Editing Quark (section)

== History ==
[[Image:MurrayGellMannJI1.jpg|right|thumb|Murray Gell-Mann (2007)]] 
[[File:George Zweig.jpg|right|thumb|George Zweig (2015)]]

The [[quark model]] was independently proposed by physicists [[Murray Gell-Mann]]<ref name="Gell-Man1964">
{{cite journal
 |author=M. Gell-Mann
 |title=A Schematic Model of Baryons and Mesons
 |journal=[[Physics Letters]]
 |volume=8 |issue=3 |pages=214–215
 |year=1964
 |bibcode=1964PhL.....8..214G
 |doi=10.1016/S0031-9163(64)92001-3
}}</ref> and [[George Zweig]]<ref name="Zweig1964a">
{{cite web
 |author=G. Zweig
 |date=17 January 1964
 |title=An SU(3) Model for Strong Interaction Symmetry and its Breaking
 |website=CERN Document Server
 |id=CERN-TH-401
 |url=https://cds.cern.ch/record/352337/files/CERN-TH-401.pdf
}}</ref><ref name="Zweig1964b">
{{cite journal
 |author=G. Zweig
 |date=21 February 1964
 |title=An SU(3) Model for Strong Interaction Symmetry and its Breaking: II
 |website=CERN Document Server
 |doi=10.17181/CERN-TH-412
 |id=CERN-TH-412
 |url=https://cds.cern.ch/record/570209
}}</ref> in 1964.<ref name="Carithers" /> The proposal came shortly after Gell-Mann's 1961 formulation of a particle classification system known as the ''[[eightfold way (physics)|Eightfold Way]]'' – or, in more technical terms, [[SU(3)]] [[flavor symmetry]], streamlining its structure.<ref>
{{cite book
 |author=M. Gell-Mann
 |year=2000 |orig-date=1964
 |chapter=The Eightfold Way: A Theory of Strong Interaction Symmetry
 |editor=M. Gell-Mann, Y. Ne'eman
 |title=The Eightfold Way
 |page=11
 |publisher=[[Westview Press]]
 |isbn=978-0-7382-0299-0
}}<br /> Original:
{{cite report<!-- Citation Bot-->
 |author=M. Gell-Mann
 |year=1961
 |title=The Eightfold Way: A Theory of Strong Interaction Symmetry
 |url=https://digital.library.unt.edu/ark:/67531/metadc867161/
 |id=CTSL-20
 |publisher=[[California Institute of Technology]] Synchrotron Laboratory
 |via=University of North Texas
 |doi=10.2172/4008239
}}</ref> Physicist [[Yuval Ne'eman]] had independently developed a scheme similar to the Eightfold Way in the same year.<ref>
{{cite book
 |author=Y. Ne'eman
 |year=2000 |orig-date=1964
 |chapter=Derivation of Strong Interactions from Gauge Invariance
 |editor=M. Gell-Mann, Y. Ne'eman
 |title=The Eightfold Way
 |publisher=[[Westview Press]]
 |isbn=978-0-7382-0299-0
}}<br />Original
{{cite journal
 |author=Y. Ne'eman
 |year=1961
 |title=Derivation of Strong Interactions from Gauge Invariance
 |journal=[[Nuclear Physics (journal)|Nuclear Physics]]
 |volume=26 |issue=2 |page=222
 |bibcode=1961NucPh..26..222N
 |doi=10.1016/0029-5582(61)90134-1
}}</ref><ref>
{{cite book
 |author1=R. C. Olby
 |author2=G. N. Cantor
 |year=1996
 |title=Companion to the History of Modern Science
 |page=673
 |publisher=[[Taylor & Francis]]
 |isbn=978-0-415-14578-7
}}</ref> An early attempt at constituent organization was available in the [[Sakata model]].

