Quark

{{short description|Elementary particle, fundamental constituent of matter}} {{about|the elementary particle and its antiparticle}} {{featured article}} {{use dmy dates|date=April 2019}} {{use American English|date=April 2019}} {{infobox particle | name = Quark | image = Quark structure proton.svg | image_size = 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 is composed of two up quarks, one down quark, and the gluons that mediate the forces "binding" them together. The color assignment of individual quarks is arbitrary, but all three colors must be present; red, blue and green are used as an analogy to the primary colors that together produce a white color. | num_types = 6 (up, down, strange, charm, bottom, and top) | composition = elementary particle | statistics = fermionic | group = | generation = 1st, 2nd, 3rd | interaction = strong, weak, electromagnetic, gravitation | particle = | antiparticle = antiquark ({{SubatomicParticle|Antiquark}}) | theorized = {{plainlist| * Murray Gell-Mann (1964) * George Zweig (1964)}} | discovered = SLAC ({{circa|1968}}) | symbol = {{SubatomicParticle|Quark}} | baryon_number = {{sfrac|1|3}} | mass = | decay_time = | decay_particle = | electric_charge = +{{sfrac|2|3}} ''e'', −{{sfrac|1|3}} ''e'' | color_charge = yes | spin = {{sfrac|1|2}} ''ħ'' | num_spin_states = }}

A '''quark''' ({{IPAc-en|'|k|w|ɔːr|k|,_|'|k|w|ɑːr|k|audio=LL-Q1860 (eng)-Naomi Persephone Amethyst (NaomiAmethyst)-quark.wav}}) is a type of elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei.<ref> {{cite encyclopedia |title=Quark (subatomic particle) |url=http://www.britannica.com/EBchecked/topic/486323/quark |encyclopedia=Encyclopædia Britannica |access-date=2008-06-29 }}</ref> All commonly observable matter is composed of up quarks, down quarks and electrons. Owing to a phenomenon known as ''color confinement'', quarks are never found in isolation; they can be found only within hadrons, which include baryons (such as protons and neutrons) and mesons, or in quark–gluon plasmas.<ref name="HyperphysicsConfinment"> {{cite web |last=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 |access-date=2008-06-29 }}</ref><ref name="HyperphysicsBagModel"> {{cite web |last=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 |access-date=2008-06-29 }}</ref><ref group="nb>There is also the theoretical possibility of more exotic phases of quark matter.</ref> For this reason, much of what is known about quarks has been drawn from observations of hadrons.

Quarks have various intrinsic properties, including electric charge, mass, color charge, and spin. They are the only elementary particles in the Standard Model of particle physics to experience all four fundamental interactions, 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.

There are six types, known as ''flavors'', of quarks: up, down, charm, strange, top, and bottom.<ref name="HyperphysicsQuark"> {{cite web |last=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 |access-date=2008-06-29 }}</ref> Up and down quarks have the lowest masses 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, bottom, and top quarks can only be produced in high energy collisions (such as those involving cosmic rays and in particle accelerators). 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 (such as the electric charge) have 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 |last1=B. Carithers |last2=P. Grannis |title=Discovery of the Top Quark |url=http://www.slac.stanford.edu/pubs/beamline/25/3/25-3-carithers.pdf |journal=Beam Line |volume=25 |issue=3 |pages=4–16 |year=1995 |access-date=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 Stanford Linear Accelerator Center in 1968.<ref name="Bloom"> {{cite journal |last=E. D. Bloom |display-authors=etal |title=High-Energy Inelastic ''e''–''p'' Scattering at 6° and 10° |journal=Physical Review Letters |volume=23 |issue=16 |pages=930–934 |year=1969 |bibcode=1969PhRvL..23..930B |doi=10.1103/PhysRevLett.23.930 |doi-access=free }}</ref><ref name="Breidenbach"> {{cite journal |last=M. Breidenbach |display-authors=etal |title=Observed Behavior of Highly Inelastic Electron–Proton Scattering |journal=Physical Review Letters |volume=23 |issue=16 |pages=935–939 |year=1969 |bibcode=1969PhRvL..23..935B |doi=10.1103/PhysRevLett.23.935 |osti=1444731 |s2cid=2575595 }}</ref> Accelerator program experiments have provided evidence for all six flavors. The top quark, first observed at Fermilab in 1995, was the last to be discovered.<ref name="Carithers"/>

== Classification == [[Image:Standard Model of Elementary Particles.svg|thumb|400px|Six of the particles in the Standard Model are quarks (shown in purple). Each of the first three columns forms a ''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 known elementary particles. This model contains six flavors of quarks ({{SubatomicParticle|quark}}), named up ({{SubatomicParticle|up quark}}), down ({{SubatomicParticle|down quark}}), strange ({{SubatomicParticle|strange quark}}), charm ({{SubatomicParticle|charm quark}}), bottom ({{SubatomicParticle|bottom quark}}), and top ({{SubatomicParticle|top quark}}).<ref name="HyperphysicsQuark"/> Antiparticles 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 charges have the opposite sign.<ref> {{cite book |last=Wong |first= S. S. M. |title=Introductory Nuclear Physics |edition=2nd |page=30 |publisher=Wiley Interscience |year=1998 |isbn=978-0-471-23973-4 |url=https://books.google.com/books?id=YgkfZgFdui8C }}</ref>

Quarks are spin-{{sfrac|1|2}} particles, which means they are fermions 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 bosons (particles with integer spin), of which any number can be in the same state.<ref> {{cite book |last=Peacock |title=The Quantum Revolution |url=https://archive.org/details/quantumrevolutio00peac |url-access=limited |page=[https://archive.org/details/quantumrevolutio00peac/page/n143 125] |publisher=Greenwood Publishing Group |year=2008 |isbn=978-0-313-33448-1 |first=K. A.}}</ref> Unlike leptons, 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 ''hadrons'' (see ''{{section link|#Strong interaction and color charge}}'' below).

