View source for Quark–gluon plasma

{{short description|Phase of quantum chromodynamics (QCD)}}
[[File:PhasDiagQGP.png|thumb|QCD phase diagram. Adapted from original made by R.S. Bhalerao.<ref>{{cite encyclopedia |last=Bhalerao |first=Rajeev S. |editor-last1=Mulders|editor-first1=M. |editor-last2=Kawagoe |editor-first2=K.|encyclopedia=1st Asia-Europe-Pacific School of High-Energy Physics |title=Relativistic heavy-ion collisions|year=2014 |publisher=CERN |series=CERN Yellow Reports: School Proceedings |volume=CERN-2014-001; KEK-Proceedings-2013–8 |location=Geneva|isbn=9789290833994 |oclc=801745660 |doi=10.5170/CERN-2014-001 |pages=219–239|s2cid=119256218 }}</ref>]]
'''Quark–gluon plasma''' ('''QGP''' or '''quark soup''') is an interacting localized assembly of [[quark]]s and [[gluon]]s at [[Thermodynamic equilibrium#Local and global equilibrium|thermal (local kinetic)]] and (close to) chemical (abundance) equilibrium. The word ''plasma'' signals that free [[color charge]]s are allowed. In a 1987 summary, [[Léon Van Hove]] pointed out the equivalence of the three terms: quark gluon plasma, quark matter and a new state of matter.<ref>{{Cite book|last=Van Hove|first=Léon Charles Prudent|url=https://cds.cern.ch/record/183417|title=Theoretical prediction of a new state of matter, the "quark-gluon plasma" (also called "quark matter")|date=1987}}</ref> Since the temperature is above the [[Hagedorn temperature]]—and thus above the scale of light u,d-quark mass—the pressure exhibits the relativistic [[Stefan–Boltzmann law|Stefan–Boltzmann]] format governed by temperature to the fourth power (<math> T^{4}</math>) and many practically massless quark and gluon constituents. It can be said that QGP emerges to be the new phase of strongly interacting matter which manifests its physical properties in terms of nearly free dynamics of practically massless gluons and quarks. Both quarks and gluons must be present in conditions near chemical (yield) equilibrium with their [[colour charge]] ''open'' for a new state of matter to be referred to as QGP.

In the Big Bang theory, quark–gluon plasma filled the entire [[Universe]] before matter as we know it was created. Theories predicting the existence of quark–gluon plasma were developed in the late 1970s and early 1980s.<ref>{{Cite book|last=Satz|first=H.|url=https://books.google.com/books?id=OY8uAAAAIAAJ|title=Statistical Mechanics of Quarks and Hadrons: Proceedings of an International Symposium Held at the University of Bielefeld, F.R.G., August 24–31, 1980|date=1981|publisher=North-Holland|isbn=978-0-444-86227-3|language=en}}</ref> Discussions around heavy ion experimentation followed suit,<ref>{{Cite encyclopedia|encyclopedia=Report of the workshop on GeV/nucleon collisions of heavy ions: how and why, November 29--December 1, 1974, Bear Mountain, New York|title=Developments at CERN|last=Cocconi|first=G.|date=January 1974|language=en|pages=78|osti=4061527}}</ref><ref>{{Cite report|last=Webb|first=C.|year=1979|title=First workshop on ultra-relativistic nuclear collisions, LBL, May 21–24, 1979|series=LBL-8957|osti=5187301|language=en}}</ref><ref>{{Cite book|last1=Nakai|first1=Kōji|url=https://books.google.com/books?id=fPMWAQAAMAAJ|title=High-energy nuclear interactions and properties of dense nuclear matter: proceedings of the Hakone Seminar (Japan-U.S. Joint Seminar) held at Hakone, from July 7 to 11, 1980|last2=Goldhaber|first2=A. S.|last3=Shinkōkai|first3=Nihon Gakujutsu|last4=Foundation (U.S.)|first4=National Science|publisher=Hayashi-Kobo|year=1980|location=Tokyo|language=en}}</ref><ref>{{Cite book|last=Darmstadt)|first=Workshop on Future Relativistic Heavy Ion Experiments (1980|url=http://repository.gsi.de/record/64831/|title=Proceedings: GSI Darmstadt, October 7–10, 1980|publisher=GSI|year=1981|language=en}}</ref><ref>{{Cite book|title=5th High Energy Heavy Ion Study, May 18–22, 1981: proceedings|publisher=Lawrence Berkeley Laboratory, University of California.|year=1981|series=LBL-12652|osti=5161227|language=en}}</ref> and the first experiment proposals were put forward at [[CERN]]<ref>{{Cite book|url=https://cds.cern.ch/record/679458|title=Letter of intent: study of particle production and target fragmentation in central <math>^{20}</math>Ne on Pb reactions at 12 GeV per nucleon energy of the CERN PS external beam|date=1980|editor-last=CERN. Geneva. Proton Synchrotron and Synchrocyclotron Committee}}</ref><ref>{{Cite book|url=https://cds.cern.ch/record/725513|title=Study of relativistic nucleus-nucleus reactions induced by <math>^{16}</math>O beams of 9–13 GeV per nucleon at the CERN PS|publisher=CERN|year=1982|editor-last=CERN. Geneva. Proton Synchrotron and Synchrocyclotron Committee|location=Geneva}}</ref><ref>{{Cite book|last=Middelkoop|first=Willem Cornelis|url=https://cds.cern.ch/record/601487/|title=Remarks on the possible use of the SPS for <math>^{16}</math>0 ion beams|publisher=CERN|others=CERN. Geneva. SPS Experiments Committee|year=1982|location=Geneva}}</ref><ref>{{Cite book|url=https://cds.cern.ch/record/604345|title=Proposal to the SPSC: use of the <math>\Omega'</math> facility for p-, <math>\alpha</math>-, and <math>^{16}</math>0-uranium collisions (CERN-SPSC-83-54)|publisher=CERN|year=1983|editor-last=CERN. Geneva. SPS Experiments Committee|location=Geneva}}</ref><ref>{{Cite encyclopedia|encyclopedia=Workshop on SPS Fixed-target Physics in the Years 1984–1989, CERN, Geneva, Switzerland, 6 – 10 Dec 1982|editor-last=Mannelli|editor-first=Italo |last=Albrow|first=M. G.|title=Experiments with nuclear beams and targets|publisher=CERN|year=1983|location=Geneva|pages=462–476|doi=10.5170/CERN-1983-002-V-2.462|series=CERN-83-02|volume=2}}</ref><ref name=":3">{{Cite journal|last=Quercigh|first=E.|year=2012|title=Four heavy-ion experiments at the CERN-SPS: A trip down memory lane|url=http://www.actaphys.uj.edu.pl/vol43/abs/v43p0771|journal=Acta Physica Polonica B|language=en|volume=43|issue=4|pages=771|doi=10.5506/APhysPolB.43.771|s2cid=126317771 |issn=0587-4254|ref=3|doi-access=free}}</ref> and [[Brookhaven National Laboratory|BNL]]<ref>{{Cite journal|year=1984|title=Report of task force for relativistic heavy ion physics|journal=Nuclear Physics A|language=en|volume=418|pages=657–668|doi=10.1016/0375-9474(84)90584-0|bibcode=1984NuPhA.418..657.|url=https://digital.library.unt.edu/ark:/67531/metadc871114/}}</ref><ref>{{Cite book|last=Laboratory|first=Brookhaven National|url=https://books.google.com/books?id=jQ7yxQEACAAJ|title=Proposal for a 15A-GeV Heavy Ion Facility at Brookhaven|publisher=Brookhaven National Laboratory|year=1983|series=BNL 32250|language=en}}</ref> in the following years. Quark–gluon plasma<ref name=":0">{{Cite book|url=https://books.google.com/books?id=8AD3GDoVaMkC|title=Quark–gluon plasma: theoretical foundations|publisher=North-Holland|year=2003|isbn=978-0-444-51110-2|editor-last=Kapusta|editor-first=J. I.|location=Amsterdam|editor-last2=Müller|editor-first2=B.|editor2-link=Berndt Müller|editor-last3=Rafelski|editor-first3=Johann|editor3-link=Johann Rafelski|ref=0}}</ref><ref>{{Cite journal|last1=Jacob|first1=M.|last2=Tran Thanh Van|first2=J.|year=1982|title=Quark matter formation and heavy ion collisions|journal=Physics Reports|language=en|volume=88|issue=5|pages=321–413|doi=10.1016/0370-1573(82)90083-7|url=https://cds.cern.ch/record/138264}}</ref> was detected for the first time in the laboratory at CERN in the year 2000.<ref name=":2" /><ref>{{cite arXiv|last1=Heinz|first1=Ulrich|last2=Jacob|first2=Maurice|date=2000-02-16|title=Evidence for a New State of Matter: An Assessment of the Results from the CERN Lead Beam Programme|eprint=nucl-th/0002042}}</ref><ref>{{Cite news|last=Glanz|first=James|date=2000-02-10|title=Particle Physicists Getting Closer To the Bang That Started It All|language=en-US|work=The New York Times|url=https://www.nytimes.com/2000/02/10/world/particle-physicists-getting-closer-to-the-bang-that-started-it-all.html|access-date=2020-05-10|issn=0362-4331}}</ref>
[[File:CERNSPSExp.pdf|thumb|upright=2.5|Timeline of the CERN-SPS relativistic heavy ion program before QGP discovery.<ref name=":2">{{Cite journal|last=Rafelski|first=Johann|year=2015|title=Melting hadrons, boiling quarks|journal=The European Physical Journal A|language=en|volume=51|issue=9|pages=114|doi=10.1140/epja/i2015-15114-0|arxiv=1508.03260|bibcode=2015EPJA...51..114R|s2cid=119191818|issn=1434-6001}}</ref>]]

