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{{About|the particle}}
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{{Infobox Particle
| name            = Quark
| image          = [[Image:Quark structure proton.svg|225px|alt=Three colored balls (symbolizing quarks) connected pairwise by springs (symbolizing gluons), all inside a gray circle (symbolizing a proton). The colors of the balls are red, green, and blue, to parallel each quark's color charge. The red and blue balls are labeled "u" (for "up" quark) and the green one is labeled "d" (for "down" quark).]]
| caption        = A proton, composed of two [[up quark]]s and one [[down quark]]. (The [[color charge|color]] assignment of individual quarks is not important, only that all three colors be present.)
| num_types      = 6 ([[up quark|up]], [[down quark|down]], [[strange quark|strange]], [[charm quark|charm]], [[bottom quark|bottom]], and [[top quark|top]])
| composition    = [[Elementary particle]]
| statistics      = [[Fermion]]ic
| group          =
| generation      = 1st, 2nd, 3rd
| interaction    = [[Electromagnetism]], [[Gravitation]], [[Strong interaction|Strong]], [[Weak interaction|Weak]]
| particle        =
| antiparticle    = Antiquark ({{SubatomicParticle|Antiquark}})
| theorized      = [[Murray Gell-Mann]] (1964) <br />[[George Zweig]] (1964)
| discovered      = [[SLAC National Accelerator Laboratory|SLAC]] (~1968)
| symbol          = {{SubatomicParticle|Quark}}
| baryon_number  = {{Frac|1|3}}
| mass            =
| decay_time      =
| decay_particle  =
| electric_charge = +{{Frac|2|3}}&nbsp;[[elementary charge|e]], −{{Frac|1|3}}&nbsp;e
| color_charge    = Yes
| spin            = {{Frac|1|2}}
| num_spin_states =
}}
 
A '''quark''' ({{IPAc-en|ˈ|k|w|ɔr|k}} or {{IPAc-en|ˈ|k|w|ɑr|k}}) is an [[elementary particle]] and a fundamental constituent of [[matter]]. Quarks combine to form [[composite particle]]s called [[hadron]]s, the most stable of which are [[proton]]s and [[neutron]]s, the components of [[atomic nucleus|atomic nuclei]].<ref>
{{cite web
|author=
|title=Quark (subatomic particle)
|url=http://www.britannica.com/EBchecked/topic/486323/quark
|work=[[Encyclopædia Britannica]]
|year=
|accessdate=2008-06-29
}}</ref> Due to a phenomenon known as ''[[color confinement]]'', quarks are never directly observed or found in isolation; they can be found only within [[hadron]]s, such as [[baryon]]s (of which protons and neutrons are examples), and [[meson]]s.<ref name="HyperphysicsConfinment">
{{cite web
|author=R. Nave
|title=Confinement of Quarks
|url=http://hyperphysics.phy-astr.gsu.edu/hbase/Particles/quark.html#c6
|work=[[HyperPhysics]]
|publisher=[[Georgia State University]], Department of Physics and Astronomy
|year=
|accessdate=2008-06-29
}}</ref><ref name="HyperphysicsBagModel">
{{cite web
|author=R. Nave
|title=Bag Model of Quark Confinement
|url=http://hyperphysics.phy-astr.gsu.edu/hbase/Particles/qbag.html#c1
|work=[[HyperPhysics]]
|publisher=[[Georgia State University]], Department of Physics and Astronomy
|year=
|accessdate=2008-06-29
}}</ref> For this reason, much of what is known about quarks has been drawn from observations of the hadrons themselves.
 
There are six types of quarks, known as ''[[flavour (particle physics)|flavors]]'': [[Up quark|up]], [[Down quark|down]], [[Strange quark|strange]], [[Charm quark|charm]], [[Bottom quark|bottom]], and [[Top quark|top]].<ref name="HyperphysicsQuark">
{{cite web
|author=R. Nave
|title=Quarks
|url=http://hyperphysics.phy-astr.gsu.edu/hbase/Particles/quark.html
|work=[[HyperPhysics]]
|publisher=[[Georgia State University]], Department of Physics and Astronomy
|year=
|accessdate=2008-06-29
}}</ref> Up and down quarks have the lowest [[mass]]es of all quarks. The heavier quarks rapidly change into up and down quarks through a process of [[particle decay]]: the transformation from a higher mass state to a lower mass state. Because of this, up and down quarks are generally stable and the most common in the [[universe]], whereas strange, charm, top, and bottom quarks can only be produced in [[high energy physics|high energy]] collisions (such as those involving [[cosmic ray]]s and in [[particle accelerator]]s).
 
Quarks have various intrinsic properties, including [[electric charge]], [[mass]], [[color charge]] and [[spin (physics)|spin]]. Quarks are the only elementary particles in the [[Standard Model]] of [[particle physics]] to experience all four [[fundamental interaction]]s, also known as ''fundamental forces'' ([[electromagnetism]], [[gravitation]], [[strong interaction]], and [[weak interaction]]), as well as the only known particles whose electric charges are not [[integer]] multiples of the [[elementary charge]]. For every quark flavor there is a corresponding type of [[antiparticle]], known as an ''antiquark'', that differs from the quark only in that some of its properties have [[additive inverse|equal magnitude but opposite sign]].
 
The [[quark model]] was independently proposed by physicists [[Murray Gell-Mann]] and [[George Zweig]] in 1964.<ref name="Carithers">
{{cite journal
|author=B. Carithers, P. Grannis
|title=Discovery of the Top Quark
|url=http://www.slac.stanford.edu/pubs/beamline/25/3/25-3-carithers.pdf |format=PDF
|journal=[[Beam Line (journal)|Beam Line]]
|volume=25 |issue=3 |pages=4–16
|publisher=[[SLAC]]
|year=1995
|accessdate=2008-09-23
}}</ref> Quarks were introduced as parts of an ordering scheme for hadrons, and there was little evidence for their physical existence until [[deep inelastic scattering]] experiments at the [[SLAC National Accelerator Laboratory|Stanford Linear Accelerator Center]] in 1968.<ref name="Bloom">
{{cite journal
|author=E.D. Bloom ''et al.''
|title=High-Energy Inelastic ''e''–''p'' Scattering at 6° and 10°
|journal=[[Physical Review Letters]]
|volume=23 |issue=16 |pages=930–934
|year=1969
|doi=10.1103/PhysRevLett.23.930
|bibcode = 1969PhRvL..23..930B }}</ref><ref name="Breidenbach">
{{cite journal
|author=M. Breidenbach ''et al.''
|title=Observed Behavior of Highly Inelastic Electron–Proton Scattering
|journal=[[Physical Review Letters]]
|volume=23 |issue=16 |pages=935–939
|year=1969
|doi=10.1103/PhysRevLett.23.935
|bibcode = 1969PhRvL..23..935B }}</ref> Accelerator experiments have provided evidence for all six flavors. The [[top quark]] was the last to be discovered at [[Fermilab]] in 1995.<ref name="Carithers"/>
 
== Classification ==
{{See also|Standard Model}}
[[Image:Standard Model of Elementary Particles.svg|thumb|300px|Six of the particles in the [[Standard Model]] are quarks (shown in purple). Each of the first three columns forms a ''[[generation (particle physics)|generation]]'' of matter.|alt=A four-by-four table of particles. Columns are three generations of matter (fermions) and one of forces (bosons). In the first three columns, two rows contain quarks and two leptons. The top two rows' columns contain up (u) and down (d) quarks, charm (c) and strange (s) quarks, top (t) and bottom (b) quarks, and photon (γ) and gluon (g), respectively. The bottom two rows' columns contain electron neutrino (ν sub e) and electron (e), muon neutrino (ν sub μ) and muon (μ), and tau neutrino (ν sub τ) and tau (τ), and Z sup 0 and W sup ± weak force. Mass, charge, and spin are listed for each particle.]]
The [[Standard Model]] is the theoretical framework describing all the currently known [[elementary particle]]s, as well as the [[Higgs boson]].<ref>
{{cite journal
|author=C. Amsler ''et al.'' ([[Particle Data Group]])
|title= Higgs Bosons: Theory and Searches
|url=http://pdg.lbl.gov/2009/reviews/rpp2009-rev-higgs-boson.pdf
|journal=[[Physics Letters B]]
|volume=667 |issue=1 |pages=1–1340
|year=2008
|doi=10.1016/j.physletb.2008.07.018
|bibcode = 2008PhLB..667....1P }}</ref> This model contains six [[flavour (particle physics)|flavors]] of quarks ({{SubatomicParticle|quark}}), named [[up quark|up]] ({{SubatomicParticle|up quark}}), [[down quark|down]] ({{SubatomicParticle|down quark}}), [[strange quark|strange]] ({{SubatomicParticle|strange quark}}), [[charm quark|charm]] ({{SubatomicParticle|charm quark}}), [[bottom quark|bottom]] ({{SubatomicParticle|bottom quark}}), and [[top quark|top]] ({{SubatomicParticle|top quark}}).<ref name="HyperphysicsQuark"/> [[Antiparticle]]s of quarks are called ''antiquarks'', and are denoted by a bar over the symbol for the corresponding quark, such as {{SubatomicParticle|Up antiquark}} for an up antiquark. As with [[antimatter]] in general, antiquarks have the same mass, [[mean lifetime]], and spin as their respective quarks, but the electric charge and other [[charge (physics)|charges]] have the opposite sign.<ref>
{{cite book
|author=S.S.M. Wong
|title=Introductory Nuclear Physics
|edition=2nd
|page=30
|publisher=[[Wiley Interscience]]
|year=1998
|isbn=0-471-23973-9
}}</ref>
 