The quarks that determine the quantum numbers of hadrons are called ''valence quarks''; apart from these, any hadron may contain an indefinite number of virtual "sea" quarks, antiquarks, and gluons, which do not influence its quantum numbers.<ref> {{cite book |last=B. Povh |last2=C. Scholz |last3=K. Rith |last4=F. Zetsche |title=Particles and Nuclei |page=98 |publisher=Springer |year=2008 |isbn=978-3-540-79367-0 }}</ref> There are two families of hadrons: baryons, with three valence quarks, and mesons, with a valence quark and an antiquark.<ref>Section 6.1. in {{cite book |last=P. C. W. Davies |title=The Forces of Nature |publisher=Cambridge University Press |year=1979 |isbn=978-0-521-22523-6 |url=https://archive.org/details/forcesofnature0000davi }}</ref> The most common baryons are the proton and the neutron, the building blocks of the atomic nucleus.<ref name="Knowing"> {{cite book |last=M. Munowitz |title=Knowing |url=https://archive.org/details/knowingnaturephy00mmun |url-access=limited |page=[https://archive.org/details/knowingnaturephy00mmun/page/n48 35] |publisher=Oxford University Press |year=2005 |isbn=978-0-19-516737-5 }}</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" hadrons with more valence quarks, such as tetraquarks ({{SubatomicParticle|quark}}{{SubatomicParticle|quark}}{{SubatomicParticle|antiquark}}{{SubatomicParticle|antiquark}}) and pentaquarks ({{SubatomicParticle|quark}}{{SubatomicParticle|quark}}{{SubatomicParticle|quark}}{{SubatomicParticle|quark}}{{SubatomicParticle|antiquark}}), was conjectured from the beginnings of the quark model<ref name="PDGTetraquarks"> {{cite journal |last=W.-M. Yao |collaboration=Particle Data Group |display-authors=etal |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 |arxiv=astro-ph/0601168 |bibcode=2006JPhG...33....1Y |doi=10.1088/0954-3899/33/1/001 |doi-access=free }}</ref> but not discovered until the early 21st century.<ref name="Belletetra"> {{cite journal |last=S.-K. Choi |collaboration=Belle Collaboration |display-authors=etal |year=2008 |title=Observation of a Resonance-like Structure in the {{Subatomic particle|Pion+-}}Ψ′ Mass Distribution in Exclusive B→K{{Subatomic particle|Pion+-}}Ψ′ decays |journal=Physical Review Letters |volume=100 |issue=14 |article-number=142001 |arxiv=0708.1790 |bibcode=2008PhRvL.100n2001C |doi=10.1103/PhysRevLett.100.142001 |pmid=18518023 |s2cid=119138620 }}</ref><ref name="Belletetrapress"> {{cite press release |year=2007 |title=Belle Discovers a New Type of Meson |url=http://www.kek.jp/intra-e/press/2007/BellePress11e.html |publisher=KEK |access-date=2009-06-20 |archive-url=https://web.archive.org/web/20090122213256/http://www.kek.jp/intra-e/press/2007/BellePress11e.html |archive-date=2009-01-22 }}</ref><ref name="LHCbtetra"> {{cite journal |last=R. Aaij |display-authors=etal. |collaboration=LHCb collaboration |year=2014 |title=Observation of the Resonant Character of the Z(4430)− State |journal=Physical Review Letters |volume=112 |issue=22 |article-number=222002 |arxiv=1404.1903 |bibcode=2014PhRvL.112v2002A |doi=10.1103/PhysRevLett.112.222002 |pmid=24949760 |s2cid=904429 }}</ref><ref name="LHCbpenta"> {{cite journal |last=R. Aaij |display-authors=etal |collaboration=LHCb collaboration |year=2015 |title=Observation of J/ψp Resonances Consistent with Pentaquark States in Λ{{su|p=0|b=b}}→J/ψK−p Decays |journal=Physical Review Letters |volume=115 |issue=7 |article-number=072001 |arxiv=1507.03414 |bibcode=2015PhRvL.115g2001A |doi=10.1103/PhysRevLett.115.072001 |pmid=26317714 |doi-access=free }}</ref>

Elementary fermions are grouped into three 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 |last=C. Amsler |collaboration=Particle Data Group |display-authors=etal |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 |bibcode=2008PhLB..667....1A |doi=10.1016/j.physletb.2008.07.018 |hdl=1854/LU-685594 |s2cid=227119789 |hdl-access=free }}</ref><ref> {{cite journal |last=C. Amsler |collaboration=Particle Data Group |display-authors=etal |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 |bibcode=2008PhLB..667....1A |doi=10.1016/j.physletb.2008.07.018 |hdl=1854/LU-685594 |hdl-access=free |s2cid=227119789 }}</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 {{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 |last=D. Decamp |collaboration=ALEPH Collaboration |display-authors=etal |title=Determination of the Number of Light Neutrino Species |url=https://cds.cern.ch/record/201511/files/198911031.pdf |journal=Physics Letters B |volume=231 |issue=4 |page=519 |year=1989 |bibcode=1989PhLB..231..519D |doi=10.1016/0370-2693(89)90704-1 |hdl=11384/1735 }}</ref><ref> {{cite journal |last=A. Fisher |title=Searching for the Beginning of Time: Cosmic Connection |url=https://books.google.com/books?id=eyPfgGGTfGgC&q=quarks+no+more+than+three+generations&pg=PA70 |journal=Popular Science |volume=238 |issue=4 |page=70 |year=1991 }}</ref><ref> {{cite book |last=J. D. Barrow |title=The Origin of the Universe |chapter=The Singularity and Other Problems |orig-date=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 decay into lower-generation particles by means of weak interactions. 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 rays), 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 accelerators.<ref> {{cite book |last=D. H. Perkins |title=Particle Astrophysics |url=https://archive.org/details/particleastrophy00perk |url-access=limited |page=[https://archive.org/details/particleastrophy00perk/page/n9 4] |publisher=Oxford University Press |year=2003 |isbn=978-0-19-850952-3 }}</ref>

Having electric charge, mass, color charge, and flavor, quarks are the only known elementary particles that engage in all four fundamental interactions 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 below for a more complete overview of the six quark flavors' properties.

