Quark–gluon plasma

Quark–gluon plasma (QGP or quark soup) is an interacting localized assembly of quarks and gluons in chemical equilibrium and local thermal equilibrium. The word plasma signals that free color charges are allowed. In normal matter quarks are confined; in the QGP quarks are deconfined. Quark–gluon plasma (QGP) occurs at energy densities high enough to melt the protons and neutrons that make up the nuclei of normal matter. It is a very low viscosity liquid composed of the elementary particles, quarks and gluons, a state of matter new to physics when it was discovered. Quark–gluon plasma is studied to understand the characteristics of the universe at about 20 μs after the Big Bang, when the universe was extremely hot and dense. Experimental groups use ultrarelativistic beams of ions colliding with other ions or protons to create this plasma in particle accelerators.

History Theories predicting the existence of quark–gluon plasma were developed in the late 1970s and early 1980s. The discovery of color confinement and asymptotic freedom properties of quantum chromodynamics lead to the realization that quarks would undergo a phase transition at high density. Using an analogy with electromagnetic plasma, in 1978 E V. Shuryak used the term "hadronic plasma" for matter much more dense than atomic nuclei, matter in which hadrons merge and the quarks act collectively. In his next paper he used "quark-gluon plasma", a name that stuck. In 2000, CERN issued a press release reporting evidence for a new state of matter based on Pb-Pb heavy ion collision studies. The evidence was consistent with "many of the characteristics of the theoretically predicted quark-gluon plasma". A competing team at Relativistic Heavy Ion Collider (RHIC) characterized the CERN results as "circumstantial" and suggested that the experiments yielded little information about the properties of this new state. A series of Au-Au collision studies from the RHIC published in 2005 showed that the collisions produce something like a liquid in contrast to early theoretical models.

Role in Standard Model QCD is one part of the modern theory of particle physics called the Standard Model. Other parts of this theory deal with electroweak interactions and neutrinos. The theory of electrodynamics has been tested and found correct to a few parts in a billion. The 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. 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 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: the first hundred microseconds or so. It is crucial to the physics goals of a new generation of observations of the universe (WMAP and its successors). It is also of relevance to Grand Unification Theories which seek to unify the three fundamental forces of nature (excluding gravity).

Occurrence The 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 10−10–10−6 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 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 electromagnetic plasma A plasma is matter in which charges are screened due to the presence of other mobile charges. For example: Coulomb's law is suppressed by the screening to yield a distance-dependent charge, Q → Q e − r / α {\displaystyle Q\rightarrow Qe^{-r/\alpha }} , 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, 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 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. The transition temperature (approximately 175 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.

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