== General introduction ==

Quark–gluon plasma is a [[state of matter]] in which the elementary particles that make up the hadrons of [[baryon]]ic matter are freed of their [[Strong interaction|strong]] attraction for one another under extremely high [[Energy density|energy densities]].<ref>{{Cite journal |last=Braun-Munzinger |first=Peter |last2=Stachel |first2=Johanna |date=July 2007 |title=The quest for the quark–gluon plasma |url=https://www.nature.com/articles/nature06080 |journal=Nature |language=en |volume=448 |issue=7151 |pages=302–309 |doi=10.1038/nature06080 |issn=0028-0836}}</ref> These particles are the [[quark]]s and gluons that compose baryonic matter.<ref>{{cite web|title=Infocenter ILGTI: Indian Lattice Gauge Theory Initiative |url=http://theory.tifr.res.in/~sgupta/ilgti/infocenter/ |access-date=May 20, 2005 |url-status=dead |archive-url=https://web.archive.org/web/20050212185849/http://theory.tifr.res.in/~sgupta/ilgti/infocenter/ |archive-date=February 12, 2005 }}</ref> In normal matter quarks are ''[[Color confinement|confined]]''; in the QGP quarks are ''[[Deconfinement|deconfined]]''. In classical [[quantum chromodynamics]] (QCD), quarks are the [[fermion]]ic components of [[hadron]]s ([[meson]]s and baryons) while the [[gluon]]s are considered the [[boson]]ic components of such particles. The gluons are the force carriers, or bosons, of the QCD color force, while the quarks by themselves are their fermionic matter counterparts.

Quark–gluon plasma is studied to recreate and understand the high energy density conditions prevailing in the Universe when matter formed from elementary degrees of freedom (quarks, gluons) at about 20&nbsp;μs after the [[Big Bang]]. Experimental groups are probing over a 'large' distance the (de)confining [[quantum vacuum]] structure, which determines prevailing form of matter and laws of nature. The experiments give insight to the origin of matter and mass: the matter and antimatter is created when the quark–gluon plasma 'hadronizes' and the mass of matter originates in the confining vacuum structure.<ref name=":2" />

== How the quark–gluon plasma fits into the general scheme of physics ==

QCD is one part of the modern theory of [[particle physics]] called the [[Standard Model]]. Other parts of this theory deal with [[electroweak interaction]]s and [[neutrino]]s. The [[quantum electrodynamics|theory of electrodynamics]] has been tested and found correct to a few parts in a billion. The [[electroweak interaction|theory of weak interactions]] has been tested and found correct to a few parts in a thousand. Perturbative forms of QCD have been tested to a few percent.<ref>{{Cite journal|last1=Tanabashi|first1=M.|last2=Hagiwara|first2=K.|last3=Hikasa|first3=K.|last4=Nakamura|first4=K.|last5=Sumino|first5=Y.|last6=Takahashi|first6=F.|last7=Tanaka|first7=J.|last8=Agashe|first8=K.|last9=Aielli|first9=G.|last10=Amsler|first10=C.|last11=Antonelli|first11=M.|year=2018|title=Review of Particle Physics|url=http://pdg.lbl.gov/2019/reviews/rpp2019-rev-qcd.pdf|journal=Physical Review D|language=en|volume=98|issue=3|pages=1–708|doi=10.1103/PhysRevD.98.030001|pmid=10020536|bibcode=2018PhRvD..98c0001T|issn=2470-0010}}</ref> Perturbative models assume relatively small changes from the ground state, i.e. relatively low temperatures and densities, which simplifies calculations at the cost of generality. In contrast, non-perturbative forms of QCD have barely been tested. The study of the QGP, which has both a high temperature and density, is part of this effort to consolidate the grand theory of particle physics.

The study of the QGP is also a testing ground for [[Thermal quantum field theory|finite temperature field theory]], a branch of theoretical physics which seeks to understand particle physics under conditions of high temperature. Such studies are important to understand the early evolution of our universe: [[Timeline of the Big Bang#Very early universe|the first hundred microseconds]] or so. It is crucial to the physics goals of a new generation of observations of the universe ([[Wilkinson Microwave Anisotropy Probe|WMAP]] and its successors). It is also of relevance to [[grand unification theory|Grand Unification Theories]] which seek to unify the three fundamental forces of nature (excluding gravity).

=== Reasons for studying the formation of quark–gluon plasma ===
The generally accepted model of the formation of the [[Universe]] states that it happened as the result of the [[Big Bang]]. In this model, in the time interval of 10<sup>−10</sup>–10<sup>−6</sup> s after the Big Bang, matter existed in the form of a quark–gluon plasma. It is possible to reproduce the density and temperature of matter existing of that time in laboratory conditions to study the characteristics of the very early Universe. So far, the only possibility is the collision of two heavy [[Atomic nucleus|atomic nuclei]] accelerated to energies of more than a hundred GeV. Using the result of a head-on collision in the volume approximately equal to the volume of the atomic nucleus, it is possible to model the density and temperature that existed in the first instants of the life of the Universe.

=== Relation to normal plasma ===

A [[Plasma (physics)|plasma]] is matter in which [[Charge (physics)|charges]] are [[Debye length|screened]] due to the presence of other mobile charges.  For example: [[Coulomb's law|Coulomb's Law]] is suppressed by the screening to yield a distance-dependent charge,  <math>Q \rightarrow Q e^{-r/\alpha}</math>, i.e., the charge Q is reduced exponentially with the distance divided by a screening length α. In a QGP, the [[color charge]] of the quarks and gluons is screened. The QGP has other analogies with a normal plasma. There are also dissimilarities because the color charge is [[non-abelian group|non-abelian]], whereas the [[electric charge]] is abelian. Outside a finite volume of QGP the color-electric field is not screened, so that a volume of QGP must still be color-neutral. It will therefore, like a nucleus, have integer electric charge.

Because of the extremely high energies involved, quark-antiquark pairs are produced by [[pair production]] and thus QGP is a roughly equal mixture of quarks and antiquarks of various flavors, with only a slight excess of quarks. This property is not a general feature of conventional plasmas, which may be too cool for pair production (see however [[pair instability supernova]]).

=== Theory ===
One consequence of this difference is that the color charge is too large for [[Perturbation theory (quantum mechanics)|perturbative]] computations which are the mainstay of QED. As a result, the main theoretical tools to explore the theory of the QGP is [[lattice gauge theory]].<ref>{{Cite journal|last=Karsch|first=F.|year=1995|title=The phase transition to the quark gluon plasma: Recent results from lattice calculations|journal=Nuclear Physics A|language=en|volume=590|issue=1–2|pages=367–381|doi=10.1016/0375-9474(95)00248-Y|arxiv=hep-lat/9503010|bibcode=1995NuPhA.590..367K|s2cid=118967199}}</ref><ref>{{Cite journal |title=The Quark–Gluon Plasma |journal=Nuclear Physics A |volume=862–863 |issue=12 |pages=4–12 |doi=10.1016/j.nuclphysa.2011.05.014|year=2011 |arxiv=1101.3937 |last1=Satz |first1=Helmut |bibcode = 2011NuPhA.862....4S |s2cid=118369368 }}</ref> The transition temperature (approximately {{val|175|ul=MeV}}) was first predicted by lattice gauge theory. Since then lattice gauge theory has been used to predict many other properties of this kind of matter. The [[AdS/CFT correspondence]] conjecture may provide insights in QGP, moreover the ultimate goal of the fluid/gravity correspondence is to understand QGP. The QGP is believed to be a phase of QCD which is completely locally thermalized and thus suitable for an effective fluid dynamic description.

=== Production ===

Production of QGP in the laboratory is achieved by colliding heavy atomic nuclei (called heavy ions as in an accelerator atoms are ionized) at relativistic energy in which matter is heated well above the Hagedorn temperature ''T''<sub>H</sub> = 150 MeV per particle, which amounts to a temperature exceeding 1.66×10<sup>12</sup> [[Kelvin|K]]. This can be accomplished by colliding two large nuclei at high energy (note that {{val|175|u=MeV}} is not the energy of the colliding beam). [[Lead]] and [[gold]] [[Atomic nucleus|nuclei]] have been used for such collisions at [[CERN]] [[Super Proton Synchrotron|SPS]] and [[Brookhaven National Laboratory|BNL]] [[Relativistic Heavy Ion Collider|RHIC]], respectively. The nuclei are accelerated to [[Ultrarelativistic limit|ultrarelativistic speeds]] ([[Length contraction|contracting their length]]) and directed towards each other, creating a "fireball", in the rare event of a collision. Hydrodynamic simulation predicts this fireball will expand under its own [[pressure]], and cool while expanding. By carefully studying the spherical and [[elliptic flow]], experimentalists put the theory to test.

