Editing Relativistic Heavy Ion Collider (section)

==Current results==
{{For|a complementary discussion|quark–gluon plasma}}

For the experimental objective of creating and studying the quark–gluon plasma, RHIC has the unique ability to provide baseline measurements for itself. This consists of both the lower energy and also lower [[mass number]] projectile combinations that do not result in the density of 200&nbsp;GeV Au&nbsp;+&nbsp;Au collisions, like the p&nbsp;+&nbsp;p and d&nbsp;+&nbsp;Au collisions of the earlier runs, and also Cu&nbsp;+&nbsp;Cu collisions in Run-5.

Using this approach, important results of the measurement of the hot QCD matter created at RHIC are:<ref>
{{cite journal
 |author1=T. Ludlam
 |author2=L. McLerran
 |year=2003
 |title=What Have We Learned from the Relativistic Heavy Ion Collider?
 |journal=[[Physics Today]]
 |volume=56 |issue=10 |page=48
 |bibcode=2003PhT....56j..48L
 |doi=10.1063/1.1629004
|doi-access=free
 }}</ref>

* '''Collective anisotropy, or [[elliptic flow]].''' The major part of the particles with lower [[momentum|momenta]] is emitted following an angular distribution <math>dn/d\phi \propto 1 + 2 v_2(p_\mathrm{T}) \cos 2 \phi</math> (''p''<sub>T</sub> is the transverse momentum, <math>\phi</math> angle with the reaction plane). This is a direct result of the elliptic shape of the nucleus overlap region during the collision and [[hydrodynamics|hydrodynamical]] property of the matter created.
* '''[[Jet quenching]].''' In the heavy ion collision event, scattering with a high transverse ''p''<sub>T</sub> can serve as a probe for the hot QCD matter, as it loses its energy while traveling through the medium. Experimentally, the quantity ''R<sub>AA</sub>'' (''A'' is the mass number) being the quotient of observed jet yield in ''A''&nbsp;+&nbsp;''A'' collisions and ''N''<sub>bin</sub>&nbsp;×&nbsp;yield in p&nbsp;+&nbsp;p collisions shows a strong damping with increasing ''A'', which is an indication of the new properties of the hot QCD matter created.
* '''[[Color glass condensate]] saturation.''' The Balitsky–Fadin–Kuraev–Lipatov (BFKL) dynamics<ref>
{{cite journal
 |author=L. N. Lipatov
 |year=1976
 |title=Reggeization of the vector meson and the vacuum singularity in nonabelian gauge theories
 |journal=[[Soviet Journal of Nuclear Physics]]
 |volume=23 |page=338
}}</ref> which are the result of a resummation of large logarithmic terms ln''(1/x)'' for [[deep inelastic scattering]] with small Bjorken-''x'', saturate at a unitarity limit <math>Q_s^2 \propto \langle N_\mathrm{part} \rangle/2</math>, with ''N''<sub>part</sub>/2 being the number of participant nucleons in a collision (as opposed to the number of binary collisions). The observed charged multiplicity follows the expected dependency of <math>n_\mathrm{ch}/A \propto 1/\alpha_s(Q_s^2)</math>, supporting the predictions of the [[color glass condensate]] model. For a detailed discussion, see e.g. [[Dmitri Kharzeev]] ''et al.'';<ref>
{{Cite journal
 |author1=D. Kharzeev
 |author2=E. Levin
 |author3=L. McLerran
 |year=2003
 |title=Parton saturation and ''N''<sub>part</sub> scaling of semi-hard processes in QCD
 |journal=[[Physics Letters B]]
 |volume=561 |issue=1–2
 |pages=93–101
 |arxiv=hep-ph/0210332
 |bibcode=2003PhLB..561...93K
 |doi=10.1016/S0370-2693(03)00420-9
|s2cid=17978566
 }}</ref> for an overview of color glass condensates, see e.g. Iancu & Venugopalan.<ref>
{{cite book
 |author1=E. Iancu
 |author2=R. Venugopalan
 |year=2003
 |chapter=The Color Glass Condensate and High Energy Scattering in QCQ
 |editor1=R. C. Hwa
 |editor2=X.-N. Wang
 |title=Quark–Gluon Plasma 3
 |url=https://archive.org/details/quarkgluonplasma03hwar
 |url-access=limited
 |page=[https://archive.org/details/quarkgluonplasma03hwar/page/n256 249]
 |publisher=[[World Scientific]]
 |arxiv=hep-ph/0303204
 |doi=10.1142/9789812795533_0005
 |isbn=978-981-238-077-7
|s2cid=117826241
 }}</ref>
* '''Particle ratios.''' The particle ratios predicted by statistical models allow the calculation of parameters such as the temperature at chemical freeze-out ''T''<sub>ch</sub> and hadron chemical potential <math>\mu_B</math>. The experimental value ''T''<sub>ch</sub> varies a bit with the model used, with most authors giving a value of 160&nbsp;MeV&nbsp;<&nbsp;''T''<sub>ch</sub>&nbsp;<&nbsp;180&nbsp;MeV, which is very close to the expected QCD [[phase transition]] value of approximately 170&nbsp;MeV obtained by lattice QCD calculations (see e.g. Karsch<ref>
{{Cite book
 |author1=F. Karsch
 |year=2002
 |chapter=Lattice QCD at High Temperature and Density
 |editor1=W. Plessas
 |editor2=L. Mathelitsch
 |title=Lectures on Quark Matter
 |series=[[Lectures Notes in Physics]]
 |volume=583 |issue=2002 |pages=209–249
 |arxiv=hep-lat/0106019
 |bibcode=2002LNP...583..209K
 |isbn=978-3-540-43234-0
|doi=10.1007/3-540-45792-5_6
 |s2cid=42124100
 }}</ref>).