== History == right|thumb|Murray Gell-Mann (2007) right|thumb|George Zweig (2015)

The quark model was independently proposed by physicists Murray Gell-Mann<ref name="Gell-Man1964"> {{cite journal |last=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 |last=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 |last=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''—or, in more technical terms, SU(3) flavor symmetry, streamlining its structure.<ref> {{cite book |last=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 |first=M.|author-link=Murray Gell-Mann}} Original: {{cite report |last=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 |first=M.|date=}}</ref> Physicist Yuval Ne'eman had independently developed a scheme similar to the Eightfold Way in the same year.<ref> {{cite book |last=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 }} Original {{cite journal |last=Y. Ne'eman |year=1961 |title=Derivation of Strong Interactions from Gauge Invariance |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 |last1=R. C. Olby |last2=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 hadrons, 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, down, and 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 |last=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 leptons, and implied a mass formula that correctly reproduced the masses of the known mesons.<ref> {{cite journal |last1=B. J. Bjorken |last2=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-like objects and was therefore not an elementary particle.<ref name="Bloom"/><ref name="Breidenbach"/><ref> {{cite web |last=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 "partons"—a term coined by Richard Feynman.<ref> {{cite journal |last=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 |first=R. P.|author-link=Richard Feynman}}</ref><ref> {{cite journal |last1=S. Kretzer |last2=H. L. Lai |last3=F. I. Olness |last4=W. K. Tung |title=CTEQ6 Parton Distributions with Heavy Quark Mass Effects |journal=Physical Review D |volume=69 |issue=11 |article-number=114005 |year=2004 |arxiv=hep-ph/0307022 |bibcode=2004PhRvD..69k4005K |doi=10.1103/PhysRevD.69.114005 |s2cid=119379329 }}</ref><ref name="Griffiths"> {{cite book |last=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 |last1=M. E. Peskin |last2=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 gluons). Richard Taylor, Henry Kendall and 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 {{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 |last=V. V. Ezhela |year=1996 |title=Particle Physics |page=2 |publisher=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 currents. This theoretical model required the existence of the as-yet undiscovered charm quark.<ref> {{cite journal |last1=S. L. Glashow |last2=J. Iliopoulos |last3=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 |last=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 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 |last1=M. Kobayashi |last2=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)—one at SLAC under Burton Richter, and one at Brookhaven National Laboratory under Samuel Ting. The charm quarks were observed 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 {{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 |last=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'' and ''bottom'' for the additional quarks.<ref name="StaleyTopBottomNames"> {{cite book |last=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 Lederman.<ref> {{cite journal |last=S. W. Herb |display-authors=etal |year=1977 |title=Observation of a Dimuon Resonance at 9.5 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 |last=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 CDF<ref name=CDF-1995> {{cite journal |last=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<ref name="D0-1995"> {{cite journal |last=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 |last=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 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}}

== 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 1939 book ''Finnegans Wake'':<ref> {{cite book |last=Joyce |title=Finnegans Wake |page=[https://archive.org/details/finneganswake00jame_0/page/383 383] |publisher=Penguin Books |year=1982 |orig-date=1939 |isbn=978-0-14-006286-1 |url=https://archive.org/details/finneganswake00jame_0/page/383 |first=J.|author-link=James Joyce}}</ref> {{blockquote|<poem> – Three quarks for Muster Mark! Sure he hasn't got much of a bark And sure any he has it's all beside the mark. </poem> }}

