:Isospin|isospin]] (G&nbsp;parity), and [[Physics:Strangeness|strangeness]]. All quarks carry an additive, conserved quantum number called a [[Physics:Baryon number|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 [[Physics:Excited state|excited state]]s known as [[Physics:Resonance (particle physics)|resonances]]. Each [[Physics:Ground state|ground state]] hadron may have several excited states; several hundreds of resonances have been observed in experiments. Resonances decay extremely quickly (within about 10{{sup|−24}}&nbsp;[[Second|second]]s) via the strong nuclear force.

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

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

[[Physics: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 [[Physics:Proton|proton]] and [[Physics:Neutron|neutron]] have three valence quarks, but [[Physics:Pentaquark|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 [[Physics:Fermion|fermion]]s, ''i.e.'', they have half-integer [[Physics:Spin|spin]]. As quarks possess [[Physics:Baryon number|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|Physics:Meson|Physics:Exotic meson}}
[[Physics:Meson|Meson]]s are hadrons containing an even number of valence quarks (at least 2).<ref name=GellMann-1964/> Most well known mesons are composed of a quark-antiquark pair, but possible [[Physics:Tetraquark|tetraquark]]s (4&nbsp;quarks) and [[Physics:Hexaquark|hexaquark]]s (6&nbsp;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 [[Physics:Exotic meson|exotic meson]] may exist which do not fall within the quark model of classification. These include [[Physics:Glueball|glueball]]s and hybrid mesons (mesons bound by excited [[Physics:Gluon|gluon]]s).

Because mesons have an even number of quarks, they are also all [[Physics:Boson|boson]]s, with integer [[Physics:Spin|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 [[Physics:Pion|pion]]s and [[Physics:Kaon|kaon]]s. Pions also play a role in holding [[Physics:Atomic nucleus|atomic nuclei]] together via the residual strong force.

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

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

}}

==Footnotes==
{{notelist}}

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

<ref name=Aaij-etal-2015-LHCb-Jψp>
{{cite journal
 |first1=R. |last1=Aaij
 |display-authors=0
 |collaboration=LHCb collaboration
 |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=[[Physics:Physical Review Letters|Physical Review Letters]]
 |volume=115  |issue=7  |pages=072001
 |doi=10.1103/PhysRevLett.115.072001 |pmid=26317714
 |arxiv = 1507.03414 |bibcode = 2015PhRvL.115g2001A |s2cid=119204136
}}
</ref>

<ref name=Amsler-etal-2008-PDG>
{{cite journal
 |first1=C. |last1=Amsler
 |display-authors=0
 |collaboration=[[Physics:Particle Data Group|Particle Data Group]]
 |year=2008
 |title=Quark Model
 |series=Review of Particle Physics
 |journal=[[Physics:Physics Letters B|Physics Letters B]]
 |volume=667  |issue=1  |pages=1–6
 |doi=10.1016/j.physletb.2008.07.018
 |bibcode = 2008PhLB..667....1A
 |url=http://pdg.lbl.gov/2008/reviews/quarkmodrpp.pdf
|hdl=1854/LU-685594
 |hdl-access=free
 }}
</ref>

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

<ref name=Choi-etal-2008-Belle>
{{cite journal
 |last1=Choi  |first1=S.-K.
 |display-authors=0
 |collaboration=[[Physics:Belle experiment|Belle Collaboration]]
 |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
 |doi=10.1103/PhysRevLett.100.142001 |pmid=18518023
 |arxiv=0708.1790  |bibcode=2008PhRvL.100n2001C  |s2cid=119138620
}}
</ref>

<ref name=GellMann-1964>
{{cite journal
 |last1=Gell-Mann |first1=M.
 |year=1964
 |title=A schematic model of baryons and mesons
 |journal=Physics Letters
 |volume=8  |issue=3  |pages=214–215
 |doi=10.1016/S0031-9163(64)92001-3
 |bibcode=1964PhL.....8..214G
}}
</ref>

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

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

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

== External links ==

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[[Category:Hadrons| ]]
[[Category:Nuclear physics]]

{{Sourceattribution|Hadron}}