=== Diagnostic tools ===
There is overwhelming evidence for production of quark–gluon plasma in relativistic heavy ion collisions.<ref>{{Cite journal|last1=Busza|first1=Wit|last2=Rajagopal|first2=Krishna|last3=van der Schee|first3=Wilke|year=2018|title=Heavy ion collisions: The big picture and the big questions|journal=[[Annual Review of Nuclear and Particle Science]]|language=en|volume=68|issue=1|pages=339–376|doi=10.1146/annurev-nucl-101917-020852| doi-access=free|arxiv=1802.04801|bibcode=2018ARNPS..68..339B|s2cid=119264938|issn=0163-8998}}</ref><ref name=":5">{{Cite journal|last=ALICE Collaboration|year=2017|title=Enhanced production of multi-strange hadrons in high-multiplicity proton–proton collisions|url=http://www.nature.com/articles/nphys4111|journal=Nature Physics|language=en|volume=13|issue=6|pages=535–539|doi=10.1038/nphys4111|arxiv=1606.07424|bibcode=2017NatPh..13..535A|s2cid=221304738 |issn=1745-2473}}</ref><ref>{{Cite journal|last1=Koch|first1=Peter|last2=Müller|first2=Berndt|last3=Rafelski|first3=Johann|year=2017|title=From strangeness enhancement to quark–gluon plasma discovery|journal=International Journal of Modern Physics A|language=en|volume=32|issue=31|pages=1730024–272|doi=10.1142/S0217751X17300241|arxiv=1708.08115|bibcode=2017IJMPA..3230024K|s2cid=119421190|issn=0217-751X}}</ref><ref name=":1" /><ref name=":4">{{Cite journal|last=The WA97 Collaboration|year=2000|title=Transverse mass spectra of strange and multi–strange particles in Pb–Pb collisions at 158 A GeV/c|journal=The European Physical Journal C|language=en|volume=14|issue=4|pages=633–641|doi=10.1007/s100520000386|bibcode=2000EPJC...14..633W|s2cid=195312472|issn=1434-6044|url=https://cds.cern.ch/record/429846}}</ref>

The important classes of experimental observations are
* [[Strangeness production]]
* [[Elliptic flow]]
* [[Jet quenching]]
* [[J/ψ#J/ψ melting|J/ψ melting]]
* [[Hanbury Brown and Twiss effect]] and [[Bose–Einstein correlations]]
* [[single particle spectrum|Single particle spectra]] (thermal [[photons]] and [[thermal dilepton]]s)

== Expected properties ==

=== Thermodynamics ===
The cross-over temperature from the normal hadronic to the QGP phase is about {{val|156|u=MeV}}.<ref>A. Bazavov, H.-T. Ding, P. Hegde, O. Kaczmarek, F. Karsch, N. Karthik, E. Laermann, Anirban Lahiri, R. Larsen, S.-T. Li, Swagato Mukherjee, H. Ohno, P. Petreczky, H. Sandmeyer, C. Schmidt, S. Sharma, P. Steinbrecher,
Chiral crossover in QCD at zero and non-zero chemical potentials,
Physics Letters B,
Volume 795,
2019,
Pages 15–21,
ISSN 0370-2693,
https://doi.org/10.1016/j.physletb.2019.05.013.</ref> This "crossover" may actually ''not'' be only a qualitative feature, but instead one may have to do with a true (second order) [[phase transition]], e.g. of the universality class of the [[Ising model#Three and four dimensions|three-dimensional Ising model]]. The phenomena involved correspond to an energy density of a little less than {{val|1|u=[[Electronvolt|GeV]]/fm<sup>3</sup>}}. For [[Relativistic particle|relativistic]] matter, pressure and temperature are not independent variables, so the [[equation of state]] is a relation between the energy density and the pressure. This has been found through lattice computations, and compared to both [[perturbation theory]] and [[string theory]]. This is still a matter of active research. Response functions such as the [[Specific heat capacity|specific heat]] and various quark number susceptibilities are currently being computed.

=== Flow ===
The discovery of the perfect liquid was a turning point in physics. Experiments at RHIC have revealed a wealth of information about this remarkable substance, which we now know to be a QGP.<ref>{{Cite web|url=https://www.bnl.gov/newsroom/news.php?a=25756|title=Celebrating a Decade of Brewing Perfection|date=26 June 2015|website=Brookhaven National Laboratory|url-status=live|archive-url=https://web.archive.org/web/20170628150247/https://www.bnl.gov/newsroom/news.php?a=25756|archive-date=28 June 2017|access-date=2020-04-15|quote=Berndt Müller, Brookhaven Lab’s Associate Laboratory Director for Nuclear and Particle Physics.}}</ref> Nuclear matter at "room temperature" is known to behave like a [[Superfluidity|superfluid]]. When heated the nuclear fluid evaporates and turns into a dilute gas of nucleons and, upon further heating, a gas of baryons and mesons (hadrons). At the critical temperature, ''T''<sub>H</sub>, the hadrons melt and the gas turns back into a liquid.  RHIC experiments have shown that this is the most perfect liquid ever observed in any laboratory experiment at any scale. The new phase of matter, consisting of dissolved hadrons, exhibits less resistance to flow than any other known substance. The experiments at RHIC have, already in 2005, shown that the Universe at its beginning was uniformly filled with this type of material—a super-liquid—which once the Universe cooled below ''T''<sub>H</sub> evaporated into a gas of hadrons. Detailed measurements show that this liquid is a quark–gluon plasma where quarks, antiquarks and gluons flow independently.<ref>Letter from Berndt Müller to Johann Rafelski, reproduced in "Discovery of Quark–Gluon Plasma: Strangeness Diaries". The European Physical Journal Special Topics. 229 (1): pp.40–41 doi:[[doi:10.1140/epjst/e2019-900263-x|10.1140/epjst/e2019-900263-x]]. ISSN 1951-6401.</ref>

[[File:Schematic representation of the interaction region formed.png|thumb|upright=1.5|Schematic representation of the interaction region formed in the first moments after the collision of heavy ions with high energies in the accelerator.<ref name=":6" />]]
In short, a quark–gluon plasma flows like a splat of liquid, and because it is not "transparent" with respect to quarks, it can attenuate [[Jet (particle physics)|jets]] emitted by collisions. Furthermore, once formed, a ball of quark–gluon plasma, like any hot object, transfers heat internally by radiation. However, unlike in everyday objects, there is enough energy available so that gluons (particles mediating the [[strong force]]) collide and produce an excess of the heavy (i.e., [[Mass-energy equivalence|high-energy]]) [[strange quarks]]. Whereas, if the QGP did not exist and there was a pure collision, the same energy would be converted into a non-equilibrium mixture containing even heavier quarks such as [[charm quark]]s or [[bottom quark]]s.<ref name=":6">{{Cite journal|last=Ollitrault|first=Jean-Yves|year=1992|title=Anisotropy as a signature of transverse collective flow|journal=Physical Review D|language=en|volume=46|issue=1|pages=229–245|doi=10.1103/PhysRevD.46.229|pmid=10014754|bibcode=1992PhRvD..46..229O|issn=0556-2821}}</ref><ref>{{Cite journal|last1=Borghini|first1=Nicolas|last2=Dinh|first2=Phuong Mai|last3=Ollitrault|first3=Jean-Yves|year=2001|title=Flow analysis from multiparticle azimuthal correlations|journal=Physical Review C|language=en|volume=64|issue=5|pages=054901|doi=10.1103/PhysRevC.64.054901|arxiv=nucl-th/0105040|bibcode=2001PhRvC..64e4901B|s2cid=119069389|issn=0556-2813}}</ref>