The word ''quark'' is an old English word meaning ''to croak''<ref> {{cite encyclopedia |title=The American Heritage Dictionary of the English Language |url=https://www.ahdictionary.com/word/search.html?q=quark |access-date=2020-10-02 }}</ref> and the above-quoted lines are about a bird choir mocking king Mark of Cornwall in the legend of Tristan and Iseult.<ref> {{cite book |last=L. Crispi |last2=S. Slote |title=How Joyce Wrote Finnegans Wake. A Chapter-by-Chapter Genetic Guide |publisher=University of Wisconsin Press |year=2007 |page=345 |isbn=978-0-299-21860-7 }}</ref> Especially in the German-speaking parts of the world there is a widespread legend, however, that Joyce had taken it from the word {{lang|de|Quark}},<ref> {{cite book |last=H. Fritzsch |title=Das absolut Unveränderliche. Die letzten Rätsel der Physik |year=2007 |publisher=Piper Verlag |isbn=978-3-492-24985-0 |page=99 }}</ref> a German word of Slavic origin which denotes a curd cheese,<ref> {{cite book |last=S. Pronk-Tiethoff |year=2013 |title=The Germanic loanwords in Proto-Slavic |url=https://books.google.com/books?id=0iWLAgAAQBAJ&pg=PA71 |publisher=Rodopi |page=71 |isbn=978-94-012-0984-7 }}</ref> but is also a colloquial term for "trivial nonsense".<ref> {{cite encyclopedia |title=What Does 'Quark' Have to Do with Finnegans Wake? |url=https://www.merriam-webster.com/words-at-play/quark |dictionary=Merriam-Webster |access-date=2018-01-17 }}</ref> In the legend it is said that he had heard it on a journey to Germany at a farmers' market in Freiburg.<ref> {{cite news |last=U. Schnabel |date=16 September 2020 |title=Quarks sind so real wie der Papst |newspaper=Die Zeit |access-date=2020-10-02 |url=https://www.zeit.de/2020/39/quarks-elementarteilchen-existenz-physik-zweifel }}</ref><ref> {{cite web |last=H. Beck |title=Alles Quark? Die Mythen der Physiker und James Joyce |url=https://www.literaturportal-bayern.de/text-debatte?task=lpbblog.default&id=1365 |work=Literaturportal Bayern |date=2 February 2017 |access-date=2020-10-02 }}</ref> Some authors, however, defend a possible German origin of Joyce's word ''quark''.<ref> {{cite web |last=G. E. P. Gillespie |title=Why Joyce Is and Is Not Responsible for the Quark in Contemporary Physics |url=http://www.siff.us.es/iberjoyce/wp-content/uploads/2013/11/POJ-3.pdf |work=Papers on Joyce 16 |access-date=2018-01-17 }}</ref> Gell-Mann went into further detail regarding the name of the quark in his 1994 book ''The Quark and the Jaguar'':<ref name="Murray"> {{cite book |last=Gell-Mann |title=The Quark and the Jaguar: Adventures in the Simple and the Complex |page=180 |publisher=Henry Holt and Co |year=1995 |isbn=978-0-8050-7253-2 |first=M.|author-link=Murray Gell-Mann}}</ref> {{blockquote|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 |last=J. Gleick |title=Genius: Richard Feynman and Modern Physics |page=390 |publisher=Little Brown and Company |year=1992 |isbn=978-0-316-90316-5 }}</ref>

The quark flavors were given their names for several reasons. The up and down quarks are named after the up and down components of isospin, which they carry.<ref name="sakurai"> {{cite book |last=J. J. Sakurai |editor=S. F. Tuan |title=Modern Quantum Mechanics |url=https://archive.org/details/modernquantummec00saku_488 |url-access=limited |page=[https://archive.org/details/modernquantummec00saku_488/page/n388 376] |edition=Revised |publisher=Addison–Wesley |year=1994 |isbn=978-0-201-53929-5 }}</ref> Strange quarks were given their name because they were discovered to be components of the strange particles 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 co-proposed the 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 |last=M. Riordan |title=The Hunting of the Quark: A True Story of Modern Physics |page=[https://archive.org/details/huntingofquarktr00mich/page/210 210] |publisher=Simon & Schuster |year=1987 |isbn=978-0-671-50466-3 |url=https://archive.org/details/huntingofquarktr00mich/page/210 }}</ref> The names "top" and "bottom", 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 |last=D. H. Perkins |title=Introduction to High Energy Physics |url=https://archive.org/details/introductiontohi00perk_790 |url-access=limited |page=[https://archive.org/details/introductiontohi00perk_790/page/n21 8] |publisher=Cambridge University Press |year=2000 |isbn=978-0-521-62196-0 }}</ref> Alternative names for top and bottom quarks are "truth" and "beauty",{{refn|group=nb|"Beauty" and "truth" are contrasted in the last lines of Keats' 1819 poem "Ode on a Grecian Urn" and may have been the origin of those names.<ref>{{cite book |url=https://archive.org/details/remnantsoffallre0000roln |url-access=registration |quote=quark keats truth beauty. |title=Remnants Of The Fall: Revelations Of Particle Secrets |last=W. B. Rolnick |page=[https://archive.org/details/remnantsoffallre0000roln/page/136 136] |publisher=World Scientific |year=2003 |isbn=978-981-238-060-9 |access-date=14 October 2018}}</ref><ref>{{cite book |url=https://books.google.com/books?id=sV1rbCXrcQ0C&q=%22quark%22+keats+truth+beauty&pg=PT191 |title=Higgs Force: Cosmic Symmetry Shattered |last=N. Mee |date=2012 |publisher=Quantum Wave Publishing |isbn=978-0-9572746-1-7 |access-date=14 October 2018}}</ref><ref>{{cite book |url=https://books.google.com/books?id=ipf5CwAAQBAJ&q=%22quark%22+keats+truth+beauty&pg=PT214 |title=May We Borrow Your Language?: How English Steals Words From All Over the World |last=P. Gooden |date=2016 |publisher=Head of Zeus |isbn=978-1-78497-798-6 |access-date=14 October 2018}}</ref>}} but these names have somewhat fallen out of use.<ref> {{cite book |last=F. Close |title=The New Cosmic Onion |page=133 |publisher=CRC Press |year=2006 |isbn=978-1-58488-798-0 }}</ref> While "truth" never did catch on, accelerator complexes devoted to massive production of bottom quarks are sometimes called "beauty factories".<ref> {{cite web |last=J. T. Volk |display-authors=etal |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 fractional electric charge values, either −{{sfrac|1|3}}, or +{{sfrac|2|3}} times the elementary charge (''e''), depending on flavor. Up, charm, and top quarks (collectively referred to as ''up-type quarks'') have a charge of +{{sfrac|2|3}} ''e''; down, strange, and bottom quarks (''down-type quarks'') have a charge of −{{sfrac|1|3}} ''e''. Antiquarks have the opposite charge to their corresponding quarks; up-type antiquarks have charges of −{{sfrac|2|3}} ''e'' and down-type antiquarks have charges of +{{sfrac|1|3}} ''e''. 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 |last=C. Quigg |chapter=Particles and the Standard Model |editor=G. Fraser |title=The New Physics for the Twenty-First Century |page=91 |publisher=Cambridge University Press |year=2006 |isbn=978-0-521-81600-7 }}</ref> For example, the hadron constituents of atomic nuclei, neutrons and protons, have charges of 0 ''e'' and +1 ''e'', 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 degree of freedom. It is sometimes visualized as the rotation of an object around its own axis (hence the name "spin"), though this notion is somewhat misguided at subatomic scales because elementary particles are believed to be point-like.<ref> {{cite web |title=The Standard Model of Particle Physics |url=https://h2g2.com/edited_entry/A666173 |publisher=BBC |year=2002 |access-date=2009-04-19 }}</ref>

