Editing Quantum chromodynamics (section)

==History==
{{main|History of quantum mechanics|History of quantum field theory}}
With the invention of [[bubble chamber]]s and [[spark chamber]]s in the 1950s, experimental [[particle physics]] discovered a large and ever-growing number of particles called [[hadron]]s. It seemed that such a large number of particles could not all be [[fundamental particles|fundamental]]. First, the particles were classified by [[Charge (physics)|charge]] and [[isospin]] by [[Eugene Wigner]] and [[Werner Heisenberg]]; then, in 1953–56,<ref>
{{cite journal
 |last1=Nakano |first1=T
 |last2=Nishijima |first2=N
 |year=1953
 |title=Charge Independence for V-particles |journal=[[Progress of Theoretical Physics]]
 |volume=10 |issue=5 |pages=581
 |doi=10.1143/PTP.10.581
|bibcode = 1953PThPh..10..581N |doi-access=free}}</ref><ref>
{{cite journal
 |last=Nishijima |first=K
 |year=1955
 |title=Charge Independence Theory of V Particles
 |journal=[[Progress of Theoretical Physics]]
 |volume=13 |issue=3 |pages=285–304 
 |doi=10.1143/PTP.13.285
|bibcode = 1955PThPh..13..285N |doi-access=free}}</ref><ref>
{{cite journal
 |last=Gell-Mann |first=M
 |year=1956
 |title=The Interpretation of the New Particles as Displaced Charged Multiplets
 |journal=[[Il Nuovo Cimento]]
 |volume=4
 |issue=S2 |pages=848–866
 |doi=10.1007/BF02748000
|bibcode=1956NCim....4S.848G|s2cid=121017243
 }}</ref> according to [[Strangeness (particle physics)|strangeness]] by [[Murray Gell-Mann]] and [[Kazuhiko Nishijima]] (see [[Gell-Mann–Nishijima formula]]). To gain greater insight, the hadrons were sorted into groups having similar properties and masses using the ''[[eightfold way (physics)|eightfold way]]'', invented in 1961 by Gell-Mann<ref>Gell-Mann, M. (1961). "The Eightfold Way: A Theory of strong interaction symmetry" (No. TID-12608; CTSL-20). California Inst. of Tech., Pasadena. Synchrotron Lab ([https://www.osti.gov/scitech/servlets/purl/4008239 online]).</ref>  and [[Yuval Ne'eman]]. Gell-Mann and [[George Zweig]], correcting an earlier approach of [[Shoichi Sakata]], went on to propose in 1963 that the structure of the groups could be explained by the existence of three [[flavour (particle physics)|flavor]]s of smaller particles inside the hadrons: the [[quark]]s. Gell-Mann also briefly discussed a field theory model in which quarks interact with gluons.<ref>{{cite journal | author= M. Gell-Mann | title=A Schematic Model of Baryons and Mesons | volume=8 | issue=3 | year=1964 | journal=[[Physics Letters]] | pages=214–215 | doi=10.1016/S0031-9163(64)92001-3| bibcode=1964PhL.....8..214G }}</ref><ref>{{cite book |author1=M. Gell-Mann |author2=H. Fritzsch | title=Murray Gell-Mann: Selected Papers | year=2010 | publisher=World Scientific| bibcode=2010mgsp.book.....F }}</ref>

Perhaps the first remark that quarks should possess an additional [[quantum number]] was made<ref>
{{cite arXiv
 |eprint=0904.0343
 |author1=Fyodor Tkachov
 |title=A contribution to the history of quarks: Boris Struminsky's 1965 JINR publication
 |class=physics.hist-ph
 |year=2009
}}</ref> as a short footnote in the preprint of [[Boris Struminsky]]<ref name="struminsky">B. V. Struminsky, Magnetic moments of baryons in the quark model. [[JINR]]-Preprint P-1939, Dubna, Russia. Submitted on January 7, 1965.</ref> in connection with the Ω<sup>−</sup> [[Omega baryon|hyperon]] being composed of three [[strange quark]]s with parallel spins (this situation was peculiar, because since quarks are [[fermion]]s, such a combination is forbidden by the [[Pauli exclusion principle]]):
{{Blockquote|text=Three identical quarks cannot form an antisymmetric S-state. In order to realize an antisymmetric orbital S-state, it is necessary for the quark to have an additional quantum number.|author=B. V. Struminsky|title=Magnetic moments of barions in the quark model|source=[[JINR]]-Preprint P-1939, Dubna, Submitted on January 7, 1965}}

Boris Struminsky was a PhD student of [[Nikolay Bogolyubov]]. The problem considered in this preprint was suggested by Nikolay Bogolyubov, who advised Boris Struminsky in this research.<ref name="struminsky" /> In the beginning of 1965, [[Nikolay Bogolyubov]], [[Boris Struminsky]] and [[Albert Tavkhelidze]] wrote a preprint with a more detailed discussion of the additional quark quantum degree of freedom.<ref>[[Nikolay Bogolyubov|N. Bogolubov]], B. Struminsky, A. Tavkhelidze. On composite models in the theory of elementary particles. [[JINR]] Preprint D-1968, [[Dubna]] 1965.</ref> This work was also presented by Albert Tavkhelidze without obtaining consent of his collaborators for doing so at an international conference in [[Trieste]] (Italy), in May 1965.<ref>A. Tavkhelidze. Proc. Seminar on High Energy Physics and Elementary Particles, Trieste, 1965, Vienna IAEA, 1965, p. 763.</ref><ref>V. A. Matveev and A. N. Tavkhelidze (INR, RAS, Moscow) [http://www.inr.ru/quantum.html The quantum number color, colored quarks and QCD] {{webarchive|url=https://web.archive.org/web/20070523073026/http://www.inr.ru/quantum.html |date=2007-05-23 }} (Dedicated to the 40th Anniversary of the Discovery of the Quantum Number Color). Report presented at the 99th Session of the JINR Scientific Council, Dubna, 19–20 January 2006.</ref>