At the time of the quark theory's inception, the "[[particle zoo]]" included a multitude of [[hadron]]s, among other particles. Gell-Mann and Zweig posited that they were not elementary particles, but were instead composed of combinations of quarks and antiquarks. Their model involved three flavors of quarks, [[up quark|up]], [[down quark|down]], and [[strange quark|strange]], to which they ascribed properties such as spin and electric charge.<ref name="Gell-Man1964"/><ref name="Zweig1964a"/><ref name="Zweig1964b"/> The initial reaction of the physics community to the proposal was mixed. There was particular contention about whether the quark was a physical entity or a mere abstraction used to explain concepts that were not fully understood at the time.<ref>
{{cite book
 |author=A. Pickering
 |title=Constructing Quarks
 |pages=114–125
 |publisher=[[University of Chicago Press]]
 |year=1984
 |isbn=978-0-226-66799-7
}}</ref>

In less than a year, extensions to the Gell-Mann–Zweig model were proposed. [[Sheldon Glashow]] and [[James Bjorken]] predicted the existence of a fourth flavor of quark, which they called ''charm''. The addition was proposed because it allowed for a better description of the [[weak interaction]] (the mechanism that allows quarks to decay), equalized the number of known quarks with the number of known [[lepton]]s, and implied a mass formula that correctly reproduced the masses of the known [[meson]]s.<ref>
{{cite journal
 |author1=B. J. Bjorken
 |author2=S. L. Glashow
 |title=Elementary Particles and SU(4)
 |journal=[[Physics Letters]]
 |volume=11 |issue=3 |pages=255–257
 |year=1964
 |bibcode=1964PhL....11..255B
 |doi=10.1016/0031-9163(64)90433-0
}}</ref>

[[Deep inelastic scattering]] experiments conducted in 1968 at the [[Stanford Linear Accelerator Center]] (SLAC) and published on October 20, 1969, showed that the proton contained much smaller, [[point particle|point-like objects]] and was therefore not an elementary particle.<ref name="Bloom" /><ref name="Breidenbach"/><ref>
{{cite web
 |author=J. I. Friedman 
 |title=The Road to the Nobel Prize 
 |url=http://www.hueuni.edu.vn/hueuni/en/news_detail.php?NewsID=1606&PHPSESSID=909807ffc5b9c0288cc8d137ff063c72 
 |publisher=[[Huế University]] 
 |access-date=2008-09-29 
 |archive-url=https://web.archive.org/web/20081225093044/http://www.hueuni.edu.vn/hueuni/en/news_detail.php?NewsID=1606&PHPSESSID=909807ffc5b9c0288cc8d137ff063c72 
 |archive-date=2008-12-25 
}}</ref> Physicists were reluctant to firmly identify these objects with quarks at the time, instead calling them "[[parton (particle physics)|partons]]" – a term coined by [[Richard Feynman]].<ref>
{{cite journal
 |author=R. P. Feynman
 |title=Very High-Energy Collisions of Hadrons
 |url=http://authors.library.caltech.edu/3871/1/FEYprl69.pdf
 |journal=[[Physical Review Letters]]
 |volume=23 |issue=24 |pages=1415–1417
 |year=1969
 |bibcode=1969PhRvL..23.1415F
 |doi=10.1103/PhysRevLett.23.1415
}}</ref><ref>
{{cite journal
 |author1=S. Kretzer
 |author2=H. L. Lai
 |author3=F. I. Olness
 |author4=W. K. Tung
 |title=CTEQ6 Parton Distributions with Heavy Quark Mass Effects
 |journal=[[Physical Review D]]
 |volume=69 |issue=11 |page=114005
 |year=2004
 |arxiv=hep-ph/0307022
 |bibcode=2004PhRvD..69k4005K
 |doi=10.1103/PhysRevD.69.114005
 |s2cid=119379329
}}</ref><ref name="Griffiths">
{{cite book
 |author=D. J. Griffiths
 |title=Introduction to Elementary Particles
 |url=https://archive.org/details/introductiontoel00grif_077
 |url-access=limited
 |page=[https://archive.org/details/introductiontoel00grif_077/page/n49 42]
 |publisher=[[John Wiley & Sons]]
 |year=1987
 |isbn=978-0-471-60386-3
}}</ref> The objects that were observed at SLAC would later be identified as up and down quarks as the other flavors were discovered.<ref>
{{cite book
 |author1=M. E. Peskin
 |author2=D. V. Schroeder
 |year=1995
 |title=An Introduction to Quantum Field Theory
 |url=https://archive.org/details/introductiontoqu0000pesk
 |url-access=registration
 |page=[https://archive.org/details/introductiontoqu0000pesk/page/556 556]
 |publisher=[[Addison–Wesley]]
 |isbn=978-0-201-50397-5
}}</ref> Nevertheless, "parton" remains in use as a collective term for the constituents of hadrons (quarks, antiquarks, and [[gluon]]s). [[Richard E. Taylor|Richard Taylor]], [[Henry Way Kendall|Henry Kendall]] and [[Jerome Isaac Friedman|Jerome Friedman]] received the 1990 Nobel Prize in physics for their work at SLAC.