The equation of state is an important input into the flow equations. The [[speed of sound]] (speed of QGP-density oscillations) is currently under investigation in lattice computations.<ref>{{Cite journal|last1=Borsányi|first1=Szabolcs|last2=Endrődi|first2=Gergely|last3=Fodor|first3=Zoltán|last4=Jakovác|first4=Antal|last5=Katz|first5=Sándor D.|last6=Krieg|first6=Stefan|last7=Ratti|first7=Claudia|last8=Szabó|first8=Kálmán K.|year=2010|title=The QCD equation of state with dynamical quarks|journal=Journal of High Energy Physics|language=en|volume=2010|issue=11|pages=77|doi=10.1007/JHEP11(2010)077|arxiv=1007.2580|bibcode=2010JHEP...11..077B|s2cid=55793321|issn=1029-8479}}</ref><ref>{{Cite journal|last1=Bazavov|first1=A.|last2=Bhattacharya|first2=Tanmoy|last3=DeTar|first3=C.|last4=Ding|first4=H.-T.|last5=Gottlieb|first5=Steven|last6=Gupta|first6=Rajan|last7=Hegde|first7=P.|last8=Heller|first8=U. M.|last9=Karsch|first9=F.|last10=Laermann|first10=E.|last11=Levkova|first11=L.|year=2014|title=Equation of state in ( 2 + 1 )-flavor QCD|journal=Physical Review D|language=en|volume=90|issue=9|pages=094503|doi=10.1103/PhysRevD.90.094503|arxiv=1407.6387|bibcode=2014PhRvD..90i4503B|s2cid=116984453|issn=1550-7998}}</ref><ref>{{Cite journal|last1=Borsanyi|first1=S.|last2=Fodor|first2=Z.|last3=Guenther|first3=J.|last4=Kampert|first4=K.-H.|last5=Katz|first5=S. D.|last6=Kawanai|first6=T.|last7=Kovacs|first7=T. G.|last8=Mages|first8=S. W.|last9=Pasztor|first9=A.|last10=Pittler|first10=F.|last11=Redondo|first11=J.|year=2016|title=Calculation of the axion mass based on high-temperature lattice quantum chromodynamics|url=http://www.nature.com/articles/nature20115|journal=Nature|language=en|volume=539|issue=7627|pages=69–71|doi=10.1038/nature20115|pmid=27808190|bibcode=2016Natur.539...69B|s2cid=2943966|issn=0028-0836}}</ref> The [[mean free path]] of quarks and gluons has been computed using [[perturbation theory]] as well as [[string theory]]. [[Lattice gauge theory|Lattice computations]] have been slower here, although the first computations of [[viscosity|transport coefficients]] have been concluded.<ref>{{Cite journal|last1=Hirano|first1=Tetsufumi|last2=Gyulassy|first2=Miklos|year=2006|title=Perfect fluidity of the quark–gluon plasma core as seen through its dissipative hadronic corona|journal=Nuclear Physics A|language=en|volume=769|pages=71–94|doi=10.1016/j.nuclphysa.2006.02.005|arxiv=nucl-th/0506049|bibcode=2006NuPhA.769...71H|s2cid=13047563}}</ref><ref>{{Cite journal|last1=Kharzeev|first1=Dmitri|last2=Tuchin|first2=Kirill|year=2008|title=Bulk viscosity of QCD matter near the critical temperature|url=http://stacks.iop.org/1126-6708/2008/i=09/a=093?key=crossref.d27fef0b80f0d0c8edb66dee6bba9889|journal=Journal of High Energy Physics|volume=2008|issue=9|pages=093|doi=10.1088/1126-6708/2008/09/093|arxiv=0705.4280|bibcode=2008JHEP...09..093K|s2cid=20224239|issn=1029-8479}}</ref> These indicate that the [[mean free time]] of quarks and gluons in the QGP may be comparable to the average interparticle spacing: hence the QGP is a liquid as far as its flow properties go. This is very much an active field of research, and these conclusions may evolve rapidly. The incorporation of dissipative phenomena into hydrodynamics is another active research area.<ref>{{Cite journal|last1=Blaizot|first1=J. P.|last2=Ollitrault|first2=J. Y.|year=1987|title=Structure of hydrodynamic flows in expanding quark–gluon plasmas|journal=Physical Review D|language=en|volume=36|issue=3|pages=916–927|doi=10.1103/PhysRevD.36.916|pmid=9958246|bibcode=1987PhRvD..36..916B|issn=0556-2821}}</ref><ref>{{Cite journal|last1=Gardim|first1=Fernando G.|last2=Grassi|first2=Frédérique|last3=Luzum|first3=Matthew|last4=Ollitrault|first4=Jean-Yves|year=2012|title=Mapping the hydrodynamic response to the initial geometry in heavy-ion collisions|journal=Physical Review C|language=en|volume=85|issue=2|pages=024908|doi=10.1103/PhysRevC.85.024908|arxiv=1111.6538|bibcode=2012PhRvC..85b4908G|s2cid=119187493|issn=0556-2813}}</ref><ref>{{Cite journal|last1=Gale|first1=Charles|last2=Jeon|first2=Sangyong|last3=Schenke|first3=Björn|year=2013|title=Hydrodynamic modeling of heavy-ion collisions|journal=International Journal of Modern Physics A|language=en|volume=28|issue=11|pages=1340011|doi=10.1142/S0217751X13400113|arxiv=1301.5893|bibcode=2013IJMPA..2840011G|s2cid=118414603|issn=0217-751X}}</ref>

=== Jet quenching effect ===
Detailed predictions were made in the late 1970s for the production of jets at the CERN [[Super Proton–Antiproton Synchrotron]].<ref>{{Cite journal|last1=Jacob|first1=M.|last2=Landshoff|first2=P.V.|year=1978|title=Large transverse momentum and jet studies|journal=Physics Reports|language=en|volume=48|issue=4|pages=285–350|doi=10.1016/0370-1573(78)90177-1|bibcode=1978PhR....48..285J|url=https://cds.cern.ch/record/262302}}</ref><ref>{{Cite journal|last=Jacob|first=M|year=1979|title=Jets in high energy collisions|journal=Physica Scripta|volume=19|issue=2|pages=69–78|doi=10.1088/0031-8949/19/2/001|bibcode=1979PhyS...19...69J|s2cid=250809871|issn=0031-8949}}</ref><ref>{{Cite journal|last1=Horgan|first1=R.|last2=Jacob|first2=M.|year=1981|title=Jet production at collider energy|journal=Nuclear Physics B|language=en|volume=179|issue=3|pages=441–460|doi=10.1016/0550-3213(81)90013-4|bibcode=1981NuPhB.179..441H|url=https://cds.cern.ch/record/134265}}</ref><ref>{{Cite journal|last1=Jacob|first1=M.|last2=Landshoff|first2=P.V.|year=1986|title=Minijets: origin and usefulness|journal=Modern Physics Letters A|language=en|volume=01|issue=12|pages=657–663|doi=10.1142/S021773238600083X|bibcode=1986MPLA....1..657J|issn=0217-7323|url=https://cds.cern.ch/record/172538}}</ref> [[UA2 experiment|UA2]] observed the first evidence for [[Jet (particle physics)|jet production]] in hadron collisions in 1981,<ref>{{Cite journal|last1=Banner|first1=M.|last2=Bloch|first2=Ph.|last3=Bonaudi|first3=F.|last4=Borer|first4=K.|last5=Borghini|first5=M.|last6=Chollet|first6=J.-C.|last7=Clark|first7=A.G.|last8=Conta|first8=C.|last9=Darriulat|first9=P.|last10=Di Lella|first10=L.|last11=Dines-Hansen|first11=J.|year=1982|title=Observation of very large transverse momentum jets at the CERN p collider|journal=Physics Letters B|language=en|volume=118|issue=1–3|pages=203–210|doi=10.1016/0370-2693(82)90629-3|bibcode=1982PhLB..118..203B|url=https://cds.cern.ch/record/904153}}</ref> which  shortly after was confirmed by [[UA1 experiment|UA1]].<ref>{{Cite journal|last1=Arnison|first1=G.|last2=Astbury|first2=A.|last3=Aubert|first3=B.|last4=Bacci|first4=C.|last5=Bernabei|first5=R.|last6=Bézaguet|first6=A.|last7=Böck|first7=R. |last8=Bowcock|first8=T.J.V.|last9=Calvetti|first9=M.|last10=Carroll|first10=T.|last11=Catz|first11=P.|year=1983|title=Observation of jets in high transverse energy events at the CERN proton antiproton collider|journal=Physics Letters B|language=en|volume=123|issue=1–2|pages=115–122|doi=10.1016/0370-2693(83)90970-X|bibcode=1983PhLB..123..115A|url=https://cds.cern.ch/record/141936}}</ref>

