# Quark–gluon plasma

Quark–gluon plasma is in the high-density, high-temperature part on this conjectured phase diagram for strong-interacting matter.[1]

A quark–gluon plasma (QGP) or quark soup[2] 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-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 atoms by electrostatic forces at ambient conditions, can move freely. Artificial quark matter, which has been produced at Brookhaven National Laboratory's Relativistic Heavy Ion Collider and CERN's Large Hadron Collider, can be produced in only 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 rays 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 stars would consist of relatively cold, but dense quark matter. It is believed that up to a few milliseconds 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.[3] Liquid or even near-perfect liquid flow with almost no frictional resistance or viscosity was claimed by research teams at RHIC[4] and LHC's Compact Muon Solenoid detector.[5] 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 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) first tried to create the QGP in the 1980s and 1990s: the results led CERN to announce indirect evidence for a "new state of matter"[6] in 2010. In 2000, 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.[7] Current experiments (2017) at the Brookhaven National Laboratory's Relativistic Heavy Ion Collider (RHIC) on Long Island (NY, USA) and at CERN's recent Large Hadron Collider near Geneva (Switzerland) are continuing this effort,[8][9] by colliding relativistically accelerated gold and other ion species (at RHIC) or lead (at LHC) with each other or with protons.[9] Three experiments running on CERN's Large Hadron Collider (LHC), on the spectrometers ALICE,[10] ATLAS and 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.[11] 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 (5.5×1012) kelvin as claimed in their Nature PR.[12]

## General introduction

Quark–gluon plasma is a state of matter in which the elementary particles that make up the hadrons of baryonic matter are freed of their strong attraction for one another under extremely high energy densities. These particles are the quarks and gluons that compose baryonic matter.[13] In normal matter quarks are confined; in the QGP quarks are deconfined. In classical QCD quarks are the fermionic components of hadrons (mesons and baryons) while the gluons are considered the bosonic 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.

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 dense fluid.[14] 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.[15][16]

### Relation to normal 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, ${\displaystyle Q\rightarrow Qe^{-r/\alpha }}$ , 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.[17][18] 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.

### Production

The QGP can be created by heating matter up to a temperature of 2×1012 K, which amounts to 175 MeV per particle. This can be accomplished by colliding two large nuclei at high energy (note that 175 MeV is not the energy of the colliding beam). Lead and gold nuclei have been used for such collisions at CERN SPS and BNL RHIC, respectively. The nuclei are accelerated to ultrarelativistic speeds (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.

### How the QGP 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 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).

## Expected properties

### Thermodynamics

The cross-over temperature from the normal hadronic to the QGP phase is about 175 MeV. 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 three-dimensional Ising model. The phenomena involved correspond to an energy density of a little less than GeV/fm3. For 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 and various quark number susceptibilities are currently being computed.

### Flow

The equation of state is an important input into the flow equations. The speed of sound is currently under investigation in lattice computations. The mean free path of quarks and gluons has been computed using perturbation theory as well as string theory. Lattice computations have been slower here, although the first computations of transport coefficients have recently been concluded. 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 recent development that is still in an active stage.

### Excitation spectrum

The study of thermodynamic and flow properties indicate that the assumption of QGP consisting almost entirely of free quarks and gluons is an over-simplification. Many ideas are currently being developed and will be put to test in the near future. It has been hypothesized recently that some mesons built from heavy quarks do not dissolve until the temperature reaches about 350 MeV. This has led to speculation that many other kinds of bound states may exist in the plasma. Some static properties of the plasma (similar to the Debye screening length) constrain the excitation spectrum.

### 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 particles are condensed into some kind of glassy (or amorphous) state, below the genuine transition between the confined state and the plasma liquid.[19] This would be analogous to the formation of metallic glasses, or amorphous alloys of them, below the genuine onset of the liquid metallic state.

