Editing AdS/CFT correspondence (section)

=== Nuclear physics ===

{{Main|AdS/QCD}}

One [[physical system]] that has been studied using the AdS/CFT correspondence is the quark–gluon plasma, an exotic [[state of matter]] produced in [[particle accelerator]]s. This state of matter arises for brief instants when heavy [[ions]] such as [[gold]] or [[lead]] [[Atomic nucleus|nuclei]] are collided at high energies. Such collisions cause the [[quarks]] that make up atomic nuclei to [[deconfinement|deconfine]] at temperatures of approximately two [[1,000,000,000,000|trillion]] [[kelvin]]s, conditions similar to those present at around 10<sup>−11</sup>&nbsp;seconds after the [[Big Bang]].{{sfn|ps=|Zwiebach|2009|p=559}}

The physics of the quark–gluon plasma is governed by quantum chromodynamics, but this theory is mathematically intractable in problems involving the quark–gluon plasma.{{refn|More precisely, one cannot apply the methods of perturbative quantum field theory.}} In an article appearing in 2005, [[Đàm Thanh Sơn]] and his collaborators showed that the AdS/CFT correspondence could be used to understand some aspects of the quark–gluon plasma by describing it in the language of string theory.{{sfn|ps=|Merali|2011|p=303}}{{sfn|ps=|Kovtun|Son|Starinets|2005}} By applying the AdS/CFT correspondence, Sơn and his collaborators were able to describe the quark gluon plasma in terms of black holes in five-dimensional spacetime. The calculation showed that the ratio of two quantities associated with the quark–gluon plasma, the [[shear viscosity]] ''η'' and volume density of [[entropy]] ''s'', should be approximately equal to a certain [[universal constant]]:
: <math>\frac{\eta}{s}\approx\frac{\hbar}{4\pi k}</math>
where ''ħ'' denotes the [[reduced Planck constant]] and ''k'' is the [[Boltzmann constant]].{{sfn|ps=|Zwiebach|2009|p=561}}{{sfn|ps=|Kovtun|Son|Starinets|2005}} In addition, the authors conjectured that this universal constant provides a [[lower bound]] for ''η''/''s'' in a large class of systems. In an experiment conducted at the [[Relativistic Heavy Ion Collider]] at [[Brookhaven National Laboratory]],  the experimental result in one model was close to this universal constant but it was not the case in another model.{{sfn|ps=|Luzum|Romatschke|2008|loc=Part IV. C}}

Another important property of the quark–gluon plasma is that very high energy quarks moving through the plasma are stopped or "quenched" after traveling only a few [[femtometre]]s. This phenomenon is characterized by a number {{overset|lh=0.3|^|''q''}} called the [[jet quenching]] parameter, which relates the energy loss of such a quark to the squared distance traveled through the plasma. Calculations based on the AdS/CFT correspondence give the estimated value {{nowrap|{{overset|lh=0.3|^|''q''}} ≈ {{val|4|u=GeV<sup>2</sup>/fm}}}}, and the experimental value of {{overset|lh=0.3|^|''q''}} lies in the range {{val|5|-|15|u=GeV<sup>2</sup>/fm}}.{{sfn|ps=|Zwiebach|2009|p=561}}