The subject was later revived at RHIC. One of the most striking physical effects obtained at RHIC energies is the effect of quenching jets.<ref>{{Cite journal|last1=Adcox|first1=K.|last2=Adler|first2=S.S.|last3=Afanasiev|first3=S.|last4=Aidala|first4=C.|last5=Ajitanand|first5=N.N.|last6=Akiba|first6=Y.|last7=Al-Jamel|first7=A.|last8=Alexander|first8=J.|last9=Amirikas|first9=R.|last10=Aoki|first10=K.|last11=Aphecetche|first11=L.|year=2005|title=Formation of dense partonic matter in relativistic nucleus–nucleus collisions at RHIC: Experimental evaluation by the PHENIX Collaboration|journal=Nuclear Physics A|language=en|volume=757|issue=1–2|pages=184–283|doi=10.1016/j.nuclphysa.2005.03.086|arxiv=nucl-ex/0410003|bibcode=2005NuPhA.757..184A|s2cid=119511423}}</ref><ref>{{Cite journal|last1=Adams|first1=J.|last2=Aggarwal|first2=M.M.|last3=Ahammed|first3=Z.|last4=Amonett|first4=J.|last5=Anderson|first5=B.D.|last6=Arkhipkin|first6=D.|last7=Averichev|first7=G.S. |last8=Badyal|first8=S.K.|last9=Bai|first9=Y.|last10=Balewski|first10=J.|last11=Barannikova|first11=O.|year=2005|title=Experimental and theoretical challenges in the search for the quark–gluon plasma: The STAR Collaboration's critical assessment of the evidence from RHIC collisions|journal=Nuclear Physics A|language=en|volume=757|issue=1–2|pages=102–183|doi=10.1016/j.nuclphysa.2005.03.085|arxiv=nucl-ex/0501009|bibcode=2005NuPhA.757..102A|s2cid=119062864}}</ref><ref>{{Cite journal|last1=Back|first1=B.B.|last2=Baker|first2=M.D.|last3=Ballintijn|first3=M.|last4=Barton|first4=D.S.|last5=Becker|first5=B.|last6=Betts|first6=R.R.|last7=Bickley|first7=A.A. |last8=Bindel|first8=R.|last9=Budzanowski|first9=A.|last10=Busza|first10=W.|last11=Carroll|first11=A.|year=2005|title=The PHOBOS perspective on discoveries at RHIC|journal=Nuclear Physics A|language=en|volume=757|issue=1–2|pages=28–101|doi=10.1016/j.nuclphysa.2005.03.084|arxiv=nucl-ex/0410022|bibcode=2005NuPhA.757...28B}}</ref> At the first stage of interaction of colliding relativistic nuclei, partons of the colliding nuclei give rise to the secondary partons with a large transverse impulse ≥&nbsp;3–6&nbsp;GeV/s. Passing through a highly heated compressed plasma, partons lose energy. The magnitude of the energy loss by the parton depends on the properties of the quark–gluon plasma (temperature, density). In addition, it is also necessary to take into account the fact that colored quarks and gluons are the elementary objects of the plasma, which differs from the energy loss by a parton in a medium consisting of colorless hadrons. Under the conditions of a quark–gluon plasma, the energy losses resulting from the RHIC energies by partons are estimated as {{tmath|\frac{dE}{dx} {{=}} 1~\text{GeV/fm} }}. This conclusion is confirmed by comparing the relative yield of hadrons with a large transverse impulse in nucleon-nucleon and nucleus-nucleus collisions at the same collision energy. The energy loss by partons with a large transverse impulse in nucleon-nucleon collisions is much smaller than in nucleus-nucleus collisions, which leads to a decrease in the yield of high-energy hadrons in nucleus-nucleus collisions. This result suggests that nuclear collisions cannot be regarded as a simple superposition of nucleon-nucleon collisions. For a short time, ~1&nbsp;μs, and in the final volume, quarks and gluons form some ideal liquid. The collective properties of this fluid are manifested during its movement as a whole. Therefore, when moving partons in this medium, it is necessary to take into account some collective properties of this quark–gluon liquid. Energy losses depend on the properties of the quark–gluon medium, on the parton density in the resulting fireball, and on the dynamics of its expansion. Losses of energy by light and heavy quarks during the passage of a fireball turn out to be approximately the same.<ref>{{Cite book|last=Schukraft|first=Jürgen|url=https://indico.cern.ch/event/114939/contributions/62599/attachments/48842/70225/CERN_HI_Dec_2010.pdf|title=ALICE—'Little Bang' : The first 3 weeks ...|year=2010}}</ref>

In November 2010, CERN announced the first direct observation of jet quenching, based on experiments with heavy-ion collisions.<ref>{{cite press release |title=LHC experiments bring new insight into primordial universe |url=http://press.web.cern.ch/press-releases/2010/11/lhc-experiments-bring-new-insight-primordial-universe|publisher=[[CERN]] |date=November 26, 2010 |access-date=December 2, 2010}}</ref><ref>{{cite journal | last=Aad | first=G. |display-authors=etal |collaboration=ATLAS Collaboration | title=Observation of a Centrality-Dependent Dijet Asymmetry in Lead-Lead Collisions at {{sqrt|s<sub>NN</sub>}} = 2.76 TeV with the ATLAS Detector at the LHC | journal=Physical Review Letters | volume=105 | issue=25 | date=13 December 2010 | doi=10.1103/physrevlett.105.252303 |doi-access=free | page=252303 | pmid=21231581 | arxiv=1011.6182 | bibcode=2010PhRvL.105y2303A }}</ref><ref>{{cite journal | last=Chatrchyan | first=S. |display-authors=etal |collaboration=CMS Collaboration | title=Observation and studies of jet quenching in Pb-Pb collisions at {{sqrt|s<sub>NN</sub>}} = 2.76 TeV | journal=Physical Review C | volume=84 | issue=2 | date=12 August 2011 | doi=10.1103/physrevc.84.024906 |doi-access=free | page=024906 | arxiv=1102.1957 | bibcode=2011PhRvC..84b4906C }}</ref><ref>{{cite web|url=http://home.web.cern.ch/about/physics/heavy-ions-and-quark-gluon-plasma|archive-url=https://web.archive.org/web/20121203180310/http://home.web.cern.ch/about/physics/heavy-ions-and-quark-gluon-plasma|url-status=live|archive-date=December 3, 2012|title=Heavy ions and quark–gluon plasma|author=CERN|date=18 July 2012}}</ref>

=== Direct photons and dileptons ===
Direct photons and [[dilepton]]s are arguably most penetrating tools to study relativistic heavy ion collisions. They are produced, by various mechanisms spanning the space-time evolution of the strongly interacting fireball. They provide in principle a snapshot on the initial stage as well. They are hard to decipher and interpret as most of the signal is originating from hadron decays long after the QGP fireball has disintegrated.<ref>{{Cite journal|last1=Albrecht|first1=R.|last2=Antonenko|first2=V.|last3=Awes|first3=T. C.|last4=Barlag|first4=C.|last5=Berger|first5=F.|last6=Bloomer|first6=M.|last7=Blume|first7=C.|last8=Bock|first8=D.|last9=Bock|first9=R.|last10=Bohne|first10=E.-M.|last11=Bucher|first11=D.|year=1996|title=Limits on the Production of Direct Photons in 200 A GeV S 32 + A u Collisions|journal=Physical Review Letters|language=en|volume=76|issue=19|pages=3506–3509|doi=10.1103/PhysRevLett.76.3506|pmid=10060985|bibcode=1996PhRvL..76.3506A|issn=0031-9007|url=https://cds.cern.ch/record/294014}}</ref><ref name="Aggarwal 2000 3595–3599">{{Cite journal|last1=Aggarwal|first1=M. M.|last2=Agnihotri|first2=A.|last3=Ahammed|first3=Z.|last4=Angelis|first4=A. L. S.|last5=Antonenko|first5=V.|last6=Arefiev|first6=V.|last7=Astakhov|first7=V.|last8=Avdeitchikov|first8=V.|last9=Awes|first9=T. C.|last10=Baba|first10=P. V. K. S.|last11=Badyal|first11=S. K.|year=2000|title=Observation of Direct Photons in Central 158 A GeV P 208 b + P 208 b Collisions|journal=Physical Review Letters|language=en|volume=85|issue=17|pages=3595–3599|doi=10.1103/PhysRevLett.85.3595|pmid=11030959|arxiv=nucl-ex/0006008|s2cid=119386387|issn=0031-9007}}</ref><ref>{{Cite journal|last1=Acharya|first1=S.|last2=Acosta|first2=F. T.-.|last3=Adamová|first3=D.|last4=Adolfsson|first4=J.|last5=Aggarwal|first5=M. M.|last6=Aglieri Rinella|first6=G.|last7=Agnello|first7=M.|last8=Agrawal|first8=N.|last9=Ahammed|first9=Z.|last10=Ahn|first10=S. U.|last11=Aiola|first11=S.|year=2019|title=Direct photon production at low transverse momentum in proton-proton collisions at s = 2.76 and 8 TeV|journal=Physical Review C|language=en|volume=99|issue=2|pages=024912|doi=10.1103/PhysRevC.99.024912|issn=2469-9985|doi-access=free|arxiv=1803.09857}}</ref>

=== Glasma hypothesis ===
Since 2008, there is a discussion about a hypothetical precursor state of the quark–gluon plasma, the so-called "Glasma", where the [[dressed particle]]s are condensed into some kind of glassy (or amorphous) state, below the genuine transition between the confined state and the plasma liquid.<ref>{{Cite journal |title=From Glasma to Quark Gluon Plasma in heavy ion collisions |year=2008 |arxiv=0806.1356 |bibcode = 2008JPhG...35j4003V |doi = 10.1088/0954-3899/35/10/104003 |volume=35 |issue=10 |journal=Journal of Physics G: Nuclear and Particle Physics |page=104003|last1=Venugopalan |first1=Raju |s2cid=15121756 }}</ref> This would be analogous to the formation of metallic glasses, or amorphous alloys of them, below the genuine onset of the liquid metallic state.