## Experimental situation

Those forms of the QGP that are easiest to compute are not those that are easiest to verify experimentally. While the balance of evidence points towards the QGP being the origin of the detailed properties of the fireball produced at SPS (CERN), in the RHIC and at LHC, this is the main barrier which prevents experimentalists from declaring a sighting of the QGP.[20]

The important classes of experimental observations are

In short, a quark–gluon plasma flows like a splat of liquid, and because it's not "transparent" with respect to quarks, it can attenuate 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 that gluons (particles mediating the strong force) collide and produce an excess of the heavy (i.e. high-energy) strange quarks. Whereas, if the QGP didn't exist and there was a pure collision, the same energy would be converted into a nonequilibrium mixture containing even heavier quarks such as charm quarks or bottom quarks.

## Formation of quark matter

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 nuclei. In the first works devoted to the formation of a quantum-gluon plasma in collisions with relativistic nuclear materials, it was discovered that at the temperature of T ≈ 170 MeV and energy density of ≈ 1 GeV / fm^3, the first-order phase transition occurs, at which the density and temperature of the medium change sharply.

### 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^(–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 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.

### Production of quark-gluon plasma in accelerators

Quark-gluon plasma experiments are conducted at the largest accelerators at the highest possible energies of colliding beams of relativistic nuclei.

The table shows the collision energies per nucleon of the colliding nuclei achieved at the Bevatron (Billions of eV Synchrotron), AGS (Alternating Gradient Synchrotron) and RHIC (Relative Heavy Ion Collider) accelerators at the BNL (Brookhaven National Laboratory) and at the LHC accelerator ( Large Hardon Collider) at CERN. [3]

Some results of the experiments:

· Experimental data obtained at the RHIC and LHC showed that the collision of heavy nuclei cannot be considered as a simple additive set of pp-collisions.

· In collisions of heavy nuclei, new previously unknown collective properties of the quark-gluon medium are manifested.

· The resulting quark-gluon medium in its properties resembles a superconducting liquid with a low viscosity coefficient.

· A comparison of the data from the STAR and ALICE collaborations shows that the lifetime of the quark-gluon medium formed in the collision of lead nuclei in the ALICE collaboration is almost two times higher than the corresponding result obtained by the STAR collaboration and amounts to 10–11 Fm/s.

Schematic representation of the interaction region formed in the first moments after the collision of heavy ions with high energies in the accelerator. [4]

· The formation of an elliptical flow and the quenching of jets in a dense quark-gluon medium are confirmed.

To study the properties of the quark-gluon medium further, more detailed information is needed on the deconfinement mechanism in the quark-gluon medium, the hadronization mechanism, quenching of jets in the quark-gluon plasma, angular distributions of products, and especially on the formation of hadrons containing heavy quarks and antiquarks.[21]

### Quark-gluon plasma density

The central issue of the formation of a quark-gluon plasma remains as follows: whether energy density can be achieved in nucleus-nucleus collisions and how does it evolve over time? The answer to this question depends on how much energy each nucleon loses, of accelerated nuclei in a collision of beams.

Usually, there are three different stages of nuclear collision distinguished:

· The maximum density is reached at the time of complete overlapping of the colliding nuclei.

· The maximum energy density transferred to particles born in a fireball.

· The maximum energy density at the moment of the local thermalization of a quark-gluon plasma.

### Jet quenching effect at RHIC energies

One of the most striking physical effects obtained at RHIC energies is the effect of quenching jets. 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 ≥ 3–6 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 dE / dx = 1 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 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.[22]

Adcox K. et al. (PHENIX collab.) // Nucl. Phys. A 2005. V.757, p.184-283

Хи N.. Kaneta M. II Nucl. Phys. 2002. V.306. P. 182301

Adler S.S. el at. II Phys. Rev. Lett. 2005. V.94. P. 122302

Adams J. et al // Phys. Rev. Lett. 2003. V.91. P. 072304, Adler С et al // Phys. Rev. Lett. 2003. V.90. P. 082302

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