Spin can be represented by a vector whose length is measured in units of the reduced Planck constant ''ħ'' (pronounced "h bar"). For quarks, a measurement of the spin vector component along any axis can only yield the values +{{sfrac|''ħ''|2}} or −{{sfrac|''ħ''|2}}; for this reason quarks are classified as spin-{{sfrac|1|2}} particles.<ref> {{cite book |last=F. Close |title=The New Cosmic Onion |pages=80–90 |publisher=CRC Press |year=2006 |isbn=978-1-58488-798-0 }}</ref> The component of spin along a given axis—by convention the ''z'' axis—is often denoted by an up arrow ↑ for the value +{{sfrac|1|2}} and a down arrow ↓ for the value −{{sfrac|1|2}}, placed after the symbol for flavor. For example, an up quark with a spin of +{{sfrac|1|2}} along the ''z'' axis is denoted by u↑.<ref> {{cite book |last=D. Lincoln |title=Understanding the Universe |url=https://archive.org/details/understandinguni0000linc |url-access=registration |page=[https://archive.org/details/understandinguni0000linc/page/116 116] |publisher=World Scientific |year=2004 |isbn=978-981-238-705-9 }}</ref>

=== Weak interaction === {{main|Weak interaction}} [[Image:Beta Negative Decay.svg|thumb|right|192px|upright|Feynman diagram of beta decay of a neutron into proton, electron, and electron antineutrino via a virtual {{SubatomicParticle|W boson-}} boson. Time is 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 interactions 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 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 {{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 |title=Weak Interactions |url=http://www2.slac.stanford.edu/vvc/theory/weakinteract.html |work=Virtual Visitor Center |publisher=Stanford Linear Accelerator Center |year=2008 |access-date=2008-09-28 |archive-date=23 November 2011 |archive-url=https://web.archive.org/web/20111123112925/http://www2.slac.stanford.edu/vvc/theory/weakinteract.html }}</ref>

{| style="margin:auto;" cellpadding="5%" |- |   {{SubatomicParticle|Neutron}}|| → ||   {{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 experiments involving neutrino detection. [[Image:Quark weak interactions.svg|thumb|271px|left|The 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 mathematical table, called the Cabibbo–Kobayashi–Maskawa matrix (CKM matrix). Enforcing unitarity, the approximate magnitudes of the entries of the CKM matrix are:<ref name="PDG2010"> {{cite journal |last=K. Nakamura |display-authors=etal |collaboration=Particle Data Group |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=Journal of Physics G |volume=37 |issue= 7A|article-number=075021 |bibcode=2010JPhG...37g5021N |doi=10.1088/0954-3899/37/7A/075021 |hdl=10481/34593 |doi-access=free }}</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''''ij'' 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 (among other variables) the decaying quark's mass, the masses of the decay products, and the corresponding element of the CKM matrix. This probability is directly proportional (but not equal) to the magnitude squared (|''V''''ij'' |2) 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 Pontecorvo–Maki–Nakagawa–Sakata matrix (PMNS matrix).<ref> {{cite journal |last1=Z. Maki |last2=M. Nakagawa |last3=S. Sakata |title=Remarks on the Unified Model of Elementary Particles |journal=Progress of Theoretical Physics |volume=28 |issue=5 |page=870 |year=1962 |bibcode=1962PThPh..28..870M |doi=10.1143/PTP.28.870 |doi-access=free }}</ref> Together, the CKM and PMNS matrices describe all flavor transformations, but the links between the two are not yet clear.<ref> {{cite journal |last1=B. C. Chauhan |last2=M. Picariello |last3=J. Pulido |last4=E. Torrente-Lujan |title=Quark–Lepton Complementarity, Neutrino and Standard Model Data Predict {{nowrap|1=θ{{su|p=PMNS|b=13}} = {{val|9|+1|-2|u=°}}}} |journal=European Physical Journal |volume=C50 |issue=3 |pages=573–578 |year=2007 |arxiv=hep-ph/0605032 |bibcode = 2007EPJC...50..573C |doi=10.1140/epjc/s10052-007-0212-z |doi-access=free|s2cid=118107624 }}</ref> {{clear}}

=== Strong interaction and color charge === {{see also|Color charge|Strong interaction}} 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. 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 |last=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 |access-date=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 carrying particles known as ''gluons''; this is discussed at length below. The theory that describes strong interactions is called quantum chromodynamics (QCD). A quark, which will have a single color value, can form a bound system with an antiquark carrying the corresponding anticolor. 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. This is analogous to the additive color model in basic optics. Similarly, the combination of three quarks, each with different color charges, or three antiquarks, each with different anticolor charges, will result in the same "white" color charge and the formation of a baryon or antibaryon.<ref> {{cite book |last=B. A. Schumm |title=Deep Down Things |pages=[https://archive.org/details/deepdownthingsbr00schu/page/131 131–132] |publisher=Johns Hopkins University Press |year=2004 |isbn=978-0-8018-7971-5 |url=https://archive.org/details/deepdownthingsbr00schu/page/131 }}</ref>