Quarks are [[spin-½|spin-{{Frac|1|2}}]] particles, implying that they are [[fermion]]s according to the [[spin-statistics theorem]]. They are subject to the [[Pauli exclusion principle]], which states that no two identical fermions can simultaneously occupy the same [[quantum state]]. This is in contrast to [[boson]]s (particles with integer spin), any number of which can be in the same state.<ref>
{{cite book
|author=K.A. Peacock
|title=The Quantum Revolution
|page=125
|publisher=[[Greenwood Publishing Group]]
|year=2008
|isbn=0-313-33448-X
}}</ref> Unlike [[lepton]]s, quarks possess [[color charge]], which causes them to engage in the [[strong interaction]]. The resulting attraction between different quarks causes the formation of composite particles known as ''[[hadron]]s'' (see "[[#Strong interaction and color charge|Strong interaction and color charge]]" below).
 
The quarks which determine the [[quantum number]]s of hadrons are called ''valence quarks''; apart from these, any hadron may contain an indefinite number of [[virtual particle|virtual]] (or ''sea'') quarks, antiquarks, and [[gluon]]s which do not influence its quantum numbers.<ref>
{{cite book
|author=B. Povh, C. Scholz, K. Rith, F. Zetsche
|title=Particles and Nuclei
|page=98
|publisher=[[Springer Science+Business Media|Springer]]
|year=2008
|isbn=3-540-79367-4
}}</ref> There are two families of hadrons: [[baryon]]s, with three valence quarks, and [[meson]]s, with a valence quark and an antiquark.<ref>Section 6.1. in
{{cite book
|author=P.C.W. Davies
|title=The Forces of Nature
|publisher=[[Cambridge University Press]]
|year=1979
|isbn=0-521-22523-X
}}</ref> The most common baryons are the proton and the neutron, the building blocks of the [[atomic nucleus]].<ref name="Knowing">
{{cite book
|author=M. Munowitz
|title=Knowing
|page=35
|publisher=[[Oxford University Press]]
|year=2005
|isbn=0-19-516737-6
}}</ref> A great number of hadrons are known (see [[list of baryons]] and [[list of mesons]]), most of them differentiated by their quark content and the properties these constituent quarks confer. The existence of [[exotic hadron|"exotic" hadrons]] with more valence quarks, such as [[tetraquark]]s ({{SubatomicParticle|quark}}{{SubatomicParticle|quark}}{{SubatomicParticle|antiquark}}{{SubatomicParticle|antiquark}}) and [[pentaquark]]s ({{SubatomicParticle|quark}}{{SubatomicParticle|quark}}{{SubatomicParticle|quark}}{{SubatomicParticle|quark}}{{SubatomicParticle|antiquark}}), has been conjectured<ref name="PDGTetraquarks">
{{cite journal
|author=W.-M. Yao ''et al.'' ([[Particle Data Group]])
|title=Review of Particle Physics: Pentaquark Update
|url=http://pdg.lbl.gov/2006/reviews/theta_b152.pdf
|journal=[[Journal of Physics G]]
|volume=33 |issue=1 |pages=1–1232
|year=2006
|doi=10.1088/0954-3899/33/1/001
|arxiv = astro-ph/0601168 |bibcode = 2006JPhG...33....1Y }}</ref> but not proven.<ref group="nb">Several research groups claimed to have proven the existence of tetraquarks and pentaquarks in the early 2000s. While the status of tetraquarks is still under debate, all known pentaquark candidates have since been established as non-existent.</ref><ref name="PDGTetraquarks"/><ref>
{{cite journal
|author=C. Amsler ''et al.'' ([[Particle Data Group]])
|title=Review of Particle Physics: Pentaquarks
|url=http://pdg.lbl.gov/2008/reviews/pentaquarks_b801.pdf
|journal=[[Physics Letters B]]
|volume=667 |issue=1 |pages=1–1340
|year=2008
|doi=10.1016/j.physletb.2008.07.018
|bibcode = 2008PhLB..667....1P }} <br />{{cite journal
|author=C. Amsler ''et al.'' ([[Particle Data Group]])
|title=Review of Particle Physics: New Charmonium-Like States
|url=http://pdg.lbl.gov/2008/reviews/rpp2008-rev-new-charmonium-like-states.pdf
|journal=[[Physics Letters B]]
|volume=667 |issue=1 |pages=1–1340
|year=2008
|doi=10.1016/j.physletb.2008.07.018
|bibcode = 2008PhLB..667....1P }} <br />{{cite book
|author=E.V. Shuryak
|title=The QCD Vacuum, Hadrons and Superdense Matter
|page=59
|publisher=[[World Scientific]]
|year=2004
|isbn=981-238-574-6
}}</ref>
 
Elementary fermions are grouped into three [[generation (particle physics)|generations]], each comprising two leptons and two quarks. The first generation includes up and down quarks, the second strange and charm quarks, and the third bottom and top quarks. All searches for a fourth generation of quarks and other elementary fermions have failed,<ref>
{{cite journal
|author=C. Amsler ''et al.'' ([[Particle Data Group]])
|title=Review of Particle Physics: b′ (4th Generation) Quarks, Searches for
|url=http://pdg.lbl.gov/2008/listings/q008.pdf
|journal=[[Physics Letters B]]
|volume=667 |issue=1 |pages=1–1340
|year=2008
|doi=10.1016/j.physletb.2008.07.018
|bibcode = 2008PhLB..667....1P }} <br />
{{cite journal
|author=C. Amsler ''et al.'' ([[Particle Data Group]])
|title=Review of Particle Physics: t′ (4th Generation) Quarks, Searches for
|url=http://pdg.lbl.gov/2008/listings/q009.pdf
|journal=[[Physics Letters B]]
|volume=667 |issue=1 |pages=1–1340
|year=2008
|doi=10.1016/j.physletb.2008.07.018
|bibcode = 2008PhLB..667....1P }}</ref> and there is strong indirect evidence that no more than three generations exist.<ref group="nb">The main evidence is based on the [[resonance width]] of the [[W and Z bosons|{{SubatomicParticle|Z boson0}} boson]], which constrains the 4th generation neutrino to have a mass greater than ~{{val|45|u=GeV/c2}}. This would be highly contrasting with the other three generations' neutrinos, whose masses cannot exceed {{val|2|u=MeV/c2}}.</ref><ref>
{{cite journal
|author=D. Decamp
|title=Determination of the number of light neutrino species
|journal=[[Physics Letters B]]
|volume=231 |issue=4 |page=519
|year=1989
|doi=10.1016/0370-2693(89)90704-1
|bibcode = 1989PhLB..231..519D
|last2=Deschizeaux
|first2=B.
|last3=Lees
|first3=J.-P.
|last4=Minard
|first4=M.-N.
|last5=Crespo
|first5=J.M.
|last6=Delfino
|first6=M.
|last7=Fernandez
|first7=E.
|last8=Martinez
|first8=M.
|last9=Miquel
|first9=R.
|displayauthors=8 }}
  <br />
{{cite journal
|author=A. Fisher
|title=Searching for the Beginning of Time: Cosmic Connection
|url=http://books.google.com/?id=eyPfgGGTfGgC&pg=PA70&dq=quarks+no+more+than+three+generations
|journal=[[Popular Science]]
|volume=238 |issue=4 |page=70
|year=1991
|doi=
}} <br />
{{cite book
|author=J.D. Barrow
|title=The Origin of the Universe
|chapter=The Singularity and Other Problems
|pages=
|origyear=1994
|edition=Reprint
|year=1997
|publisher=[[Basic Books]]
|isbn=978-0-465-05314-8
}}</ref> Particles in higher generations generally have greater mass and less stability, causing them to [[particle decay|decay]] into lower-generation particles by means of [[weak interaction]]s. Only first-generation (up and down) quarks occur commonly in nature. Heavier quarks can only be created in high-energy collisions (such as in those involving [[cosmic ray]]s), and decay quickly; however, they are thought to have been present during the first fractions of a second after the [[Big Bang]], when the universe was in an extremely hot and dense phase (the [[quark epoch]]). Studies of heavier quarks are conducted in artificially created conditions, such as in [[particle accelerator]]s.<ref>
{{cite book
|author=D.H. Perkins
|title=Particle Astrophysics
|page=4
|publisher=[[Oxford University Press]]
|year=2003
|isbn=0-19-850952-9
}}</ref>
 