[[Image:Charmed-dia-w.png|thumb|left|Photograph of the event that led to the discovery of the [[Charmed sigma baryon|{{SubatomicParticle|Charmed sigma++}} baryon]], at the [[Brookhaven National Laboratory]] in 1974|alt=Photo of bubble chamber tracks next to diagram of same tracks. A neutrino (unseen in photo) enters from below and collides with a proton, producing a negatively charged muon, three positively charged pions, and one negatively charged pion, as well as a neutral lambda baryon (unseen in photograph). The lambda baryon then decays into a proton and a negative pion, producing a "V" pattern.]]

The strange quark's existence was indirectly validated by SLAC's scattering experiments: not only was it a necessary component of Gell-Mann and Zweig's three-quark model, but it provided an explanation for the [[kaon]] ({{SubatomicParticle|Kaon}}) and [[pion]] ({{SubatomicParticle|Pion}}) hadrons discovered in cosmic rays in 1947.<ref>
{{cite book
 |author=V. V. Ezhela
 |year=1996
 |title=Particle Physics
 |page=2
 |publisher=[[Springer Science+Business Media|Springer]]
 |isbn=978-1-56396-642-2
}}</ref>

In a 1970 paper, Glashow, [[John Iliopoulos]] and [[Luciano Maiani]] presented the [[GIM mechanism]] (named from their initials) to explain the experimental non-observation of [[flavor-changing neutral current]]s. This theoretical model required the existence of the as-yet undiscovered [[charm quark]].<ref>
{{cite journal
 |author1=S. L. Glashow
 |author2=J. Iliopoulos
 |author3=L. Maiani
 |title=Weak Interactions with Lepton–Hadron Symmetry
 |journal=[[Physical Review D]]
 |volume=2 |issue=7 |pages=1285–1292
 |year=1970
 |bibcode=1970PhRvD...2.1285G
 |doi=10.1103/PhysRevD.2.1285
}}</ref><ref>
{{cite book
 |author=D. J. Griffiths
 |title=Introduction to Elementary Particles
 |url=https://archive.org/details/introductiontoel00grif_077
 |url-access=limited
 |page=[https://archive.org/details/introductiontoel00grif_077/page/n51 44]
 |publisher=[[John Wiley & Sons]]
 |year=1987
 |isbn=978-0-471-60386-3
}}</ref> The number of supposed quark flavors grew to the current six in 1973, when [[Makoto Kobayashi (physicist)|Makoto Kobayashi]] and [[Toshihide Maskawa]] noted that the experimental observation of [[CP violation]]<ref group=nb>CP violation is a phenomenon that causes weak interactions to behave differently when left and right are swapped ([[P symmetry]]) and particles are replaced with their corresponding antiparticles ([[C symmetry]]).</ref><ref name="KM">
{{cite journal
 |author1=M. Kobayashi 
 |author2=T. Maskawa 
 |title=CP-Violation in the Renormalizable Theory of Weak Interaction 
 |journal=[[Progress of Theoretical Physics]] 
 |volume=49 
 |issue=2 
 |pages=652–657 
 |year=1973 
 |bibcode=1973PThPh..49..652K 
 |doi=10.1143/PTP.49.652 
 |doi-access=free 
 |hdl=2433/66179 
 |hdl-access=free 
}}</ref> could be explained if there were another pair of quarks.