In modern particle physics, gauge symmetries—a kind of symmetry group—relate interactions between particles (see gauge theories). Color SU(3) (commonly abbreviated to SU(3)c) 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 |last1=M. E. Peskin |last2=D. V. Schroeder |title=An Introduction to Quantum Field Theory |url=https://archive.org/details/introductiontoqu0000pesk |url-access=registration |publisher=Addison–Wesley |year=1995 |isbn=978-0-201-50397-5 }}</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)c color transformations correspond to "rotations" in color space (which, mathematically speaking, is a complex space). Every quark flavor ''f'', each with subtypes ''f''B, ''f''G, ''f''R corresponding to the quark colors,<ref> {{cite book |last=V. Icke |title=The Force of Symmetry |url=https://archive.org/details/forceofsymmetry0000icke |url-access=registration |page=[https://archive.org/details/forceofsymmetry0000icke/page/216 216] |publisher=Cambridge University Press |year=1995 |isbn=978-0-521-45591-6 }}</ref> forms a triplet: a three-component quantum field that transforms under the fundamental representation of SU(3)c.<ref> {{cite book |last=M. Y. Han |title=A Story of Light |url=https://archive.org/details/storylightshorti00hanm_264 |url-access=limited |page=[https://archive.org/details/storylightshorti00hanm_264/page/n86 78] |publisher=World Scientific |year=2004 |isbn=978-981-256-034-6 }}</ref> The requirement that SU(3)c 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, it implies the existence of eight gluon types to act as its force carriers.<ref name="PeskinSchroeder"/><ref> {{cite encyclopedia |last=C. Sutton |title=Quantum Chromodynamics (physics) |url=http://www.britannica.com/EBchecked/topic/486191/quantum-chromodynamics#ref=ref892183 |encyclopedia=Encyclopædia Britannica Online |access-date=2009-05-12 }}</ref>

=== Mass === [[Image:Quark masses as balls.svg|thumb|Current quark masses for all six flavors in comparison, as balls of proportional volumes. Proton (gray) and electron (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 particle field surrounding the quark.<ref> {{cite book |last=A. Watson |title=The Quantum Quark |pages=285–286 |publisher=Cambridge University Press |year=2004 |isbn=978-0-521-82907-6 }}</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 {{val|938|ul=MeV/c2}}, of which the rest mass of its three valence quarks only contributes about {{val|9|u=MeV/c2}}; much of the remainder can be attributed to the field energy of the gluons<ref name=PDGQuarks/><ref> {{cite book |last1=W. Weise |last2=A. M. Green |title=Quarks and Nuclei |pages=65–66 |publisher=World Scientific |year=1984 |isbn=978-9971-966-61-4 }}</ref> (see chiral symmetry breaking). The Standard Model posits that elementary particles derive their masses from the Higgs mechanism, which is associated to the Higgs boson. It is hoped that further research into the reasons for the top quark's large mass of ~{{val|173|u=GeV/c2}}, almost the mass of a gold atom,<ref name=PDGQuarks/><ref> {{cite book |last=D. McMahon |title=Quantum Field Theory Demystified |url=https://archive.org/details/quantumfieldtheo00mcma_095 |url-access=limited |page=[https://archive.org/details/quantumfieldtheo00mcma_095/page/n35 17] |publisher=McGraw–Hill |year=2008 |isbn=978-0-07-154382-8 }}</ref> might reveal more about the origin of the mass of quarks and other elementary particles.<ref> {{cite book |last=S. G. Roth |title=Precision Electroweak Physics at Electron–Positron Colliders |page=VI |publisher=Springer |year=2007 |isbn=978-3-540-35164-1 }}</ref>

=== Size ===

In QCD, quarks are considered to be point-like entities, with no structure. As of 2014, experimental evidence indicates they have no structure greater than 10−4 times the size of a proton, i.e. less than {{val|e=-19|u=m}}.<ref>{{cite web| url = https://www.pbs.org/wgbh/nova/article/smaller-than-small/| title = Smaller than Small: Looking for Something New With the LHC by Don Lincoln ''PBS Nova'' blog 28 October 2014| website = PBS| date = 28 October 2014}}</ref>