Having electric charge, mass, color charge, and flavor, quarks are the only known elementary particles that engage in all four [[fundamental interaction]]s of contemporary physics: electromagnetism, gravitation, strong interaction, and weak interaction.<ref name="Knowing" /> Gravitation is too weak to be relevant to individual particle interactions except at extremes of energy ([[Planck energy]]) and distance scales ([[Planck distance]]). However, since no successful [[quantum theory of gravity]] exists, gravitation is not described by the Standard Model.
 
See the [[#Table of properties|table of properties below]] for a more complete overview of the six quark flavors' properties.
 
== History ==
[[Image:Murray Gell-Mann.jpg|right|thumb|upright|[[Murray Gell-Mann]] at [[TED (conference)|TED]] in 2007. Gell-Mann and [[George Zweig]] proposed the quark model in 1964.|alt=Half-length portrait of a white-haired man in his seventies talking. A painting of Beethoven is in the background.]]
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
|doi=10.1016/S0031-9163(64)92001-3
|bibcode = 1964PhL.....8..214G }}</ref>
and [[George Zweig]]<ref name="Zweig1964a">
{{cite journal
|author=G. Zweig
|title=An SU(3) Model for Strong Interaction Symmetry and its Breaking
|journal=CERN Report No.8182/TH.401
|url=http://cds.cern.ch/record/352337/files/CERN-TH-401.pdf
|year=1964
}}</ref><ref name="Zweig1964b">
{{cite journal
|author=G. Zweig
|title=An SU(3) Model for Strong Interaction Symmetry and its Breaking: II
|journal=CERN Report No.8419/TH.412
|url=http://lib-www.lanl.gov/la-pubs/00323548.pdf
|year=1964
}}</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]].<ref>
{{cite book
|author=M. Gell-Mann
|year=2000 |origyear=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=0-7382-0299-1
}}<br /> Original: {{cite journal
|author=M. Gell-Mann
|year=1961
|title=The Eightfold Way: A theory of strong interaction symmetry
|work=[[Synchroton Laboratory]] Report CTSL-20
|publisher=[[California Institute of Technology]]
}}</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 |origyear=1964
|chapter=Derivation of strong interactions from gauge invariance
|editor=M. Gell-Mann, Y. Ne'eman
|title=The Eightfold Way
|publisher=[[Westview Press]]
|isbn=0-7382-0299-1
}}<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 |page=222
|doi=10.1016/0029-5582(61)90134-1
|bibcode = 1961NucPh..26..222N
|issue=2 }}</ref><ref>
{{cite book
|first=R.C. Olby, G.N. Cantor
|year=1996
|title=Companion to the History of Modern Science
|page=673
|publisher=[[Taylor & Francis]]
|isbn= 0-415-14578-3
}}</ref>
 
At the time of the quark theory's inception, the "[[particle zoo]]" included, amongst other particles, a multitude of [[hadron]]s. 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 an abstraction used to explain concepts that were not properly understood at the time.<ref>
{{cite book
|author=A. Pickering
|title=Constructing Quarks
|pages=114–125
|publisher=[[University of Chicago Press]]
|year=1984
|isbn=0-226-66799-5
}}</ref>
 
In less than a year, extensions to the Gell-Mann–Zweig model were proposed. [[Sheldon Lee 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
|author=B.J. Bjorken, S.L. Glashow
|title=Elementary Particles and SU(4)
|journal=[[Physics Letters]]
|volume=11 |issue=3 |pages=255–257
|year=1964
|doi=10.1016/0031-9163(64)90433-0
|bibcode = 1964PhL....11..255B }}</ref>
 
In 1968, [[deep inelastic scattering]] experiments at the [[Stanford Linear Accelerator Center]] (SLAC) 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=[[Hue University]]
|year=
|accessdate=2008-09-29
}}</ref> Physicists were reluctant to 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
|journal=[[Physical Review Letters]]
|volume=23 |issue=24 |pages=1415–1417
|year=1969
|doi=10.1103/PhysRevLett.23.1415
|bibcode = 1969PhRvL..23.1415F }}</ref><ref>
{{cite journal
|author=S. Kretzer ''et al.''
|title=CTEQ6 Parton Distributions with Heavy Quark Mass Effects
|journal=[[Physical Review D]]
|volume=69 |issue=11 |page=114005
|year=2004
|doi=10.1103/PhysRevD.69.114005
|arxiv = hep-ph/0307022 |bibcode = 2004PhRvD..69k4005K }}</ref><ref name="Griffiths">
{{cite book
|author=D.J. Griffiths
|title=Introduction to Elementary Particles
|page=42
|publisher=[[John Wiley & Sons]]
|year=1987
|isbn=0-471-60386-4
}}</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
|author=M.E. Peskin, D.V. Schroeder
|year=1995
|title=An introduction to quantum field theory
|page=556
|publisher=[[Addison–Wesley]]
|isbn=0-201-50397-2
}}</ref> Nevertheless, "parton" remains in use as a collective term for the constituents of hadrons (quarks, antiquarks, and [[gluon]]s).
 
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=1-56396-642-5
}}</ref>
 
In a 1970 paper, Glashow, [[John Iliopoulos]] and [[Luciano Maiani]] presented further reasoning for the existence of the as-yet undiscovered [[charm quark]].<ref>
{{cite journal
|author=S.L. Glashow, J. Iliopoulos, L. Maiani
|title=Weak Interactions with Lepton–Hadron Symmetry
|journal=[[Physical Review D]]
|volume=2 |issue=7 |pages=1285–1292
|year=1970
|doi=10.1103/PhysRevD.2.1285
|bibcode = 1970PhRvD...2.1285G }}</ref><ref>
{{cite book
|author=D.J. Griffiths
|title=Introduction to Elementary Particles
|page=44
|publisher=[[John Wiley & Sons]]
|year=1987
|isbn=0-471-60386-4
}}</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 which 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
|author=M. Kobayashi, T. Maskawa
|title=CP-Violation in the Renormalizable Theory of Weak Interaction
|url=http://ptp.ipap.jp/link?PTP/49/652/pdf
|journal=[[Progress of Theoretical Physics]]
|volume=49 |issue=2 |pages=652–657
|year=1973
|doi=10.1143/PTP.49.652
|bibcode = 1973PThPh..49..652K }}</ref> could be explained if there were another pair of quarks.
 
[[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.]]
 
