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== Classification == {{See also|Standard Model}} [[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 (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 known [[elementary particle]]s. 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=978-0-471-23973-4 |url=https://books.google.com/books?id=YgkfZgFdui8C }}</ref> Quarks are [[spin-1/2|spin-{{sfrac|1|2}}]] particles, which means 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), of which any number can be in the same state.<ref> {{cite book |author=K. A. 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 }}</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 ''{{slink|#Strong interaction and color charge}}'' below). The quarks that 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]] "[[#Sea quarks|sea]]" quarks, antiquarks, and [[gluon]]s, which do not influence its quantum numbers.<ref> {{cite book |author=B. Povh |author2=C. Scholz |author3=K. Rith |author4=F. Zetsche |title=Particles and Nuclei |page=98 |publisher=[[Springer Science+Business Media|Springer]] |year=2008 |isbn=978-3-540-79367-0 }}</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=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 |author=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 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}}), was conjectured from the beginnings of the quark model<ref name="PDGTetraquarks"> {{cite journal |author=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 |author=S.-K. Choi |collaboration=[[Belle experiment|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 |page=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 |author=R. Aaij |display-authors=etal. |collaboration=[[LHCb|LHCb collaboration]] |year=2014 |title=Observation of the Resonant Character of the Z(4430)<sup>−</sup> State |journal=[[Physical Review Letters]] |volume=112 |issue=22 |page=222002 |arxiv=1404.1903 |bibcode=2014PhRvL.112v2002A |doi=10.1103/PhysRevLett.112.222002 |pmid=24949760 |s2cid=904429 }}</ref><ref name="LHCbpenta"> {{cite journal |author=R. Aaij |display-authors=etal |collaboration=[[LHCb|LHCb collaboration]] |year=2015 |title=Observation of J/ψp Resonances Consistent with Pentaquark States in Λ{{su|p=0|b=b}}→J/ψK<sup>−</sup>p Decays |journal=[[Physical Review Letters]] |volume=115 |issue=7 |page=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 [[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 |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 |author=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 |s2cid=227119789 |hdl-access=free }}</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 |collaboration=[[ALEPH experiment|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 }}</ref><ref> {{cite journal |author=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 |author=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 [[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 |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 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.
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