Editing Gluon (section)

== Experimental observations ==
[[Quark]]s and gluons (colored) manifest themselves by fragmenting into more quarks and gluons, which in turn hadronize into normal (colorless) particles, correlated in jets. As revealed in 1978&nbsp;summer conferences,<ref name="SMY"/> the [[PLUTO detector]] at the electron-positron collider DORIS ([[DESY]]) produced the first evidence that the hadronic decays of the very narrow resonance Υ(9.46) could be interpreted as [[three-jet event]] topologies produced by three gluons. Later, published analyses by the same experiment confirmed this interpretation and also the spin&nbsp;=&nbsp;1 nature of the gluon<ref>{{cite journal |author1=Berger, Ch. |display-authors=etal |collaboration=PLUTO collaboration |year=1979 |title=Jet analysis of the Υ(9.46) decay into charged hadrons |journal=[[Physics Letters B]] |volume=82 |page=449 |bibcode=1979PhLB...82..449B |doi=10.1016/0370-2693(79)90265-X |issue=3–4 |df=dmy-all}}</ref><ref>{{cite journal |author=Berger, Ch. |display-authors=etal |collaboration=PLUTO collaboration |year=1981 |title=Topology of the Υ decay |journal=[[Zeitschrift für Physik C]] |volume=8 |page=101 |bibcode=1981ZPhyC...8..101B |doi=10.1007/BF01547873 |issue=2 |s2cid=124931350 |df=dmy-all}}</ref> (see also the recollection<ref name="SMY"/> and [[PLUTO experiments]]).

In summer&nbsp;1979, at higher energies at the electron-positron collider [[PETRA]] (DESY), again three-jet topologies were observed, now clearly visible and interpreted as q{{overline|q}} gluon [[bremsstrahlung]], by [[TASSO]],<ref>{{cite journal |author=Brandelik, R. |display-authors=etal |collaboration=[[TASSO collaboration]] |year=1979 |title=Evidence for Planar Events in e<sup>+</sup>e<sup>−</sup> annihilation at High Energies |journal=[[Physics Letters B]] |volume=86 |issue=2 |pages=243–249 |bibcode=1979PhLB...86..243B |doi=10.1016/0370-2693(79)90830-X}}</ref> [[MARK-J]]<ref>{{cite journal |author1=Barber, D.P. |s2cid=13903005 |display-authors=etal |collaboration=MARK-J collaboration |year=1979 |title=Discovery of Three-Jet Events and a Test of Quantum Chromodynamics at PETRA |journal=[[Physical Review Letters]] |volume=43 |page=830 |bibcode=1979PhRvL..43..830B |doi= 10.1103/PhysRevLett.43.830 |issue=12 |df=dmy-all}}</ref> and PLUTO experiments<ref>{{cite journal |author=Berger, Ch. |display-authors=etal |collaboration=PLUTO collaboration |year=1979 |title=Evidence for Gluon Bremsstrahlung in e<sup>+</sup>e<sup>−</sup> Annihilations at High Energies |journal=[[Physics Letters B]] |volume=86 |page=418 |bibcode=1979PhLB...86..418B |doi=10.1016/0370-2693(79)90869-4 |issue=3–4 |df=dmy-all}}</ref> (later in 1980 also by [[JADE (particle detector)|JADE]]<ref>{{cite journal |author1=Bartel, W. |display-authors=etal |collaboration=JADE collaboration |year=1980 |title=Observation of planar three-jet events in e<sup>+&nbsp;</sup>e<sup>&minus;</sup> annihilation and evidence for gluon bremsstrahlung |journal=[[Physics Letters B]] |volume=91 |issue=1 |page=142 |bibcode=1980PhLB...91..142B |doi=10.1016/0370-2693(80)90680-2 |url=http://bib-pubdb1.desy.de/search?p=id:%22PUBDB-2017-02984%22 |df=dmy-all}}</ref>). The spin&nbsp;=&nbsp;1 property of the gluon was confirmed in 1980 by TASSO<ref>{{cite journal |author1=Brandelik, R. |display-authors=etal |collaboration=[[TASSO collaboration]] |year=1980 |title=Evidence for a spin-1 gluon in three-jet events |journal=[[Physics Letters B]] |volume=97 |issue=3–4 |page=453 |bibcode=1980PhLB...97..453B |doi=10.1016/0370-2693(80)90639-5 |df=dmy-all}}</ref> and PLUTO experiments<ref>{{cite journal |author1=Berger, Ch. |display-authors=etal |collaboration=PLUTO collaboration |year=1980 |title=A study of multi-jet events in e<sup>+&nbsp;</sup>e<sup>&minus;</sup> annihilation |journal=[[Physics Letters B]] |volume=97 |issue=3–4 |page=459 |bibcode=1980PhLB...97..459B |doi=10.1016/0370-2693(80)90640-1 |df=dmy-all}}</ref> (see also the review<ref name="SOE"/>). In 1991 a subsequent experiment at the [[LEP]] storage ring at [[CERN]] again confirmed this result.<ref>{{cite journal |author1=Alexander, G. |display-authors=etal |collaboration=[[OPAL detector|OPAL collaboration]] |year=1991 |title=Measurement of three-jet distributions sensitive to the gluon spin in e<sup>+&nbsp;</sup>e<sup>&minus;</sup> Annihilations at √s = 91&nbsp;GeV |journal=[[Zeitschrift für Physik C]] |volume=52 |issue=4 |page=543 |bibcode=1991ZPhyC..52..543A |doi=10.1007/BF01562326 |s2cid=51746005 |url=https://repository.ubn.ru.nl//bitstream/handle/2066/124457/124457.pdf |df=dmy-all}}</ref>