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=57B |page=265
|doi=10.1016/0370-2693(75)90072-6
|bibcode = 1975PhLB...57..265H
|issue=3 }}</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=http://books.google.com/?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 ''et al''.
|year=1977
|title=Observation of a Dimuon Resonance at 9.5 GeV in 400-GeV Proton-Nucleus Collisions |journal=[[Physical Review Letters]]
|volume=39 |page=252
|doi=10.1103/PhysRevLett.39.252
|bibcode = 1977PhRvL..39..252H
|issue=5 }}</ref><ref>{{cite book
|author=M. Bartusiak
|title=A Positron named Priscilla
|page=245
|publisher=[[National Academies Press]]
|year=1994
|isbn=0-309-04893-1
}}</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. However, 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 ''et al''. ([[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 |pages=2626–2631
|doi=10.1103/PhysRevLett.74.2626
|pmid=10057978
|bibcode=1995PhRvL..74.2626A
|issue=14
}}</ref> and [[DØ experiment|DØ]]<ref name=D0-1995>
{{cite journal
|author=S. Abachi ''et al''. ([[DØ Collaboration]])
|year=1995
|title=Search for High Mass Top Quark Production in {{SubatomicParticle|Proton}}{{SubatomicParticle|Antiproton}} Collisions at {{radical|''s''}}&nbsp;=&nbsp;1.8&nbsp;TeV
|journal=[[Physical Review Letters]]
|volume=74 |pages=2422–2426
|doi=10.1103/PhysRevLett.74.2422
|bibcode=1995PhRvL..74.2422A
|issue=13
}}</ref> teams at Fermilab.<ref name="Carithers"/> It had a mass much greater than had been previously expected<ref>
{{cite book
|author=K.W. Staley
|title=The Evidence for the Top Quark
|url=http://books.google.com/?id=K7z2oUBzB_wC
|page=144
|publisher=[[Cambridge University Press]]
|year=2004
|isbn=0-521-82710-8
}}</ref> – almost as great as a [[gold]] atom.<ref name="BNLTop">
{{cite web
|author=
|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
|accessdate=2013-11-03
}}</ref>
{{clr}}
 
== Etymology ==
For some time, Gell-Mann was undecided on an actual spelling for the term he intended to coin, until he found the word ''quark'' in [[James Joyce]]'s book ''[[Finnegans Wake]]'':
{{quote|<poem>
Three quarks for Muster Mark!
Sure he has not got much of a bark
And sure any he has it's all beside the mark.
</poem>
|sign=James Joyce, ''Finnegans Wake''<ref>
{{cite book
|author=J. Joyce
|title=Finnegans Wake
|page=383
|publisher=[[Penguin Books]]
|year=1982
|origyear=1939
|isbn=0-14-006286-6
|lccn=59354
}}</ref><!-- If the novel is divided into chapters or stuff like that, especially if it's not the original edition, specifying the chapter (or the smallest division thereof) would be useful for readers having an edition with different page numbers. -->
}}
 
Gell-Mann went into further detail regarding the name of the quark in his book, ''[[The Quark and the Jaguar]]'':<ref name="Murray">
{{cite book
|author=M. Gell-Mann
|title=The Quark and the Jaguar: Adventures in the Simple and the Complex
|page=180
|publisher=[[Henry Holt and Co.|Henry Holt and Co]]
|year=1995
|isbn=978-0-8050-7253-2
}}</ref>
{{quote|In 1963, when I assigned the name "quark" to the fundamental constituents of the nucleon, I had the sound first, without the spelling, which could have been "kwork". Then, in one of my occasional perusals of ''Finnegans Wake'', by James Joyce, I came across the word "quark" in the phrase "Three quarks for Muster Mark". Since "quark" (meaning, for one thing, the cry of the gull) was clearly intended to rhyme with "Mark", as well as "bark" and other such words, I had to find an excuse to pronounce it as "kwork". But the book represents the dream of a publican named Humphrey Chimpden Earwicker. Words in the text are typically drawn from several sources at once, like the "[[portmanteau]]" words in "Through the Looking-Glass". From time to time, phrases occur in the book that are partially determined by calls for drinks at the bar. I argued, therefore, that perhaps one of the multiple sources of the cry "Three quarks for Muster Mark" might be "Three quarts for Mister Mark", in which case the pronunciation "kwork" would not be totally unjustified. In any case, the number three fitted perfectly the way quarks occur in nature.}}
 
Zweig preferred the name ''ace'' for the particle he had theorized, but Gell-Mann's terminology came to prominence once the quark model had been commonly accepted.<ref>
{{cite book
|author=J. Gleick
|title=Genius: Richard Feynman and modern physics
|page=390
|publisher=[[Little Brown and Company]]
|year=1992
|isbn=0-316-90316-7
}}</ref>
 
The quark flavors were given their names for a number of reasons. The up and down quarks are named after the up and down components of [[isospin]], which they carry.<ref name="sakurai">
{{cite book
|author=J.J. Sakurai
|editor=S.F Tuan
|title=Modern Quantum Mechanics
|page=376
|edition=Revised
|publisher=[[Addison–Wesley]]
|year=1994
|isbn=0-201-53929-2
}}</ref> Strange quarks were given their name because they were discovered to be components of the [[strange particle]]s discovered in cosmic rays years before the quark model was proposed; these particles were deemed "strange" because they had unusually long lifetimes.<ref name="DHPerkins" /> Glashow, who coproposed charm quark with Bjorken, is quoted as saying, "We called our construct the 'charmed quark', for we were fascinated and pleased by the symmetry it brought to the subnuclear world."<ref>
{{cite book
|author=M. Riordan
|title=The Hunting of the Quark: A True Story of Modern Physics
|page=210
|publisher=[[Simon & Schuster]]
|year=1987
|isbn=978-0-671-50466-3
}}</ref> The names "bottom" and "top", coined by Harari, were chosen because they are "logical partners for up and down quarks".<ref name="Harari"/><ref name="StaleyTopBottomNames"/><ref name="DHPerkins">
{{cite book
|author=D.H. Perkins
|title=Introduction to high energy physics
|page=8
|publisher=[[Cambridge University Press]]
|year=2000
|isbn=0-521-62196-8
}}</ref> In the past, bottom and top quarks were sometimes referred to as "beauty" and "truth" respectively, but these names have somewhat fallen out of use.<ref>
{{cite book
|author=F. Close
|title=The New Cosmic Onion
|page=133
|publisher=[[CRC Press]]
|year=2006
|isbn=1-58488-798-2
}}</ref> While "truth" never did catch on, accelerator complexes devoted to massive production of bottom quarks are sometimes called "[[B-factory|beauty factories]]".<ref>
{{cite journal
|author=J.T. Volk ''et al.''
|year=1987
|title=Letter of Intent for a Tevatron Beauty Factory
|url=http://lss.fnal.gov/archive/test-proposal/0000/fermilab-proposal-0783.pdf
|id=Fermilab Proposal #783
}}</ref>
 
== Properties ==
 
=== Electric charge ===
{{See also|Electric charge}}
Quarks have [[fraction (mathematics)|fractional]] electric charge values – either {{Frac|1|3}} or {{Frac|2|3}} times the [[elementary charge]], depending on flavor. Up, charm, and top quarks (collectively referred to as ''up-type quarks'') have a charge of +{{Frac|2|3}}, while down, strange, and bottom quarks (''down-type quarks'') have −{{Frac|1|3}}. Antiquarks have the opposite charge to their corresponding quarks; up-type antiquarks have charges of −{{Frac|2|3}} and down-type antiquarks have charges of +{{Frac|1|3}}. Since the electric charge of a [[hadron]] is the sum of the charges of the constituent quarks, all hadrons have integer charges: the combination of three quarks (baryons), three antiquarks (antibaryons), or a quark and an antiquark (mesons) always results in integer charges.<ref>
{{cite book
|author=G. Fraser
|title=The New Physics for the Twenty-First Century
|page=91
|publisher=[[Cambridge University Press]]
|year=2006
|isbn=0-521-81600-9
}}</ref> For example, the hadron constituents of atomic nuclei, neutrons and protons, have charges of 0 and +1 respectively; the neutron is composed of two down quarks and one up quark, and the proton of two up quarks and one down quark.<ref name="Knowing" />
 
=== Spin ===
{{See also|Spin (physics)}}
Spin is an intrinsic property of elementary particles, and its direction is an important [[Degrees of freedom (physics and chemistry)|degree of freedom]]. It is sometimes visualized as the rotation of an object around its own axis (hence the name "[[Wikt:spin|spin]]"), though this notion is somewhat misguided at subatomic scales because elementary particles are believed to be [[point particle|point-like]].<ref>
{{cite web
|author=
|title=The Standard Model of Particle Physics
|url=http://www.bbc.co.uk/dna/h2g2/A666173
|publisher=BBC
|year=2002
|accessdate=2009-04-19
}}</ref>
 