Although the experimental high temperatures and densities predicted as producing a quark–gluon plasma have been realized in the laboratory, the resulting matter does ''not'' behave as a quasi-ideal state of free quarks and gluons, but, rather, as an almost [[Perfect fluid|perfect]] dense fluid.<ref name="Zajc">{{Cite journal|author=WA Zajc|year=2008|title=The fluid nature of quark–gluon plasma|journal=Nuclear Physics A|volume=805|issue=1–4|pages=283c–294c|arxiv=0802.3552|bibcode=2008NuPhA.805..283Z|doi=10.1016/j.nuclphysa.2008.02.285|s2cid=119273920}}</ref> Actually, the fact that the quark–gluon plasma will not yet be "free" at temperatures realized at present accelerators was predicted in 1984, as a consequence of the remnant effects of confinement.<ref>{{Cite journal|last1=Plümer|first1=M.|last2=Raha|first2=S.|last3=Weiner|first3=R. M.|name-list-style=amp|year=1984|title=How free is the quark–gluon plasma|journal=Nucl. Phys. A|volume=418|pages=549–557|bibcode=1984NuPhA.418..549P|doi=10.1016/0375-9474(84)90575-X}}</ref><ref>{{Cite journal|last1=Plümer|first1=M.|last2=Raha|first2=S.|last3=Weiner|first3=R. M.|name-list-style=amp|year=1984|title=Effect of confinement on the sound velocity in a quark–gluon plasma|journal=Phys. Lett. B|volume=139|issue=3|pages=198–202|bibcode=1984PhLB..139..198P|doi=10.1016/0370-2693(84)91244-9}}</ref>

=== Neutron stars ===
It has been hypothesized that the core of some massive [[neutron star]]s may be a quark–gluon plasma.<ref>{{Cite journal |last1=Annala |first1=Eemeli |last2=Gorda |first2=Tyler |last3=Hirvonen |first3=Joonas |last4=Komoltsev |first4=Oleg |last5=Kurkela |first5=Aleksi |last6=Nättilä |first6=Joonas |last7=Vuorinen |first7=Aleksi |date=2023-12-19 |title=Strongly interacting matter exhibits deconfined behavior in massive neutron stars |journal=Nature Communications |language=en |volume=14 |issue=1 |pages=8451 |doi=10.1038/s41467-023-44051-y |pmid=38114461 |issn=2041-1723|pmc=10730725 }}</ref>

== In-laboratory formation of deconfined&nbsp;matter ==
A quark–gluon plasma (QGP)<ref>{{Cite book|last=Wang|first=Xin-Nian|title=Quark–Gluon Plasma 5|publisher=World Scientific|year=2016|isbn=978-981-4663-70-0|language=en|doi=10.1142/9533|bibcode=2016qgpf.book.....W}}</ref> or quark soup<ref>{{Cite journal|last1=Harris|first1=John W.|last2=Müller|first2=Berndt|year=1996|title=The search for the quark–gluon plasma|journal=[[Annual Review of Nuclear and Particle Science]]|language=en|volume=46|issue=1|pages=71–107|arxiv=hep-ph/9602235|doi=10.1146/annurev.nucl.46.1.71| doi-access=free|bibcode=1996ARNPS..46...71H|s2cid=2213461|issn=0163-8998}}</ref><ref>{{cite journal|author1=Bohr, Henrik|author2=Nielsen, H. B.|year=1977|title=Hadron production from a boiling quark soup: quark model predicting particle ratios in hadronic collisions|journal=Nuclear Physics B|volume=128|issue=2|page=275|bibcode=1977NuPhB.128..275B|doi=10.1016/0550-3213(77)90032-3}}</ref> is a [[state of matter]] in quantum chromodynamics (QCD) which exists at extremely high [[temperature]] and/or [[density]]. This state is thought to consist of [[asymptotically free]] [[Strong force|strong-interacting]] quarks and gluons, which are ordinarily confined by [[color confinement]] inside atomic nuclei or other hadrons. This is in analogy with the conventional plasma where nuclei and electrons, confined inside [[atom]]s by [[electrostatic forces]] at ambient conditions, can move freely. Experiments to create artificial quark matter started at CERN in 1986/87, resulting in first claims that were published in 1991.<ref>{{Cite journal|last1=Abatzis|first1=S.|last2=Antinori|first2=F.|last3=Barnes|first3=R.P.|last4=Benayoun|first4=M.|last5=Beusch|first5=W.|last6=Bloodworth|first6=I.J.|last7=Bravar|first7=A.|last8=Carney|first8=J. N.|last9=Di Bari|first9=D.|last10=Dufey|first10=J. P.|last11=Evans|first11=D.|year=1991|title=Production of multistrange baryons and antibaryons in sulphur-tungsten interactions at 200 GeV/c per nucleon|journal=Physics Letters B|language=en|volume=259|issue=4|pages=508–510|doi=10.1016/0370-2693(91)91666-J|bibcode=1991PhLB..259..508A|url=https://cds.cern.ch/record/213853}}</ref><ref>{{Cite journal|last1=Abatzis|first1=S.|last2=Antinori|first2=F.|last3=Barnes|first3=R. P.|last4=Benayoun|first4=M.|last5=Beusch|first5=W.|last6=Bloodworth|first6=I.J.|last7=Bravar|first7=A.|last8=Carney|first8=J. N.|last9=de la Cruz|first9=B.|last10=Di Bari|first10=D.|last11=Dufey|first11=J.P.|year=1991|title=production in sulphur-tungsten interactions at 200 GeV/c per nucleon|journal=Physics Letters B|language=en|volume=270|issue=1|pages=123–127|doi=10.1016/0370-2693(91)91548-A|url=https://cds.cern.ch/record/222848}}</ref> It took several years before the idea became accepted in the community of particle and nuclear physicists. Formation of a new state of matter in Pb–Pb collisions was officially announced at CERN in view of the convincing experimental results presented by the CERN [[Super Proton Synchrotron|SPS]] WA97 experiment in 1999,<ref>{{Cite journal|last1=Andersen|first1=E.|last2=Antinori|first2=F.|last3=Armenise|first3=N.|last4=Bakke|first4=H.|last5=Bán|first5=J.|last6=Barberis|first6=D.|last7=Beker|first7=H.|last8=Beusch|first8=W.|last9=Bloodworth|first9=I. J.|last10=Böhm|first10=J.|last11=Caliandro|first11=R.|year=1999|title=Strangeness enhancement at mid-rapidity in Pb–Pb collisions at 158 A GeV/c|journal=Physics Letters B|language=en|volume=449|issue=3–4|pages=401–406|doi=10.1016/S0370-2693(99)00140-9|bibcode=1999PhLB..449..401W|url=https://cds.cern.ch/record/380839}}</ref><ref name=":4" /><ref>{{Citation|last=Müller|first=Berndt|title=Melting Hadrons, Boiling Quarks - from Hagedorn Temperature to Ultra-Relativistic Heavy-Ion Collisions at CERN |chapter=A New Phase of Matter: Quark-Gluon Plasma Beyond the Hagedorn Critical Temperature |date=2016|pages=107–116|editor-last=Rafelski|editor-first=Johann|publisher=Springer International Publishing|language=en|doi=10.1007/978-3-319-17545-4_14|isbn=978-3-319-17544-7|s2cid=119120988|doi-access=free|arxiv=1501.06077}}</ref> and later elaborated by [[Brookhaven National Laboratory|Brookhaven National Laboratory's]] [[Relativistic Heavy Ion Collider]].<ref>{{Cite web|url=http://www.eurekalert.org/pub_releases/2005-04/du-dtp041505.php|title=Duke theorists play role in search for superhot 'quark–gluon plasma'|website=EurekAlert!|language=en|access-date=2020-03-17}}</ref><ref>{{Cite journal|last1=Jacak|first1=Barbara|last2=Steinberg|first2=Peter|year=2010|title=Creating the perfect liquid in heavy-ion collisions|journal=Physics Today|language=en|volume=63|issue=5|pages=39–43|doi=10.1063/1.3431330|bibcode=2010PhT....63e..39J|issn=0031-9228}}</ref><ref name=":1" /> Quark matter can only be produced in minute quantities and is unstable and impossible to contain, and will radioactively decay within a fraction of a second into stable particles through [[hadronization]]; the produced hadrons or their decay products and [[gamma ray]]s can then be detected. In the [[quark matter]] phase diagram, QGP is placed in the high-temperature, high-density regime, whereas ordinary matter is a cold and rarefied mixture of nuclei and vacuum, and the hypothetical [[quark star]]s would consist of relatively cold, but dense quark matter. It is believed that up to a few microseconds (10<sup>−12</sup> to 10<sup>−6</sup> seconds) after the Big Bang, known as the [[quark epoch]], the Universe was in a quark–gluon plasma state.

