Editing Quark (section)

== 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=[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
}}</ref><ref name="Veltman45">
{{cite book
 |author=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
 |author1=F. Wilczek
 |author2=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
 |author1=F. Wilczek
 |author2=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 [[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="Veltman295">
{{cite book
 |author=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
 |author=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
 |author=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 ===
<!-- 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 contain, along with the ''[[valence quark]]s'' ({{SubatomicParticle|valence quark}}) that contribute to their [[quantum number]]s, [[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
 |url=https://archive.org/details/learningaboutpar00stei_561
 |url-access=limited
 |page=[https://archive.org/details/learningaboutpar00stei_561/page/n136 130]
 |publisher=[[Springer Science+Business Media|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
 |author=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
 |author1=S. B. Rüester
 |author2=V. Werth
 |author3=M. Buballa
 |author4=I. A. Shovkovy
 |author5=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
 |author1=M. G. Alford
 |author2=K. Rajagopal
 |author3=T. Schaefer
 |author4=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 (physics)|plasma]] of freely moving quarks and gluons. 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 |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
 |author1=Z. Fodor
 |author2=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
 |author1=U. Heinz
 |author2=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<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=978-0-7503-0765-9
}}</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
 |author1=A. Sedrakian
 |author2=J. W. Clark
 |author3=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>

{{Portal|Physics}}