Spin can be represented by a [[euclidean vector|vector]] whose length is measured in units of the [[reduced Planck constant]] ''ħ'' (pronounced "h bar"). For quarks, a measurement of the spin vector [[vector projection|component]] along any axis can only yield the values +''ħ''/2 or −''ħ''/2; for this reason quarks are classified as [[spin-½|spin-{{Frac|1|2}}]] particles.<ref>
{{cite book
|author=F. Close
|title=The New Cosmic Onion
|pages=80–90
|publisher=[[CRC Press]]
|year=2006
|isbn=1-58488-798-2
}}</ref> The component of spin along a given axis – by convention the ''z'' axis – is often denoted by an up arrow ↑ for the value +{{Frac|1|2}} and down arrow ↓ for the value −{{Frac|1|2}}, placed after the symbol for flavor. For example, an up quark with a spin of +{{Frac|1|2}} along the ''z'' axis is denoted by u↑.<ref>
{{cite book
|author=D. Lincoln
|title=Understanding the Universe
|page=116
|publisher=[[World Scientific]]
|year=2004
|isbn=981-238-705-6
}}</ref>
 
=== Weak interaction ===
{{Main|Weak interaction}}
[[Image:Beta Negative Decay.svg|thumb|right|192px|upright|[[Feynman diagram]] of [[beta decay]] with time flowing upwards. The CKM matrix (discussed below) encodes the probability of this and other quark decays.|alt=A tree diagram consisting mostly of straight arrows. A down quark forks into an up quark and a wavy-arrow W[superscript minus] boson, the latter forking into an electron and reversed-arrow electron antineutrino.]]
A quark of one flavor can transform into a quark of another flavor only through the weak interaction, one of the four [[fundamental interaction]]s in particle physics. By absorbing or emitting a [[W boson]], any up-type quark (up, charm, and top quarks) can change into any down-type quark (down, strange, and bottom quarks) and vice versa. This flavor transformation mechanism causes the [[radioactive decay|radioactive]] process of [[beta decay]], in which a neutron ({{SubatomicParticle|neutron}}) "splits" into a proton ({{SubatomicParticle|proton}}), an [[electron]] ({{SubatomicParticle|electron}}) and an [[electron antineutrino]] ({{SubatomicParticle|electron antineutrino}}) (see picture). This occurs when one of the down quarks in the neutron ({{SubatomicParticle|up quark}}{{SubatomicParticle|down quark}}{{SubatomicParticle|down quark}}) decays into an up quark by emitting a [[virtual particle|virtual]] {{SubatomicParticle|W boson-}} boson, transforming the neutron into a proton ({{SubatomicParticle|up quark}}{{SubatomicParticle|up quark}}{{SubatomicParticle|down quark}}). The {{SubatomicParticle|W boson-}} boson then decays into an electron and an electron antineutrino.<ref name="SLAC">
{{cite web
|author=
|title=Weak Interactions
|url=http://www2.slac.stanford.edu/vvc/theory/weakinteract.html
|work=Virtual Visitor Center
|publisher=[[Stanford Linear Accelerator Center]]
|year=2008
|accessdate=2008-09-28
}}</ref>
 
{| style="margin:auto;" cellpadding="5%"
|-
| &nbsp; {{SubatomicParticle|Neutron}}|| → || &nbsp; {{SubatomicParticle|Proton}} ||+|| {{SubatomicParticle|electron}} ||+|| {{SubatomicParticle|electron antineutrino}} || (Beta decay, hadron notation)
|-
| {{SubatomicParticle|up quark}}{{SubatomicParticle|down quark}}{{SubatomicParticle|down quark}} || → || {{SubatomicParticle|up quark}}{{SubatomicParticle|up quark}}{{SubatomicParticle|down quark}} ||+|| {{SubatomicParticle|electron}} ||+|| {{SubatomicParticle|electron antineutrino}} || (Beta decay, quark notation)
|}
 
Both beta decay and the inverse process of ''[[inverse beta decay]]'' are routinely used in medical applications such as [[positron emission tomography]] (PET) and in high-energy experiments such as [[neutrino detector|neutrino detection]].
[[Image:Quark weak interactions.svg|thumb|271px|left|The [[coupling|strengths]] of the weak interactions between the six quarks. The "intensities" of the lines are determined by the elements of the [[CKM matrix]].|alt=Three balls "u", "c", and "t" noted "up-type quarks" stand above three balls "d", "s", "b" noted "down-type quark". The "u", "c", and "t" balls are vertically aligned with the "d", "s", and b" balls respectively. Colored lines connect the "up-type" and "down-type" quarks, with the darkness of the color indicating the strength of the weak interaction between the two; The lines "d" to "u", "c" to "s", and "t" to "b" are dark; The lines "c" to "d" and "s" to "u" are grayish; and the lines "b" to "u", "b" to "c", "t" to "d", and "t" to "s" are almost white.]]
 
While the process of flavor transformation is the same for all quarks, each quark has a preference to transform into the quark of its own generation. The relative tendencies of all flavor transformations are described by a [[matrix (mathematics)|mathematical table]], called the [[Cabibbo–Kobayashi–Maskawa matrix]] (CKM matrix). Enforcing [[Unitary operator|unitarity]], the approximate [[absolute value|magnitudes]] of the entries of the CKM matrix are:<ref name="PDG2010">
{{cite journal
|author=K. Nakamura ''et al.''
|year=2010
|title=Review of Particles Physics: The CKM Quark-Mixing Matrix
|url=http://pdg.lbl.gov/2010/reviews/rpp2010-rev-ckm-matrix.pdf
|journal=J. Phys. G
|volume=37 |issue=75021 |page=150
|doi=
}}</ref>
:<math alt="|V_ud| ≅ 0.974; |V_us| ≅ 0.225; |V_ub| ≅ 0.003; |V_cd| ≅ 0.225; |V_cs| ≅ 0.973; |V_cb| ≅ 0.041; |V_td| ≅ 0.009; |V_ts| ≅ 0.040; |V_tb| ≅ 0.999.">
\begin{bmatrix} |V_\mathrm {ud}| & |V_\mathrm {us}| & |V_\mathrm {ub}| \\ |V_\mathrm {cd}| & |V_\mathrm {cs}| & |V_\mathrm {cb}| \\ |V_\mathrm {td}| & |V_\mathrm {ts}| & |V_\mathrm {tb}| \end{bmatrix} \approx
\begin{bmatrix} 0.974 & 0.225 & 0.003 \\ 0.225 & 0.973 & 0.041 \\ 0.009 & 0.040 & 0.999 \end{bmatrix},</math>
where ''V''<sub>''ij''</sub> represents the tendency of a quark of flavor ''i'' to change into a quark of flavor ''j'' (or vice versa).<ref group="nb">The actual probability of decay of one quark to another is a complicated function of (amongst other variables) the decaying quark's mass, the masses of the [[decay product]]s, and the corresponding element of the CKM matrix. This probability is directly proportional (but not equal) to the magnitude squared (|''V''<sub>''ij''</sub>|<sup>2</sup>) of the corresponding CKM entry.</ref>
 
There exists an equivalent weak interaction matrix for leptons (right side of the W boson on the above beta decay diagram), called the [[PMNS matrix|Pontecorvo–Maki–Nakagawa–Sakata matrix]] (PMNS matrix).<ref>
{{cite journal
|author=Z. Maki, M. Nakagawa, S. Sakata
|title=Remarks on the Unified Model of Elementary Particles
|url=http://ptp.ipap.jp/link?PTP/28/870/pdf
|journal=[[Progress of Theoretical Physics]]
|volume=28 |issue=5 |page=870
|year=1962
|doi=10.1143/PTP.28.870
|bibcode = 1962PThPh..28..870M }}</ref> Together, the CKM and PMNS matrices describe all flavor transformations, but the links between the two are not yet clear.<ref>
{{cite journal
|author=B.C. Chauhan, M. Picariello, J. Pulido, E. Torrente-Lujan
|title=Quark–lepton complementarity, neutrino and standard model data predict {{PhysicsParticle|θ|TR=PMNS|BR=13}} = {{val|9|+1|-2|u=°}}<!-- See Section 2 -->
|journal=[[European Physical Journal]]
|volume=C50 |issue=3 |pages=573–578
|arxiv=hep-ph/0605032
|doi=10.1140/epjc/s10052-007-0212-z
|year=2007
|bibcode = 2007EPJC...50..573C }}</ref>
{{clr}}
 