=== Table of properties === {{see also|Flavour (particle physics)}} The following table summarizes the key properties of the six quarks. Flavor quantum numbers (isospin (''I''3), 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 +{{sfrac|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''3, ''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> {{cite journal |last1=K. A. Olive |display-authors=etal |collaboration=Particle Data Group |title=Review of Particle Physics |journal=Chinese Physics C |volume=38 |issue=9 |pages=1–708 |year=2014 |bibcode=2014ChPhC..38i0001O |doi=10.1088/1674-1137/38/9/090001 |pmid=10020536 |doi-access=free |arxiv=1412.1408 }}</ref> ! colspan="2" | Particle ! rowspan="2" | Mass* {{br}}{{bracket|{{val|ul=MeV/c2}}}} ! rowspan="2" width="50"| ''J'' {{br}}{{bracket|''ħ''}} ! rowspan="2" width="50"| ''B'' ! rowspan="2" width="50"| ''Q'' {{br}}{{bracket|''e''}} ! rowspan="2" width="50"| ''I''3 ! rowspan="2" width="50"| ''C'' ! rowspan="2" width="50"| ''S'' ! rowspan="2" width="50"| ''T'' ! rowspan="2" width="50"| ''B′'' ! colspan="2" | Antiparticle |- ! Name ! Symbol ! Name ! Symbol |- |colspan="13"|'''''First generation''''' |- | up | {{SubatomicParticle|Up quark}} | {{val|2.3|0.7}} ± 0.5 | {{sfrac|1|2}} | +{{sfrac|1|3}} | +{{sfrac|2|3}} | +{{sfrac|1|2}} | 0 | 0 | 0 | 0 | antiup | {{SubatomicParticle|Up antiquark}} |- | down | {{SubatomicParticle|Down quark}} | {{val|4.8|0.5}} ± 0.3 | {{sfrac|1|2}} | +{{sfrac|1|3}} | −{{sfrac|1|3}} | −{{sfrac|1|2}} | 0 | 0 | 0 | 0 | antidown | {{SubatomicParticle|Down antiquark}} |- |colspan="13"|'''''Second generation''''' |- | charm | {{SubatomicParticle|Charm quark}} | {{val|1275|25}} | {{sfrac|1|2}} | +{{sfrac|1|3}} | +{{sfrac|2|3}} | 0 | +1 | 0 | 0 | 0 | anticharm | {{SubatomicParticle|Charm antiquark}} |- | strange | {{SubatomicParticle|Strange quark}} | {{val|95|5}} | {{sfrac|1|2}} | +{{sfrac|1|3}} | −{{sfrac|1|3}} | 0 | 0 | −1 | 0 | 0 | antistrange | {{SubatomicParticle|Strange antiquark}} |- |colspan="13"|'''''Third generation''''' |- | top | {{SubatomicParticle|Top quark}} | {{val|173210|510}} ± 710 * | {{sfrac|1|2}} | +{{sfrac|1|3}} | +{{sfrac|2|3}} | 0 | 0 | 0 | +1 | 0 | antitop | {{SubatomicParticle|Top antiquark}} |- | bottom | {{SubatomicParticle|Bottom quark}} | {{val|4180|30}} | {{sfrac|1|2}} | +{{sfrac|1|3}} | −{{sfrac|1|3}} | 0 | 0 | 0 | 0 | −1 | antibottom | {{SubatomicParticle|Bottom antiquark}} |} {{center|1= ''J'': total angular momentum, ''B'': baryon number, ''Q'': electric charge, ''I''3: isospin, ''C'': charm, ''S'': strangeness, ''T'': topness, ''B''′: bottomness. * Notation such as {{val|173210|510}} ± 710, in the case of the top quark, denotes two types of measurement uncertainty: The first uncertainty is statistical in nature, and the second is systematic.}}

== Interacting quarks == {{see also|Color confinement|Gluon}}

As described by quantum chromodynamics, the strong interaction between quarks is mediated by gluons, massless vector gauge bosons. 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), gluons are constantly exchanged between quarks through a 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 |last=Feynman |title=QED: The Strange Theory of Light and Matter |edition=1st |pages=[https://archive.org/details/qedstrangetheory00feyn_822/page/n140 136]–137 |publisher=Princeton University Press |year=1985 |isbn=978-0-691-08388-9 |title-link=QED: The Strange Theory of Light and Matter |first=R. P.|author-link=Richard Feynman}}</ref><ref name="Veltman45"> {{cite book |last=M. Veltman |title=Facts and Mysteries in Elementary Particle Physics |url=https://archive.org/details/factsmysteriesin0000velt |url-access=registration |pages=[https://archive.org/details/factsmysteriesin0000velt/page/45 45–47] |publisher=World Scientific |year=2003 |isbn=978-981-238-149-1 }}</ref><ref> {{cite book |last1=F. Wilczek |last2=B. Devine |title=Fantastic Realities |url=https://archive.org/details/fantasticrealiti00wilc |url-access=limited |page=[https://archive.org/details/fantasticrealiti00wilc/page/n94 85] |publisher=World Scientific |year=2006 |isbn=978-981-256-649-2 }}</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 |last1=F. Wilczek |last2=B. Devine |title=Fantastic Realities |pages=400ff |publisher=World Scientific |year=2006 |isbn=978-981-256-649-2 }}</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 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="Veltman295"> {{cite book |last=M. Veltman |title=Facts and Mysteries in Elementary Particle Physics |url=https://archive.org/details/factsmysteriesin0000velt |url-access=registration |pages=[https://archive.org/details/factsmysteriesin0000velt/page/295 295–297] |publisher=World Scientific |year=2003 |isbn=978-981-238-149-1 }}</ref><ref> {{cite book |last=T. Yulsman |title=Origin |page=55 |publisher=CRC Press |year=2002 |isbn=978-0-7503-0765-9 }}</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="PDB-top-quark"> {{cite journal |last=P. A. Zyla |display-authors=et al. |collaboration=Particle Data Group |title=Top quark |journal=Progress of Theoretical and Experimental Physics |volume=2020 |date=2020 |pages=083C01 |url=http://pdg.lbl.gov/2020/reviews/rpp2020-rev-top-quark.pdf }}</ref>

=== Sea quarks ===  Hadrons contain, along with the ''valence quarks'' ({{SubatomicParticle|valence quark}}) that contribute to their quantum numbers, 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 |last=J. Steinberger |title=Learning about Particles |url=https://archive.org/details/learningaboutpar00stei_561 |url-access=limited |page=[https://archive.org/details/learningaboutpar00stei_561/page/n136 130] |publisher=Springer |year=2005 |isbn=978-3-540-21329-1 }}</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 |last=C.-Y. Wong |title=Introduction to High-energy Heavy-ion Collisions |page=149 |publisher=World Scientific |year=1994 |isbn=978-981-02-0263-7 }}</ref>