A similar mysterious situation was with the [[Delta baryon|Δ<sup>++</sup> baryon]]; in the quark model, it is composed of three [[up quark]]s with parallel spins. In 1964–65, [[Oscar W. Greenberg|Greenberg]]<ref>{{cite journal |first=O. W. |last=Greenberg |title=Spin and Unitary Spin Independence in a Paraquark Model of Baryons and Mesons |journal=Phys. Rev. Lett. |volume=13 |issue= 20|pages=598–602 |year=1964 |doi=10.1103/PhysRevLett.13.598 |bibcode=1964PhRvL..13..598G }}</ref> and [[Moo-Young Han|Han]]–[[Yoichiro Nambu|Nambu]]<ref>{{cite journal |first1=M. Y. |last1=Han |first2=Y. |last2=Nambu |title=Three-Triplet Model with Double SU(3) Symmetry |journal=Phys. Rev. |volume=139 |issue= 4B|pages=B1006–B1010 |year=1965 |doi=10.1103/PhysRev.139.B1006 |bibcode=1965PhRv..139.1006H |url=https://digital.library.unt.edu/ark:/67531/metadc1031342/ |url-access=subscription }}</ref> independently resolved the problem by proposing that quarks possess an additional [[special unitary group|SU(3)]] [[gauge theory|gauge]] [[degrees of freedom (physics and chemistry)|degree of freedom]], later called color charge. Han and Nambu noted that quarks might interact via an octet of vector [[gauge boson]]s: the [[gluon]]s.

Since free quark searches consistently failed to turn up any evidence for the new particles, and because an elementary particle back then was ''defined'' as a particle that could be separated and isolated, Gell-Mann often said that quarks were merely convenient mathematical constructs, not real particles. The meaning of this statement was usually clear in context: He meant quarks are confined, but he also was implying that the strong interactions could probably not be fully described by quantum field theory.

[[Richard Feynman]] argued that high energy experiments showed quarks are real particles: he called them ''[[Parton (particle physics)|partons]]'' (since they were parts of hadrons). By particles, Feynman meant objects that travel along paths, elementary particles in a field theory.

The difference between Feynman's and Gell-Mann's approaches reflected a deep split in the theoretical physics community. Feynman thought the quarks have a distribution of position or momentum, like any other particle, and he (correctly) believed that the diffusion of parton momentum explained [[pomeron|diffractive scattering]]. Although Gell-Mann believed that certain quark charges could be localized, he was open to the possibility that the quarks themselves could not be localized because space and time break down. This was the more radical approach of [[S-matrix theory]].

[[James Daniel Bjorken|James Bjorken]] proposed that pointlike partons would imply certain relations in [[deep inelastic scattering]] of [[electron]]s and protons, which were verified in experiments at [[SLAC]] in 1969. This led physicists to abandon the S-matrix approach for the strong interactions.

In 1973 the concept of [[Color charge|color]] as the source of a "strong field" was developed into the theory of QCD by physicists [[Harald Fritzsch]] and [[Heinrich Leutwyler]], together with physicist [[Murray Gell-Mann]].<ref>{{cite journal | last1 = Fritzsch | first1 = H. | last2 = Gell-Mann | first2 = M. | last3 = Leutwyler | first3 = H. | title = Advantages of the color octet gluon picture | journal = Physics Letters | volume = 47B | issue = 4| pages = 365–368 | year = 1973 | doi=10.1016/0370-2693(73)90625-4| bibcode = 1973PhLB...47..365F | citeseerx = 10.1.1.453.4712 }}</ref> In particular, they employed the general field theory developed in 1954 by [[Chen Ning Yang]] and [[Robert Mills (physicist)|Robert Mills]]<ref>{{cite journal |author-link1=Chen-Ning Yang |first1=C. N. |last1=Yang |author-link2=Robert Mills (physicist) |first2=R. |last2=Mills |title=Conservation of Isotopic Spin and Isotopic Gauge Invariance |journal=[[Physical Review]] |volume=96 |issue=1 |pages=191–195 |year=1954 |doi=10.1103/PhysRev.96.191|bibcode = 1954PhRv...96..191Y |doi-access=free }}</ref> (see [[Yang–Mills theory]]), in which the carrier particles of a force can themselves radiate further carrier particles. (This is different from QED, where the photons that carry the electromagnetic force do not radiate further photons.)

The discovery of [[asymptotic freedom]] in the strong interactions by [[David Gross]], [[David Politzer]] and [[Frank Wilczek]] allowed physicists to make precise predictions of the results of many high energy experiments using the quantum field theory technique of [[perturbation theory (quantum mechanics)|perturbation theory]]. Evidence of gluons was discovered in [[three-jet event]]s at [[PETRA]] in 1979. These experiments became more and more precise, culminating in the verification of [[perturbative QCD]] at the level of a few percent at [[LEP]],  at [[CERN]].

The other side of asymptotic freedom is [[Color confinement|confinement]]. Since the force between color charges does not decrease with distance, it is believed that quarks and gluons can never be liberated from hadrons. This aspect of the theory is verified within [[lattice QCD]] computations, but is not mathematically proven. One of the [[Millennium Prize Problems]] announced by the [[Clay Mathematics Institute]] requires a claimant to produce such a proof. Other aspects of [[non-perturbative]] QCD are the exploration of phases of [[QCD matter|quark matter]], including the [[quark–gluon plasma]].