The gluons play an important role in the elementary strong interactions between [[quark]]s and gluons, described by QCD and studied particularly at the electron-proton collider [[HERA]] at DESY. The number and momentum distribution of the gluons in the [[proton]] (gluon density) have been measured by two experiments, [[H1 (particle detector)|H1]] and [[ZEUS]],<ref>{{cite journal |author=Lindeman, L. |collaboration=H1 and ZEUS collaborations |year=1997 |title=Proton structure functions and gluon density at HERA |journal=[[Nuclear Physics B: Proceedings Supplements]] |volume=64 |issue=1 |pages=179–183 |bibcode=1998NuPhS..64..179L |doi=10.1016/S0920-5632(97)01057-8 |df=dmy-all}}</ref> in the years 1996–2007. The gluon contribution to the proton spin has been studied by the [[HERMES experiment]] at HERA.<ref>{{cite web |url=http://www-hermes.desy.de |title=The spinning world at DESY |website=www-hermes.desy.de |access-date=26 March 2018 |df=dmy-all |archive-date=25 May 2021 |archive-url=https://web.archive.org/web/20210525042229/http://www-hermes.desy.de/ |url-status=dead }}</ref> The gluon density in the proton (when behaving hadronically) also has been measured.<ref>{{cite journal |author1=Adloff, C. |display-authors=etal |collaboration=H1 collaboration |year=1999 |title=Charged particle cross sections in the photoproduction and extraction of the gluon density in the photon |journal=[[European Physical Journal C]] |volume=10 |issue=3 |pages=363–372 |arxiv=hep-ex/9810020 |bibcode=1999EPJC...10..363H |doi=10.1007/s100520050761 |s2cid=17420774 |df=dmy-all}}</ref>

[[Color confinement]] is verified by the failure of [[free quark]] searches (searches of fractional charges). Quarks are normally produced in pairs (quark + antiquark) to compensate the quantum color and flavor numbers; however at [[Fermilab]] single production of [[top quark]]s has been shown.{{efn|Technically the single [[top quark]]  production at [[Fermilab]] still involves a pair production, but the quark and antiquark are of different flavors.}}<ref>{{cite web |author=Chalmers, M. |date=6 March 2009 |title=Top result for Tevatron |url=https://physicsworld.com/a/top-result-for-tevatron/ <!-- dead link http://physicsworld.com/cws/article/news/38140 --> |work=[[Physics World]] |access-date=2012-04-02 |df=dmy-all}}</ref> No [[glueball]] has been demonstrated.