The strength of the [[color force]] means that unlike the gas-like plasma, quark–gluon plasma behaves as a near-ideal [[Fermi liquid]], although research on flow characteristics is ongoing.<ref>{{cite web|url=http://physicsworld.com/cws/article/news/2005/apr/19/quark-gluon-plasma-goes-liquid|archive-url=https://web.archive.org/web/20121107045757/http://physicsworld.com/cws/article/news/2005/apr/19/quark-gluon-plasma-goes-liquid|url-status=live|archive-date=November 7, 2012|title=Quark–gluon plasma goes liquid|publisher=physicsworld.com|access-date=2016-03-04}}</ref> Liquid or even near-perfect liquid flow with almost no frictional resistance or viscosity was claimed by research teams at RHIC<ref>{{Cite web|url=https://www.bnl.gov/newsroom/news.php?a=110303|work=BNL Newsroom |title=RHIC Scientists Serve Up 'Perfect' Liquid|access-date=2017-04-21}}</ref> and LHC's [[Compact Muon Solenoid]] detector.<ref>{{cite web|url=http://earthsky.org/human-world/lhc-creates-liquid-from-big-bang|title=LHC creates liquid from Big Bang &#124; Human World|author=Eleanor Imster|date=15 September 2015|publisher=EarthSky|access-date=2016-03-04}}</ref> QGP differs from a "free" collision event by several features; for example, its particle content is indicative of a temporary [[chemical equilibrium]] producing an excess of middle-energy [[Strangeness production|strange quarks]] vs. a nonequilibrium distribution mixing light and heavy quarks ("strangeness production"), and it does not allow particle jets to pass through ("jet quenching").

Experiments at CERN's Super Proton Synchrotron (SPS) begun experiments to create QGP in the 1980s and 1990s: the results led CERN to announce evidence for a "new state of matter"<ref>{{Cite web|url=https://home.cern/news/press-release/cern/new-state-matter-created-cern|title=New State of Matter created at CERN|date=10 February 2000|website=CERN|language=en|access-date=2020-03-25}}</ref> in 2000.<ref>{{Cite web|url=https://indico.cern.ch/event/457044/timetable/|title=30 Years of Heavy ions : ...what next?|date=9 November 2016|website=Indico|publisher=CERN|access-date=2020-04-07}}</ref> Scientists at Brookhaven National Laboratory's Relativistic Heavy Ion Collider announced they had created quark–gluon plasma by colliding gold ions at nearly the speed of light, reaching temperatures of 4 trillion degrees Celsius.<ref>{{Cite news|last=Overbye|first=Dennis|url=https://www.nytimes.com/2010/02/16/science/16quark.html|title=In Brookhaven Collider, Briefly Breaking a Law of Nature|date=2010-02-15|work=The New York Times|access-date=2017-04-21|issn=0362-4331}}</ref> Current experiments (2017) at the [[Brookhaven National Laboratory]]'s Relativistic Heavy Ion Collider (RHIC) on Long Island (New York, USA) and at CERN's recent [[Large Hadron Collider]] near Geneva (Switzerland) are continuing this effort,<ref>{{cite web|url=http://www.bnl.gov/rhic/|title=RHIC &#124; Relativistic Heavy Ion Collider|publisher=BNL|access-date=2016-03-04}}</ref><ref name="RHIC">[http://www.bnl.gov/rhic/news2/news.asp?a=1074&t=pr {{"'}}Perfect' Liquid Hot Enough to be Quark Soup"]. {{Webarchive|url=https://web.archive.org/web/20110806045228/http://www.bnl.gov/rhic/news2/news.asp?a=1074&t=pr |date=2011-08-06}}.</ref> by colliding relativistically accelerated gold and other ion species (at RHIC) or lead (at LHC) with each other or with protons.<ref name="RHIC" /> Three experiments running on CERN's Large Hadron Collider (LHC), on the spectrometers [[A Large Ion Collider Experiment|ALICE]],<ref>{{cite web|url=http://aliceinfo.cern.ch/index.html|title=Alice Experiment: The ALICE Portal|url-status=dead|archive-url=https://web.archive.org/web/20060213023750/http://aliceinfo.cern.ch/index.html|archive-date=February 13, 2006|access-date=July 12, 2005}}</ref> [[ATLAS experiment|ATLAS]] and [[Compact Muon Solenoid|CMS]], have continued studying the properties of QGP. CERN temporarily ceased colliding [[protons]], and began colliding [[lead]] ions for the ALICE experiment in 2011, in order to create a QGP.<ref>{{cite web|url=http://press.cern/press-releases/2010/11/lhc-enters-new-phase|title=The LHC enters a new phase|access-date=November 23, 2016}}</ref> A new record breaking temperature was set by [[ALICE: A Large Ion Collider Experiment]] at CERN in August 2012 in the ranges of 5.5 trillion ({{val|5.5|e=12}}) kelvin as claimed in their Nature PR.<ref>{{cite web|url=http://blogs.nature.com/news/2012/08/hot-stuff-cern-physicists-create-record-breaking-subatomic-soup.html|title=Hot stuff: CERN physicists create record-breaking subatomic soup |work=Nature News Blog|date=2012-08-13|access-date=2016-03-04|archive-date=2016-03-04|archive-url=https://web.archive.org/web/20160304203826/http://blogs.nature.com/news/2012/08/hot-stuff-cern-physicists-create-record-breaking-subatomic-soup.html|url-status=dead}}</ref>

The formation of a quark–gluon plasma occurs as a result of a [[strong interaction]] between the [[partons]] (quarks, gluons) that make up the nucleons of the colliding heavy nuclei called heavy ions. Therefore, experiments are referred to as relativistic heavy ion collision experiments. Theoretical and experimental works show that the formation of a quark–gluon plasma occurs at the temperature of T ≈ 150–160 MeV, the Hagedorn temperature, and an energy density of ≈ 0.4–1 GeV / fm<sup>3</sup>. While at first a phase transition was expected, present day theoretical interpretations propose a phase transformation similar to the process of ionisation of normal matter into ionic and electron plasma.<ref>{{Cite book|last1=Hwa|first1=Rudolph C|title=Quark–Gluon Plasma 4|last2=Wang|first2=Xin-Nian|publisher=World Scientific|year=2010|isbn=978-981-4293-28-0|language=en|doi=10.1142/7588|bibcode=2010qgp4.book.....H}}</ref><ref>{{cite journal |journal=CERN Courier|volume=60|pages=40–46 |last=Mangano |first=Michelangelo |arxiv=2003.05976 |title=LHC at 10: the physics legacy|url=https://cds.cern.ch/record/2712176|issue=2|year=2020|bibcode=2020arXiv200305976M}}</ref><ref>{{Cite journal|last=Shuryak|first=Edward|year=2017|title=Strongly coupled quark–gluon plasma in heavy ion collisions|journal=Reviews of Modern Physics|language=en|volume=89|issue=3|pages=035001|doi=10.1103/RevModPhys.89.035001|bibcode=2017RvMP...89c5001S|issn=0034-6861|doi-access=free|arxiv=1412.8393}}</ref><ref>{{Cite journal|last1=Pasechnik|first1=Roman|last2=Šumbera|first2=Michal|year=2017|title=Phenomenological review on quark–gluon plasma: concepts vs. observations|journal=Universe|language=en|volume=3|issue=1|pages=7|doi=10.3390/universe3010007|arxiv=1611.01533|bibcode=2017Univ....3....7P|s2cid=17657668|issn=2218-1997|doi-access=free}}</ref><ref name=":1">{{Cite report|last1=Ludlam|first1=T.|last2=Aronson|first2=S.|year=2005|title=Hunting the quark gluon plasma|url=https://www.bnl.gov/isd/documents/29263.pdf|docket=BNL-73847-2005|publisher=Brookhaven National Laboratory|doi=10.2172/15015225}}</ref>

=== Quark–gluon plasma and the onset of deconfinement ===
The central issue of the formation of a quark–gluon plasma is the research for the [[onset of deconfinement]]. From the beginning of the research on formation of QGP, the issue was whether [[energy density]] can be achieved in nucleus-nucleus collisions. This depends on how much energy each nucleon loses. An influential reaction picture was the scaling solution presented by [[James Bjorken|Bjorken]].<ref>{{Cite journal|last=Bjorken|first=J. D.|year=1983|title=Highly relativistic nucleus-nucleus collisions: The central rapidity region|journal=Physical Review D|language=en|volume=27|issue=1|pages=140–151|doi=10.1103/PhysRevD.27.140|bibcode=1983PhRvD..27..140B|issn=0556-2821}}</ref> This model applies to ultra-high energy collisions. In experiments carried out at CERN SPS and BNL RHIC more complex situation arose, usually divided into three stages:<ref name=":7">{{Cite book|last1=Letessier|first1=Jean|url=https://books.google.com/books?id=SAlbKkdor1gC|title=Hadrons and Quark–Gluon Plasma|last2=Rafelski|first2=Johann|author2-link=Johann Rafelski|date=2002-05-30|publisher=Cambridge University Press|isbn=978-1-139-43303-7|language=en|ref=7}}</ref>
* Primary parton collisions and baryon stopping at the time of complete overlapping of the colliding nuclei.
* Redistribution of particle energy and new particles born in the QGP fireball.
* The fireball of QGP matter equilibrates and expands before hadronizing.