===Strong interaction and color charge===
{{See also|Color charge|Strong interaction}}
[[Image:Hadron colors.svg|right|thumb|upright|All types of hadrons have zero total color charge.|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.]]
[[File:Strong force charges.svg|200px|left|thumb|The pattern of strong charges for the three colors of quark, three antiquarks, and eight gluons (with two of zero charge overlapping).]]
According to [[quantum chromodynamics|QCD]], quarks possess a property called ''[[color charge]]''. There are three types of color charge, arbitrarily labeled ''blue'', ''green'', and ''red''.<ref group="nb">Despite its name, color charge is not related to the color spectrum of visible light.</ref> Each of them is complemented by an anticolor – ''antiblue'', ''antigreen'', and ''antired''. Every quark carries a color, while every antiquark carries an anticolor.<ref>
{{cite web
|author=R. Nave
|title=The Color Force
|url=http://hyperphysics.phy-astr.gsu.edu/hbase/forces/color.html#c2
|work=[[HyperPhysics]]
|publisher=[[Georgia State University]], Department of Physics and Astronomy
|year=
|accessdate=2009-04-26
}}</ref>
 
The system of attraction and repulsion between quarks charged with different combinations of the three colors is called [[strong interaction]], which is mediated by [[force carrier|force carrying particles]] known as ''[[gluon]]s''; this is discussed at length below. The theory that describes strong interactions is called [[quantum chromodynamics]] (QCD). A quark charged with one color value can form a [[bound state|bound system]] with an antiquark carrying the corresponding anticolor; three (anti)quarks, one of each (anti)color, will similarly be bound together. The result of two attracting quarks will be color neutrality: a quark with color charge ''ξ'' plus an antiquark with color charge −''ξ'' will result in a color charge of 0 (or "white" color) and the formation of a meson. Analogous to the [[additive color]] model in basic [[optics]], the combination of three quarks or three antiquarks, each with different color charges, will result in the same "white" color charge and the formation of a baryon or antibaryon.<ref>
{{cite book
|author=B.A. Schumm
|title=Deep Down Things
|pages=131–132
|publisher=[[Johns Hopkins University Press]]
|year=2004
|isbn=0-8018-7971-X
|oclc=55229065
}}</ref>
 
In modern particle physics, [[gauge symmetry|gauge symmetries]] – a kind of [[symmetry group]] – relate interactions between particles (see [[gauge theories]]). Color [[SU(3)]] (commonly abbreviated to SU(3)<sub>c</sub>) is the gauge symmetry that relates the color charge in quarks and is the defining symmetry for quantum chromodynamics.<ref name="PeskinSchroeder">Part III of
{{cite book
|title=An Introduction to Quantum Field Theory
|author=M.E. Peskin, D.V. Schroeder
|publisher=[[Addison–Wesley]]
|year=1995
|isbn=0-201-50397-2
}}</ref> Just as the laws of physics are independent of which directions in space are designated ''x'', ''y'', and ''z'', and remain unchanged if the coordinate axes are rotated to a new orientation, the physics of quantum chromodynamics is independent of which directions in three-dimensional color space are identified as blue, red, and green. SU(3)<sub>c</sub> color transformations correspond to "rotations" in color space (which, mathematically speaking, is a [[complex space]]). Every quark flavor ''f'', each with subtypes ''f''<sub>B</sub>, ''f''<sub>G</sub>, ''f''<sub>R</sub> corresponding to the quark colors,<ref>
{{cite book
|author=V. Icke
|title=The force of symmetry
|page=216
|publisher=[[Cambridge University Press]]
|year=1995
|isbn=0-521-45591-X
}}</ref> forms a triplet: a three-component [[quantum field]] which transforms under the fundamental [[representation theory|representation]] of SU(3)<sub>c</sub>.<ref>
{{cite book
|author=M.Y. Han
|title=A story of light
|page=78
|publisher=[[World Scientific]]
|year=2004
|isbn=981-256-034-3
}}</ref> The requirement that SU(3)<sub>c</sub> should be local – that is, that its transformations be allowed to vary with space and time – determines the properties of the strong interaction, in particular the existence of [[Gluon#Eight gluon colors|eight gluon types]] to act as its force carriers.<ref name="PeskinSchroeder"/><ref>
{{cite web
|author=C. Sutton
|title=Quantum chromodynamics (physics)
|url=http://www.britannica.com/EBchecked/topic/486191/quantum-chromodynamics#ref=ref892183
|work=[[Encyclopædia Britannica Online]]
|accessdate=2009-05-12
}}</ref>
 
=== Mass ===
[[Image:Quark masses as balls.svg|thumb|Current quark masses for all six flavors in comparison, as [[w:ball (mathematics)|balls]] of proportional volumes. [[Proton]] and [[electron]]&nbsp;(red) are shown in bottom left corner for scale]]
{{See also|Invariant mass}}
Two terms are used in referring to a quark's mass: ''[[current quark]] mass'' refers to the mass of a quark by itself, while ''[[constituent quark]] mass'' refers to the current quark mass plus the mass of the [[gluon]] [[quantum field theory|particle field]] surrounding the quark.<ref>
{{cite book
|author=A. Watson
|title=The Quantum Quark
|pages=285–286
|publisher=[[Cambridge University Press]]
|year=2004
|isbn=0-521-82907-0
}}</ref> These masses typically have very different values. Most of a hadron's mass comes from the gluons that bind the constituent quarks together, rather than from the quarks themselves. While gluons are inherently massless, they possess energy – more specifically, [[quantum chromodynamics binding energy]] (QCBE) – and it is this that contributes so greatly to the overall mass of the hadron (see [[mass in special relativity]]). For example, a proton has a mass of approximately 938&nbsp;[[Electron volt#Mass|MeV/c<sup>2</sup>]], of which the rest mass of its three valence quarks only contributes about 11&nbsp;MeV/c<sup>2</sup>; much of the remainder can be attributed to the gluons' QCBE.<ref name=PDGQuarks/><ref>
{{cite book
|author=W. Weise, A.M. Green
|title=Quarks and Nuclei
|pages=65–66
|publisher=[[World Scientific]]
|year=1984
|isbn=9971-966-61-1
}}</ref>
 
The Standard Model posits that elementary particles derive their masses from the [[Higgs mechanism]], which is related to the [[Higgs boson]]. Physicists hope that further research into the reasons for the top quark's large mass of ~173&nbsp;GeV/c<sup>2</sup>, almost the mass of a gold atom,<ref name=PDGQuarks/><ref>
{{cite book
|author=D. McMahon
|title=Quantum Field Theory Demystified
|page=17
|publisher=[[McGraw–Hill]]
|year=2008
|isbn=0-07-154382-1
}}</ref> might reveal more about the origin of the mass of quarks and other elementary particles.<ref>
{{cite book
|author=S.G. Roth
|title=Precision electroweak physics at electron–positron colliders
|page=VI
|publisher=[[Springer Science+Business Media|Springer]]
|year=2007
|isbn=3-540-35164-7}}</ref>
 
=== Table of properties ===
{{See also|Flavor (particle physics)}}
The following table summarizes the key properties of the six quarks. [[Flavour quantum numbers|Flavor quantum numbers]] ([[isospin]] (''I''<sub>3</sub>), [[Charm (quantum number)|charm]] (''C''), [[strangeness]] (''S'', not to be confused with spin), [[topness]] (''T''), and [[bottomness]] (''B''′)) are assigned to certain quark flavors, and denote qualities of quark-based systems and hadrons. The [[baryon number]] (''B'') is +{{Frac|1|3}} for all quarks, as baryons are made of three quarks. For antiquarks, the electric charge (''Q'') and all flavor quantum numbers (''B'', ''I''<sub>3</sub>, ''C'', ''S'', ''T'', and ''B''′) are of opposite sign. Mass and [[total angular momentum]] (''J''; equal to spin for point particles) do not change sign for the antiquarks.
 