Charm quarks were produced almost simultaneously by two teams in November 1974 (see [[November Revolution (physics)|November Revolution]]) – one at SLAC under [[Burton Richter]], and one at [[Brookhaven National Laboratory]] under [[Samuel C. C. Ting|Samuel Ting]]. The charm quarks were observed [[bound state|bound]] with charm antiquarks in mesons. The two parties had assigned the discovered meson two different symbols, J and ψ; thus, it became formally known as the [[J/ψ meson|{{SubatomicParticle|J/Psi}} meson]]. The discovery finally convinced the physics community of the quark model's validity.<ref name="Griffiths"/>

In the following years a number of suggestions appeared for extending the quark model to six quarks. Of these, the 1975 paper by [[Haim Harari]]<ref name="Harari">
{{cite journal
 |author=H. Harari
 |year=1975
 |title=A New Quark Model for hadrons
 |journal=[[Physics Letters B]]
 |volume=57 |issue=3 |page=265
 |bibcode=1975PhLB...57..265H
 |doi=10.1016/0370-2693(75)90072-6
}}</ref> was the first to coin the terms ''[[top quark|top]]'' and ''[[bottom quark|bottom]]'' for the additional quarks.<ref name="StaleyTopBottomNames">
{{cite book
 |author=K. W. Staley
 |year=2004
 |title=The Evidence for the Top Quark
 |url=https://books.google.com/books?id=K7z2oUBzB_wC
 |pages=31–33
 |publisher=[[Cambridge University Press]]
 |isbn=978-0-521-82710-2
}}</ref>

In 1977, the bottom quark was observed by a team at [[Fermilab]] led by [[Leon M. Lederman|Leon Lederman]].<ref>
{{cite journal
 |author=S. W. Herb
 |display-authors=etal
 |year=1977
 |title=Observation of a Dimuon Resonance at 9.5&nbps;GeV in 400-GeV Proton–Nucleus Collisions
 |journal=[[Physical Review Letters]]
 |volume=39 |issue=5 |page=252
 |bibcode=1977PhRvL..39..252H
 |doi=10.1103/PhysRevLett.39.252
 |osti=1155396
}}</ref><ref>
{{cite book
 |author=M. Bartusiak
 |title=A Positron named Priscilla
 |page=[https://archive.org/details/positronnamedpri00marc/page/245 245]
 |publisher=[[National Academies Press]]
 |year=1994
 |isbn=978-0-309-04893-4
 |url=https://archive.org/details/positronnamedpri00marc/page/245
}}</ref> This was a strong indicator of the top quark's existence: without the top quark, the bottom quark would have been without a partner. It was not until 1995 that the top quark was finally observed, also by the [[Collider Detector at Fermilab|CDF]]<ref name=CDF-1995>
{{cite journal
 |author=F. Abe
 |display-authors=etal
 |collaboration=[[CDF Collaboration]]
 |year=1995
 |title=Observation of Top Quark Production in {{SubatomicParticle|Antiproton}}{{SubatomicParticle|Proton}} Collisions with the Collider Detector at Fermilab
 |journal=[[Physical Review Letters]]
 |volume=74 |issue=14 |pages=2626–2631
 |bibcode=1995PhRvL..74.2626A
 |doi=10.1103/PhysRevLett.74.2626
 |pmid=10057978
 |arxiv=hep-ex/9503002
 |s2cid=119451328
}}</ref> and [[DØ experiment|DØ]]<ref name="D0-1995">
{{cite journal
 |author=S. Abachi
 |display-authors=et al
 |collaboration=[[DØ Collaboration]]
 |year=1995
 |title=Observation of the Top Quark
 |journal=[[Physical Review Letters]]
 |volume=74 |issue=14 |pages=2632–2637
 |arxiv=hep-ex/9503003
 |doi=10.1103/PhysRevLett.74.2632
 |pmid=10057979
 |bibcode=1995PhRvL..74.2632A
 |s2cid=42826202
}}</ref> teams at Fermilab.<ref name="Carithers"/> It had a mass much larger than expected,<ref>
{{cite book
 |author=K. W. Staley
 |title=The Evidence for the Top Quark
 |page=144
 |publisher=[[Cambridge University Press]]
 |year=2004
 |isbn=978-0-521-82710-2
}}</ref> almost as large as that of a [[gold]] atom.<ref name="BNLTop">
{{cite web
 |title=New Precision Measurement of Top Quark Mass
 |url=http://www.bnl.gov/newsroom/news.php?a=1190
 |publisher=[[Brookhaven National Laboratory|Brookhaven National Laboratory News]]
 |year=2004
 |access-date=2013-11-03
 |archive-url=https://web.archive.org/web/20160305012525/https://www.bnl.gov/newsroom/news.php?a=1190
 |archive-date=5 March 2016
 }}</ref>
{{clear}}