=== Other phases of quark matter === {{main|QCD matter}} [[File:QCDphasediagram.svg|upright=1.15|thumb|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 |last1=S. B. Rüester |last2=V. Werth |last3=M. Buballa |last4=I. A. Shovkovy |last5=D. H. Rischke |title=The Phase Diagram of Neutral Quark Natter: Self-consistent Treatment of Quark Masses |journal=Physical Review D |volume=72 |issue=3 |page=034003 |year=2005 |arxiv=hep-ph/0503184 |bibcode = 2005PhRvD..72c4004R |doi=10.1103/PhysRevD.72.034004 |s2cid=10487860 }}</ref><ref name=Alford> {{cite journal |last1=M. G. Alford |last2=K. Rajagopal |last3=T. Schaefer |last4=A. Schmitt |title=Color Superconductivity in Dense Quark Matter |journal=Reviews of Modern Physics |volume=80 |issue=4 |pages=1455–1515 |year=2008 |arxiv=0709.4635 |bibcode=2008RvMP...80.1455A |doi=10.1103/RevModPhys.80.1455 |s2cid=14117263 }}</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" out of bound states and propagate as thermalized "free" excitations in the larger medium. In the course of asymptotic freedom, the strong interaction becomes weaker at increasing temperatures. Eventually, color confinement would be effectively lost in an extremely hot plasma of freely moving quarks and gluons. This theoretical phase of matter is called quark–gluon plasma.<ref> {{cite journal |last=S. Mrowczynski |journal=Acta Physica Polonica B |title=Quark–Gluon Plasma |volume=29 |issue=12 |page=3711 |year=1998 |arxiv=nucl-th/9905005 |bibcode=1998AcPPB..29.3711M |bibcode-access=free }}</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. An estimate puts the needed temperature at {{val|1.90|0.02|e=12}} kelvin.<ref> {{cite journal |last1=Z. Fodor |last2=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 |issue=4 |page=50 |year=2004 |arxiv=hep-lat/0402006 |bibcode=2004JHEP...04..050F |doi=10.1088/1126-6708/2004/04/050 |doi-access=free }}</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 |last1=U. Heinz |last2=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 }}</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 |year = 2005 |title = RHIC Scientists Serve Up "Perfect" Liquid |url = https://www.bnl.gov/rhic/news2/news.asp?a=303&t=pr |access-date = 2009-05-22 |publisher = Brookhaven National Laboratory |archive-url = https://web.archive.org/web/20130415062818/http://www.bnl.gov/rhic/news2/news.asp?a=303&t=pr |archive-date = 2013-04-15 }}</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 10−6 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 |last=T. Yulsman |title=Origins: The Quest for Our Cosmic Roots |page=75 |publisher=CRC Press |year=2002 |isbn=978-0-7503-0765-9 }}</ref>

Given sufficiently high baryon densities and relatively low temperatures—possibly comparable to those found in neutron stars—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 pairs, thereby breaking the local SU(3)c symmetry. Because quark Cooper pairs harbor color charge, such a phase of quark matter would be color superconductive; that is, color charge would be able to pass through it with no resistance.<ref> {{cite book |last1=A. Sedrakian |last2=J. W. Clark |last3=M. G. Alford |title=Pairing in Fermionic Systems |url=https://archive.org/details/pairingfermionic00sedr |url-access=limited |pages=[https://archive.org/details/pairingfermionic00sedr/page/n12 2]–3 |publisher=World Scientific |year=2007 |isbn=978-981-256-907-3 }}</ref>

== See also == {{portal|Physics}} {{div col begin|colwidth=24em}} * Color–flavor locking * Koide formula * Nucleon magnetic moment * Preons * Quarkonium * Quark star * Quark–lepton complementarity {{div col end}}

== Explanatory notes == {{reflist|group="nb"}}

== References == {{reflist|30em}}

== Further reading == * {{cite journal |last1=A. Ali |last2=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 |s2cid=54062126 }} * {{cite web |last1=R. Bowley |last2=E. Copeland |title=Quarks |url=http://www.sixtysymbols.com/videos/quarks.htm |work=Sixty Symbols |publisher=Brady Haran for the University of Nottingham }} * {{cite book |last=D. J. Griffiths |author-link=David Griffiths (physicist) |title=Introduction to Elementary Particles |edition=2nd |publisher=Wiley–VCH |year=2008 |isbn=978-3-527-40601-2 }} * {{cite book |last=I. S. Hughes |author-link=Ian Simpson Hughes |title=Elementary Particles |edition=2nd |publisher=Cambridge University Press |year=1985 |isbn=978-0-521-26092-3 |url=https://archive.org/details/elementarypartic00hugh }} * {{cite book |last=R. Oerter |author-link=Robert Oerter |title=The Theory of Almost Everything: The Standard Model, the Unsung Triumph of Modern Physics |url=https://archive.org/details/theoryofalmostev0000oert |url-access=registration |publisher=Pi Press |year=2005 |isbn=978-0-13-236678-6 }} * {{cite book |last=A. Pickering |author-link=Andrew Pickering |title=Constructing Quarks: A Sociological History of Particle Physics |publisher=The University of Chicago Press |year=1984 |isbn=978-0-226-66799-7 }} * {{cite book |last=B. Povh |author-link=Bogdan Povh |title=Particles and Nuclei: An Introduction to the Physical Concepts |publisher=Springer-Verlag |year=1995 |isbn=978-0-387-59439-2 }} * {{cite book |last=M. Riordan |author-link=Michael Riordan (scientist) |title=The Hunting of the Quark: A True Story of Modern Physics |url=https://archive.org/details/huntingofquarktr00mich |url-access=registration |publisher=Simon & Schuster |year=1987 |isbn=978-0-671-64884-8 }} * {{cite book |last=B. A. Schumm |author-link=Bruce A. Schumm |title=Deep Down Things: The Breathtaking Beauty of Particle Physics |publisher=Johns Hopkins University Press |year=2004 |isbn=978-0-8018-7971-5 |url=https://archive.org/details/deepdownthingsbr00schu }}

== External links == {{commons}} {{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] – A description of CERN's experiment to count the families of quarks. * [https://bigthink.com/starts-with-a-bang/what-rules-the-proton-quarks-or-gluons/ Think Big website, Quarks and Gluons] * [https://bigthink.com/starts-with-a-bang/there-are-no-free-quarks/ Think Big website, Quarks 2019]

Category:Quarks Category:Elementary particles Category:Finnegans Wake