While in the first years, theorists were eager to claim that RHIC has discovered the quark–gluon plasma (e.g. Gyulassy & McLarren<ref>
{{Cite journal
 |author1=M. Gyulassy
 |author2=L. McLerran
 |year=2005
 |title=New Forms of QCD Matter Discovered at RHIC
 |journal=[[Nuclear Physics A]]
 |volume=750 |pages=30–63
 |arxiv=nucl-th/0405013
 |bibcode=2005NuPhA.750...30G
 |doi=10.1016/j.nuclphysa.2004.10.034
|s2cid=14175774
 }}</ref>), the experimental groups were more careful not to jump to conclusions, citing various variables still in need of further measurement.<ref>
{{cite web
 |author=K. McNulty Walsh
 |date=2004
 |url=http://www.bnl.gov/discover/Spring_04/RHIC_1.asp
 |title=Latest RHIC Results Make News Headlines at Quark Matter 2004
 |work=Discover Brookhaven
 |volume=2 |issue=1 |pages=14&ndash;17
 |archive-url=https://web.archive.org/web/20141011130758/http://www.bnl.gov/discover/Spring_04/RHIC_1.asp
 |archive-date=2014-10-11
}}</ref> The present results shows that the matter created is a fluid with a viscosity near the quantum limit, but is unlike a weakly interacting plasma (a widespread yet not quantitatively unfounded belief on how quark–gluon plasma looks).

A recent overview of the physics result is provided by the [http://www.phenix.bnl.gov/WWW/info/comment/ RHIC Experimental Evaluations 2004] {{Webarchive|url=https://web.archive.org/web/20170202030039/http://www.phenix.bnl.gov/WWW/info/comment/ |date=2017-02-02 }}, a community-wide effort of RHIC experiments to evaluate the current data in the context of implication for formation of a new state of matter.<ref>
{{Cite journal
 |author1=I. Arsene
 |display-authors=etal
 |collaboration=BRAHMS collaboration
 |title=Quark Gluon Plasma an Color Glass Condensate at RHIC? The perspective from the BRAHMS experiment
 |journal=[[Nuclear Physics A]]
 |volume=757 |issue=1–2
 |pages=1–27
 |year=2005
 |arxiv=nucl-ex/0410020
 |bibcode=2005NuPhA.757....1A
 |doi=10.1016/j.nuclphysa.2005.02.130
|s2cid=204924453
 }}</ref><ref>
{{Cite journal
 |author1=K. Adcox
 |display-authors=etal.
 |collaboration=PHENIX Collaboration
 |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]]
 |volume=757 |issue=1–2
 |pages=184–283
 |arxiv=nucl-ex/0410003
 |bibcode=2005NuPhA.757..184A
 |doi=10.1016/j.nuclphysa.2005.03.086
|s2cid=119511423
 }}</ref><ref>
{{Cite journal
 |author1=B. B. Back 
 |display-authors=etal
 |collaboration=PHOBOS Collaboration
 |year=2005
 |title=The PHOBOS Perspective on Discoveries at RHIC
 |journal=[[Nuclear Physics A]]
 |volume=757 |issue=1–2
 |pages=28–101
 |arxiv=nucl-ex/0410022
 |bibcode=2005NuPhA.757...28B
 |doi=10.1016/j.nuclphysa.2005.03.084
}}</ref><ref>
{{Cite journal
 |author1=J. Adams
 |collaboration=STAR Collaboration
 |display-authors=etal
 |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]]
 |volume=757 |issue=1–2
 |pages=102–183
 |arxiv=nucl-ex/0501009
 |bibcode=2005NuPhA.757..102A
 |doi=10.1016/j.nuclphysa.2005.03.085
|s2cid=119062864
 }}</ref> These results are from the first three years of data collection at RHIC.

New results were published in ''[[Physical Review Letters]]'' on February 16, 2010, stating the discovery of the first hints of [[symmetry transformation]]s, and that the observations may suggest that bubbles formed in the aftermath of the collisions created in the RHIC may break [[Parity (physics)|parity symmetry]], which normally characterizes [[color charge|interactions]] between [[quark]]s and [[gluon]]s.<ref name=symmetry>
{{cite web
 |author=K. Melville
 |date=16 February 2010
 |title=Mirror Symmetry Broken at 7 Trillion Degrees
 |url=http://www.scienceagogo.com/news/20100115233339data_trunc_sys.shtml
 |work=Science a Go Go
 |access-date=2010-02-16
}}</ref><ref>
{{cite news
 |author=D. Overbye
 |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
 |work=[[The New York Times]]
 |access-date=2010-02-16
}}</ref>

The RHIC physicists announced new temperature measurements for these experiments of up to 4 trillion kelvins, the highest temperature ever achieved in a laboratory.<ref>
{{cite web
 |date=15 February 2010
 |title=Perfect Liquid Hot Enough to be Quark Soup
 |url=https://www.bnl.gov/newsroom/news.php?a=111074
 |publisher=[[Brookhaven National Laboratory]]
 |access-date=2017-01-24
}}</ref> It is described as a recreation of the conditions that existed during the [[birth of the Universe]].<ref name=temperature>
{{cite web
 |author=D. Vergano
 |date=16 February 2010
 |title=Scientists Re-create High Temperatures from Big Bang
 |url=https://www.usatoday.com/tech/science/2010-02-16-RHIC16_ST_N.htm
 |work=[[USA Today]]
 |access-date=2010-02-16
}}</ref>