More and more experimental evidence points to the strength of QGP formation mechanisms—operating even in LHC-energy scale proton-proton collisions.<ref name=":5" />

== Further reading ==

=== Books ===
* {{Cite book|title=Quark-Gluon Plasma, Heavy Ion Collisions and Hadrons|publisher=World Scientific|year=2024|isbn=978-981-128234-8|author-last=Shuryak|author-first=Edward|series=World Scientific Lecture Notes in Physics |volume=85 |location=Singapore|language=en|doi=10.1142/13570}}
* {{Cite book |title=Melting Hadrons, Boiling Quarks – From Hagedorn Temperature to Ultra-Relativistic Heavy-Ion Collisions at CERN |publisher=Springer International Publishing |year=2016 |isbn=978-3319175447 |editor-last=Rafelski |editor-first=Johann |editor-link=Johann Rafelski |location=Cham |language=en |doi=10.1007/978-3-319-17545-4 |doi-access=free |bibcode=2016mhbq.book.....R}}
* {{Cite book|last=E|first=Fortov Vladimr|url=https://books.google.com/books?id=AwvyCwAAQBAJ|title=Thermodynamics And Equations Of State For Matter: From Ideal Gas To Quark–gluon Plasma|publisher=World Scientific|year=2016|isbn=978-9814749213|language=en|location=Singapore}}
* {{Cite book|last1=Yagi|first1=Kohsuke|url=https://books.google.com/books?id=C2bpxwUXJngC|title=Quark–Gluon Plasma: From Big Bang to Little Bang|last2=Hatsuda|first2=Tetsuo|last3=Miake|first3=Yasuo|publisher=Cambridge University Press|year=2005|isbn=978-0521561082|series=Cambridge monographs on particle physics, nuclear physics, and cosmology|location=Cambridge}}
* {{Cite book|last=Florkowski|first=Wojciech|url=https://books.google.com/books?id=GhY8DQAAQBAJ|title=Phenomenology of ultra-relativistic heavy-ion collisions|publisher=World Scientific|year=2010|isbn=978-9814280662|location=Singapore}}
* {{Cite book|date=2010|isbn=978-3642022852|editor1-last=Sarkar|editor1-first=Sourav|volume=785|location=Berlin; Heidelberg|language=en|doi=10.1007/978-3-642-02286-9|arxiv = 0810.3553|editor-last2=Satz|editor-first2=Helmut|editor-last3=Sinha|editor-first3=Bikash|last1=Banerjee|first1=Debasish|last2=Nayak|first2=Jajati K.|last3=Venugopalan|first3=Raju|title=The Physics of the Quark-Gluon Plasma |series=Lecture Notes in Physics|pages=105–137}}
* {{Cite book|url=http://materials.springer.com/bp/docs/978-3-642-01539-7|title=Relativistic Heavy Ion Physics|publisher=Springer: Berlin; Heidelberg|year=2010|isbn=978-3642015380|editor-last=Stock|editor-first=R.|series=Landolt-Börnstein – Group I Elementary Particles, Nuclei and Atoms|volume=23|location=Berlin; Heidelberg|doi=10.1007/978-3-642-01539-7|citeseerx=10.1.1.314.4982}}
* {{Cite book|last1=Sahu|first1=P. K.|url=https://books.google.com/books?id=dk5bPgAACAAJ|title=Quark Gluon Plasma and Hadron Physics|last2=Phatak|first2=S. C.|last3=Viyogi|first3=Yogendra Pathak|author3-link=Y. P. Viyogi|publisher=Narosa|year=2009|isbn=978-8173199578|language=en}}
* {{Cite book|title=The Physics of the Quark-Gluon Plasma|date=1985|publisher=Springer Berlin; Heidelberg|isbn=978-3540152118|series=Lecture Notes in Physics|volume=225|location=Berlin; Heidelberg|language=en|doi=10.1007/bfb0114317|arxiv=hep-ph/9509334 |last1=Müller |first1=Berndt |author-link=Berndt Müller }}

=== Review articles with a historical perspective of the field ===
* {{cite journal|last1=Gazdzicki|first1=Marek|last2=Gorenstein|first2=Mark|last3=Seyboth|first3=Peter|year=2020|title=Brief history of the search for critical structures in heavy-ion collisions|journal=Acta Physica Polonica B|volume=51|issue=5|page=1033|doi=10.5506/APhysPolB.51.1033|arxiv=2004.02255|bibcode=2020AcPPB..51.1033G|s2cid=214802159}}
* {{Cite journal|last=Rafelski|first=Johann|year=2020|title=Discovery of Quark–Gluon Plasma: Strangeness Diaries|journal=The European Physical Journal Special Topics|language=en|volume=229|issue=1|pages=1–140|doi=10.1140/epjst/e2019-900263-x|arxiv=1911.00831|bibcode=2020EPJST.229....1R|s2cid=207869782|issn=1951-6401}}
* {{Cite journal|last1=Pasechnik|first1=Roman|last2=Šumbera|first2=Michal|year=2017|title=Phenomenological Review on Quark–Gluon Plasma: Concepts vs. Observations|journal=Universe|language=en|volume=3|issue=1|pages=7|doi=10.3390/universe3010007|arxiv=1611.01533|bibcode=2017Univ....3....7P|s2cid=17657668|issn=2218-1997|doi-access=free}}
* {{Cite journal|last1=Satz|first1=Helmut|last2=Stock|first2=Reinhard|year=2016|title=Quark Matter: The Beginning|journal=Nuclear Physics A|language=en|volume=956|pages=898–901|doi=10.1016/j.nuclphysa.2016.06.002|bibcode=2016NuPhA.956..898S}}
* {{Cite journal|last=Gazdzicki|first=M.|year=2012|title=On the history of multi-particle production in high energy collisions|url=http://www.actaphys.uj.edu.pl/vol43/abs/v43p0791|journal=Acta Physica Polonica B|language=en|volume=43|issue=4|page=791|doi=10.5506/APhysPolB.43.791|arxiv=1201.0485|bibcode=2012arXiv1201.0485G|s2cid=118418649|issn=0587-4254}}
* {{Cite journal|last=Müller|first=B.|year=2012|title=Strangeness and the quark–gluon plasma: thirty years of discovery|url=http://www.actaphys.uj.edu.pl/vol43/abs/v43p0761|journal=Acta Physica Polonica B|language=en|volume=43|issue=4|page=761|doi=10.5506/APhysPolB.43.761|arxiv=1112.5382|s2cid=119280137|issn=0587-4254}}
* {{Cite journal|last=Heinz|first=Ulrich|year=2008|title=From SPS to RHIC: Maurice and the CERN heavy-ion programme|url=http://stacks.iop.org/1402-4896/78/i=2/a=028005?key=crossref.8671cc766b779ec166b546c05ebbf3dc|journal=Physica Scripta|volume=78|issue=2|page=028005|doi=10.1088/0031-8949/78/02/028005|arxiv=0805.4572|bibcode=2008PhyS...78b8005H|s2cid=13833990|issn=0031-8949}}
* {{Cite journal|last=Baym|first=G.|year=2002|title=RHIC: From dreams to beams in two decades|journal=Nuclear Physics A|language=en|volume=698|issue=1–4|pages=xxiii–xxxii|doi=10.1016/S0375-9474(01)01342-2|arxiv=hep-ph/0104138|bibcode=2002NuPhA.698D..23B|s2cid=12028950}}

== See also ==
{{div col |colwidth=27em}}
* [[Color confinement]]
* [[Color-glass condensate]]
* [[Hadron]]s (that is [[meson]]s and [[baryon]]s)
* [[Hadronization]]
* [[Hagedorn temperature]]
* [[Neutron star]]
* [[Plasma (physics)|Plasma physics]]
* [[QCD matter]]
* [[Quantum electrodynamics]]
* [[Quantum chromodynamics]]
* [[Quantum hydrodynamics]]
* [[Relativistic plasma]]
* [[Relativistic nuclear collision]]
* [[Strangeness production]]
* [[Strange matter]]
* [[List of unsolved problems in physics]]
{{div col end}}

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

== External links ==
*{{Commons category-inline}}
* [http://www.bnl.gov/rhic/ The Relativistic Heavy Ion Collider] at [http://www.bnl.gov/ Brookhaven National Laboratory]
* [http://aliceinfo.cern.ch/ The Alice Experiment] {{Webarchive|url=https://web.archive.org/web/20110602001439/http://aliceinfo.cern.ch/ |date=2011-06-02 }} at [http://www.cern.ch/ CERN]
* [http://theory.tifr.res.in/~sgupta/ilgti The Indian Lattice Gauge Theory Initiative]
* Quark matter reviews: [https://arxiv.org/abs/hep-ph/0402251 2004 theory], [https://arxiv.org/abs/nucl-ex/0405007 2004 experiment]
* Quark–Gluon Plasma reviews: [https://arxiv.org/abs/1101.3937 2011 theory]
* Lattice reviews: [https://arxiv.org/abs/hep-ph/0303042 2003], [https://arxiv.org/abs/hep-ph/0505073 2005]
* [http://news.bbc.co.uk/1/hi/sci/tech/4462209.stm BBC article mentioning Brookhaven results (2005)]
* [https://web.archive.org/web/20050423224100/http://www.aip.org/pnu/2005/split/728-1.html Physics News Update article on the quark–gluon liquid, with links to preprints]
* "Hadrons and Quark–Gluon Plasma" by Jean Letessier and [[Johann Rafelski]]   Cambridge University Press (2002) {{ISBN|0-521-38536-9}}, Cambridge, UK;
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