{| class="wikitable" style="margin: 0 auto; text-align:center"
|+'''Quark flavor properties'''<ref name=PDGQuarks>K. Nakamura ''et al.'' (Particle Data Group), JP G '''37''', 075021 (2010) and 2011 partial update for the 2012 edition (URL: http://pdg.lbl.gov)</ref>
! Name
! Symbol
! Mass ([[Electronvolt#As a unit of mass|MeV/''c''<sup>2</sup>]])<sup>*</sup>
!width="50"|''J''
!width="50"|''B''
!width="50"|''Q''
!width="50"|''I''<sub>3</sub>
!width="50"|''C''
!width="50"|''S''
!width="50"|''T''
!width="50"|''B′''
! Antiparticle
! Antiparticle symbol
|-
|colspan="13"|'''''First generation'''''
|-
| Up
| {{SubatomicParticle|Up quark}}
| 1.7 to 3.1
| {{Frac|1|2}}
| +{{Frac|1|3}}
| +{{Frac|2|3}}
| +{{Frac|1|2}}
| 0
| 0
| 0
| 0
| Antiup
| {{SubatomicParticle|Up antiquark}}
|-
| Down
| {{SubatomicParticle|Down quark}}
| 4.1 to 5.7
| {{Frac|1|2}}
| +{{Frac|1|3}}
| −{{Frac|1|3}}
| −{{Frac|1|2}}
| 0
| 0
| 0
| 0
| Antidown
| {{SubatomicParticle|Down antiquark}}
|-
|colspan="13"|'''''Second generation'''''
|-
| Charm
| {{SubatomicParticle|Charm quark}}
| {{val|1290|+50|-110}}
| {{Frac|1|2}}
| +{{Frac|1|3}}
| +{{Frac|2|3}}
| 0
| +1
| 0
| 0
| 0
| Anticharm
| {{SubatomicParticle|Charm antiquark}}
|-
| Strange
| {{SubatomicParticle|Strange quark}}
| {{val|100|+30|-20}}
| {{Frac|1|2}}
| +{{Frac|1|3}}
| −{{Frac|1|3}}
| 0
| 0
| −1
| 0
| 0
| Antistrange
| {{SubatomicParticle|Strange antiquark}}
|-
|colspan="13"|'''''Third generation'''''
|-
| Top
| {{SubatomicParticle|Top quark}}
| {{val|172900|600}}&nbsp;±&nbsp;900
| {{Frac|1|2}}
| +{{Frac|1|3}}
| +{{Frac|2|3}}
| 0
| 0
| 0
| +1
| 0
| Antitop
| {{SubatomicParticle|Top antiquark}}
|-
| Bottom
| {{SubatomicParticle|Bottom quark}}
| {{val|4190|+180|-60}}
| {{Frac|1|2}}
| +{{Frac|1|3}}
| −{{Frac|1|3}}
| 0
| 0
| 0
| 0
| −1
| Antibottom
| {{SubatomicParticle|Bottom antiquark}}
|}
<small><center>''J'' = [[total angular momentum]], ''B'' = [[baryon number]], ''Q'' = [[electric charge]], ''I''<sub>3</sub> = [[isospin]], ''C'' = [[Charm (quantum number)|charm]], ''S'' = [[strangeness]], ''T'' = [[topness]], ''B''′ = [[bottomness]]. <br />* Notation such as {{val|4190|+180|-60}} denotes [[measurement uncertainty]]. In the case of the top quark, the first uncertainty is [[statistical error|statistical]] in nature, and the second is [[systematic error|systematic]].</center></small>
 
==Interacting quarks==
{{See also|Color confinement|Gluon}}
 
As described by [[quantum chromodynamics]], the [[strong interaction]] between quarks is mediated by gluons, massless [[vector boson|vector]] [[gauge boson]]s. Each gluon carries one color charge and one anticolor charge. In the standard framework of particle interactions (part of a more general formulation known as [[Perturbation theory (quantum mechanics)|perturbation theory]]), gluons are constantly exchanged between quarks through a [[virtual particle|virtual]] emission and absorption process. When a gluon is transferred between quarks, a color change occurs in both; for example, if a red quark emits a red–antigreen gluon, it becomes green, and if a green quark absorbs a red–antigreen gluon, it becomes red. Therefore, while each quark's color constantly changes, their strong interaction is preserved.<ref>
{{cite book
|author=R.P. Feynman
|title=[[QED: The Strange Theory of Light and Matter]]
|edition=1st
|pages=136–137
|publisher=[[Princeton University Press]]
|year=1985
|isbn=0-691-08388-6
}}</ref><ref name="Veltman">
{{cite book
|author=M. Veltman
|title=Facts and Mysteries in Elementary Particle Physics
|pages=45–47
|publisher=[[World Scientific]]
|year=2003
|isbn=981-238-149-X
}}</ref><ref>
{{cite book
|author=F. Wilczek, B. Devine
|title=Fantastic Realities
|page=85
|publisher=[[World Scientific]]
|year=2006
|isbn=981-256-649-X
}}</ref>
 
Since gluons carry color charge, they themselves are able to emit and absorb other gluons. This causes ''[[asymptotic freedom]]'': as quarks come closer to each other, the chromodynamic binding force between them weakens.<ref>
{{cite book
|author=F. Wilczek, B. Devine
|title=Fantastic Realities
|pages=400ff
|publisher=[[World Scientific]]
|year=2006
|isbn=981-256-649-X
}}</ref> Conversely, as the distance between quarks increases, the binding force strengthens. The color field becomes stressed, much as an elastic band is stressed when stretched, and more gluons of appropriate color are spontaneously created to strengthen the field. Above a certain energy threshold, pairs of quarks and antiquarks [[pair creation|are created]]. These pairs bind with the quarks being separated, causing new hadrons to form. This phenomenon is known as ''[[color confinement]]'': quarks never appear in isolation.<ref name="Veltman">
{{cite book
|author=M. Veltman
|title=Facts and Mysteries in Elementary Particle Physics
|pages=295–297
|publisher=[[World Scientific]]
|year=2003
|isbn=981-238-149-X
}}</ref><ref>
{{cite book
|author=T. Yulsman
|title=Origin
|page=55
|publisher=[[CRC Press]]
|year=2002
|isbn=0-7503-0765-X
}}</ref> This process of [[hadronization]] occurs before quarks, formed in a high energy collision, are able to interact in any other way. The only exception is the top quark, which may decay before it hadronizes.<ref name=Garberson>
{{cite arXiv
|author=F. Garberson
|title=Top Quark Mass and Cross Section Results from the Tevatron
|year=2008
|class=hep-ex
|eprint=0808.0273
}}</ref>
 
=== Sea quarks ===
<!-- Referenced from redirects at Sea quark and elsewhere in this article - if you change this section heading you must change it in those places too.-->
Hadrons, along with the ''[[valence quark]]s'' ({{SubatomicParticle|valence quark}}) that contribute to their [[quantum number]]s, contain [[virtual particle|virtual]] quark–antiquark ({{SubatomicParticle|quark}}{{SubatomicParticle|antiquark}}) pairs known as ''sea quarks'' ({{SubatomicParticle|sea quark}}). Sea quarks form when a gluon of the hadron's color field splits; this process also works in reverse in that the [[annihilation]] of two sea quarks produces a gluon. The result is a constant flux of gluon splits and creations colloquially known as "the sea".<ref>
{{cite book
|author=J. Steinberger
|title=Learning about Particles
|page=130
|publisher=[[Springer Science+Business Media|Springer]]
|year=2005
|isbn=3-540-21329-5
}}</ref> Sea quarks are much less stable than their valence counterparts, and they typically annihilate each other within the interior of the hadron. Despite this, sea quarks can hadronize into baryonic or mesonic particles under certain circumstances.<ref>
{{cite book
|author=C.-Y. Wong
|title=Introduction to High-energy Heavy-ion Collisions
|page=149
|publisher=[[World Scientific]]
|year=1994
|isbn=981-02-0263-6
}}</ref>
 