[[Deconfinement]] was claimed in 2000 at CERN SPS<ref>{{cite journal |author1=Abreu, M.C. |display-authors=etal |collaboration=NA50 collaboration |year=2000 |title=Evidence for deconfinement of quark and antiquark from the J/Ψ suppression pattern measured in Pb-Pb collisions at the CERN SpS |journal=[[Physics Letters B]] |volume=477 |issue=1–3 |pages=28–36 |bibcode=2000PhLB..477...28A  |doi=10.1016/S0370-2693(00)00237-9 |df=dmy-all|url=https://cds.cern.ch/record/427590 }}</ref> in [[heavy-ion collisions]], and it implies a new state of matter: [[quark–gluon plasma]], less interactive than in the [[Atomic nucleus|nucleus]], almost as in a liquid. It was found at the [[Relativistic Heavy Ion Collider]] (RHIC) at Brookhaven in the years 2004–2010 by four contemporaneous experiments.<ref>{{cite news |author=Overbye, D. |date=15 February 2010 |title=In Brookhaven collider, scientists briefly break a law of nature |url=https://www.nytimes.com/2010/02/16/science/16quark.html |archive-url=https://ghostarchive.org/archive/20220102/https://www.nytimes.com/2010/02/16/science/16quark.html |archive-date=2022-01-02 |url-access=limited |url-status=live |work=[[The New York Times]] |access-date=2012-04-02 |df=dmy-all}}{{cbignore}}</ref> A quark–gluon plasma state has been confirmed at the [[CERN]] Large Hadron Collider (LHC) by the three experiments [[A Large Ion Collider Experiment|ALICE]], [[ATLAS experiment|ATLAS]] and [[Compact Muon Solenoid|CMS]] in 2010.<ref>{{cite press release |date=26 November 2010 |title=LHC experiments bring new insight into primordial universe |url=http://press.cern/press-releases/2010/11/lhc-experiments-bring-new-insight-primordial-universe |publisher=[[CERN]] |access-date=2016-11-20 |df=dmy-all}}</ref>

[[Jefferson Lab]]'s [[Continuous Electron Beam Accelerator Facility]], in [[Newport News, Virginia]],{{efn|[[Jefferson Lab]] is a [[nickname]] for the [[Thomas Jefferson National Accelerator Facility]] in [[Newport News, Virginia]].}} is one of 10&nbsp;[[United States Department of Energy|Department of Energy]] facilities doing research on gluons. The Virginia lab was competing with another facility – [[Brookhaven National Laboratory]], on Long Island, New York – for funds to build a new [[electron-ion collider]].<ref>{{cite news |last1=Nolan |first1=Jim |title=State hopes for big economic bang as Jeff Lab bids for ion collider |url=http://www.richmond.com/news/virginia/government-politics/article_fc5216f3-13e8-5ebe-85a6-64e054358389.html |access-date=19 October 2015 |newspaper=Richmond Times-Dispatch |date=October 19, 2015 |pages=A1, A7 |quote=Those clues can give scientists a better understanding of what holds the universe together. |df=dmy-all}}</ref> In December 2019, the US Department of Energy selected the [[Brookhaven National Laboratory]] to host the [[electron-ion collider]].<ref>{{cite press release |date=9 January 2020 |title=U.S. Department of Energy selects Brookhaven National Laboratory to host major new nuclear physics facility |url=https://www.energy.gov/articles/us-department-energy-selects-brookhaven-national-laboratory-host-major-new-nuclear-physics |publisher=[[United States Department of Energy|DOE]] |access-date=2020-06-01 |df=dmy-all}}</ref>