=== Other phases of quark matter ===
{{Main|QCD matter}}
[[File:QCDphasediagram.svg|right|thumb|300px|A qualitative rendering of the [[phase diagram]] of quark matter. The precise details of the diagram are the subject of ongoing research.<ref name=Ruester>
{{cite journal
|author=S.B. Rüester, V. Werth, M. Buballa, I.A. Shovkovy, D.H. Rischke
|title=The phase diagram of neutral quark matter: Self-consistent treatment of quark masses
|journal=[[Physical Review D]]
|volume=72 |page=034003
|year=2005
|doi=10.1103/PhysRevD.72.034004
|arxiv=hep-ph/0503184
|bibcode = 2005PhRvD..72c4004R
|issue=3 }}</ref><ref name=Alford>
{{cite journal
|author=M.G. Alford, K. Rajagopal, T. Schaefer, A. Schmitt
|title=Color superconductivity in dense quark matter
|journal=[[Reviews of Modern Physics]]
|volume=80 |pages=1455–1515
|year=2008
|doi= 10.1103/RevModPhys.80.1455
|arxiv=0709.4635
|bibcode = 2008RvMP...80.1455A
|issue=4 }}</ref>|alt=Quark–gluon plasma exists at very high temperatures; the hadronic phase exists at lower temperatures and baryonic densities, in particular nuclear matter for relatively low temperatures and intermediate densities; color superconductivity exists at sufficiently low temperatures and high densities.]]
Under sufficiently extreme conditions, quarks may become deconfined and exist as free particles. In the course of [[asymptotic freedom]], the strong interaction becomes weaker at higher temperatures. Eventually, color confinement would be lost and an extremely hot [[plasma (physics)|plasma]] of freely moving quarks and gluons would be formed. This theoretical phase of matter is called [[quark–gluon plasma]].<ref>
{{cite journal
|author=S. Mrowczynski
|journal=[[Acta Physica Polonica B]]
|title=Quark–Gluon Plasma
|volume=29 | page=3711
|year=1998
|arxiv=nucl-th/9905005
|doi=
|bibcode = 1998AcPPB..29.3711M }}</ref> The exact conditions needed to give rise to this state are unknown and have been the subject of a great deal of speculation and experimentation. A recent estimate puts the needed temperature at {{val|1.90|0.02|e=12}} [[Kelvin]].<ref>
{{cite journal
|author = Z. Fodor, S.D. Katz
|title = Critical point of QCD at finite T and μ, lattice results for physical quark masses
|journal = [[Journal of High Energy Physics]]
|volume = 2004 |page=50
|year = 2004
|arxiv = hep-lat/0402006
|doi = 10.1088/1126-6708/2004/04/050
|bibcode = 2004JHEP...04..050F
|issue = 4 }}</ref> While a state of entirely free quarks and gluons has never been achieved (despite numerous attempts by [[CERN]] in the 1980s and 1990s),<ref>
{{cite arXiv
|author=U. Heinz, M. Jacob
|year=2000
|title=Evidence for a New State of Matter: An Assessment of the Results from the CERN Lead Beam Programme
|eprint=nucl-th/0002042
|class=nucl-th
}}</ref> recent experiments at the [[Relativistic Heavy Ion Collider]] have yielded evidence for liquid-like quark matter exhibiting "nearly perfect" [[fluid motion]].<ref name=RHIC >
{{cite web
|url=http://www.bnl.gov/bnlweb/pubaf/pr/PR_display.asp?prID=05-38
|title=RHIC Scientists Serve Up "Perfect" Liquid
|year=2005
|accessdate=2009-05-22
|publisher=[[Brookhaven National Laboratory|Brookhaven National Laboratory News]]
}}</ref>
 
The quark–gluon plasma would be characterized by a great increase in the number of heavier quark pairs in relation to the number of up and down quark pairs. It is believed that in the period prior to 10<sup>−6</sup> seconds after the [[Big Bang]] (the [[quark epoch]]), the universe was filled with quark–gluon plasma, as the temperature was too high for hadrons to be stable.<ref>
{{cite book
|author=T. Yulsman
|title=Origins: The Quest for Our Cosmic Roots
|page=75
|publisher=[[CRC Press]]
|year=2002
|isbn=0-7503-0765-X
}}</ref>
 
Given sufficiently high baryon densities and relatively low temperatures – possibly comparable to those found in [[neutron star]]s – quark matter is expected to degenerate into a [[Fermi liquid]] of weakly interacting quarks. This liquid would be characterized by a [[condensation]] of colored quark [[Cooper pair]]s, thereby [[spontaneous symmetry breaking|breaking the local SU(3)<sub>c</sub> symmetry]]. Because quark Cooper pairs harbor color charge, such a phase of quark matter would be [[color superconductivity|color superconductive]]; that is, color charge would be able to pass through it with no resistance.<ref>
{{cite book
|author=A. Sedrakian, J.W. Clark, M.G. Alford
|title=Pairing in fermionic systems
|pages=2–3
|publisher=[[World Scientific]]
|year=2007
|isbn=981-256-907-3
}}</ref>
 
== See also ==
{{Portal|Physics}}
{{Wikipedia books
|1=Quarks
|3=Particles of the Standard Model
|5=Hadronic Matter
}}
* [[Color–flavor locking]]
* [[Neutron magnetic moment]]
* [[Lepton]]s
* [[Preon]]s – Hypothetical particles which were once postulated to be subcomponents of quarks and leptons
* [[Quarkonium]] – Mesons made of a quark and antiquark of the same flavor
* [[Quark star]] – A hypothetical degenerate [[neutron star]] with extreme density
* [[Quark–lepton complementarity]] – Possible fundamental relation between quarks and leptons
 
== Notes ==
<references group="nb" />
 
==References==
{{Reflist|30em}}
 
== Further reading ==
* {{cite journal
|author=A. Ali, G. Kramer
|year=2011
|title=JETS and QCD: A historical review of the discovery of the quark and gluon jets and its impact on QCD
|journal=[[European Physical Journal H]]
|volume=36 |issue=2 |page=245
|arxiv =1012.2288
|bibcode=2011EPJH...36..245A
|doi=10.1140/epjh/e2011-10047-1
}}
* {{cite book
|author=D.J. Griffiths
|title=Introduction to Elementary Particles
|edition=2nd
|publisher=[[Wiley–VCH]]
|year=2008
|isbn=3-527-40601-8
|authorlink=David Griffiths (physicist)
}}
* {{cite book
|author=I.S. Hughes
|title=Elementary particles
|edition=2nd
|publisher=[[Cambridge University Press]]
|year=1985
|isbn=0-521-26092-2
|authorlink=Ian Simpson Hughes
}}
* {{cite book
|author=R. Oerter
|title=The Theory of Almost Everything: The Standard Model, the Unsung Triumph of Modern Physics
|publisher=[[Pi Press]]
|year=2005
|isbn=0-13-236678-9
|authorlink=Robert Oerter
}}
* {{cite book
|author=A. Pickering
|title=Constructing Quarks: A Sociological History of Particle Physics
|publisher=[[The University of Chicago Press]]
|year=1984
|isbn=0-226-66799-5
|authorlink=Andrew Pickering
}}
* {{cite book
|author=B. Povh
|title=Particles and Nuclei: An Introduction to the Physical Concepts
|publisher=[[Springer–Verlag]]
|year=1995
|isbn=0-387-59439-6
|authorlink=Bogdan Povh
}}
* {{cite book
|author=M. Riordan
|title=The Hunting of the Quark: A true story of modern physics
|publisher=[[Simon & Schuster]]
|year=1987
|isbn=0-671-64884-5
|authorlink=Michael Riordan (scientist)
}}
* {{cite book
|author=B.A. Schumm
|title=Deep Down Things: The Breathtaking Beauty of Particle Physics
|publisher=[[Johns Hopkins University Press]]
|year=2004
|isbn=0-8018-7971-X
|authorlink=Bruce A. Schumm
}}
 
==External links==
{{Commons|Quark}}
{{Wiktionary|quark}}
* [http://nobelprize.org/nobel_prizes/physics/laureates/1969/index.html 1969 Physics Nobel Prize lecture by Murray Gell-Mann]
* [http://nobelprize.org/nobel_prizes/physics/laureates/1976/richter-lecture.html 1976 Physics Nobel Prize lecture by Burton Richter]
* [http://nobelprize.org/nobel_prizes/physics/laureates/1976/ting-lecture.html 1976 Physics Nobel Prize lecture by Samuel C.C. Ting]
* [http://nobelprize.org/nobel_prizes/physics/laureates/2008/kobayashi-lecture.html 2008 Physics Nobel Prize lecture by Makoto Kobayashi]
* [http://nobelprize.org/nobel_prizes/physics/laureates/2008/maskawa-lecture.html 2008 Physics Nobel Prize lecture by Toshihide Maskawa]
* [http://books.nap.edu/openbook.php?isbn=0-309-04893-1&page=236 The Top Quark And The Higgs Particle by T.A. Heppenheimer]&nbsp;– A description of [[CERN]]'s experiment to count the families of quarks.
* {{cite web|last=Bowley|first=Roger|title=Quarks|url=http://www.sixtysymbols.com/videos/quarks.htm|work=Sixty Symbols|publisher=[[Brady Haran]] for the [[University of Nottingham]]|coauthors=Copeland, Ed}}
 
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{{Composition}}
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