Quark–gluon plasma or QGP is an interacting localized assembly
of quarks and gluons at thermal (local kinetic) and (close to)
chemical (abundance) equilibrium. The word plasma signals that
free color charges are allowed. In a 1987-summary, Léon van
Hove pointed out the equivalence of the three terms: quark
gluon plasma, quark matter and a new state of matter.[2] Since
the temperature is above the Hagedorn temperature—and thus above
the scale of light u,d-quark mass—the pressure exhibits the
relativistic Stefan-Boltzmann format governed by fourth power of
temperature and many practically mass free quark and gluon constituents.
We can say that QGP emerges to be the new phase of strongly interacting
matter which manifests its physical properties in terms of nearly free
dynamics of practically massless gluons and quarks. Both quarks and gluons,
must be present in conditions near chemical (yield) equilibrium with their
colour charge open for a new state of matter to be referred to as QGP.

Quark–gluon plasma filled the entire Universe before matter was created.
Theories predicting the existence of quark–gluon plasma were developed in
the late 1970s and early 1980s.[3] Discussions around heavy ion experimentation
followed suit[4][5][6][7][8] and the first experiment proposals were put forward
at CERN[9][10][11][12][13][14] and BNL[15][16] in the following years.
Quark–gluon plasma[17][18] was detected for the first time in the laboratory at
CERN in the year 2000.[19][20][21]

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.[22] 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.

We study quark–gluon plasma to recreate and understand the high energy density
conditions prevailing in the Universe when matter formed from elementary degrees
of freedom (quarks, gluons) at about 20μs after the Big Bang. Experimental groups
are probing over a ‘large’ distance the (de)confining quantum vacuum structure,
the present day relativistic æther, which determines prevailing form of matter and
laws of nature. The experiments give insight to the origin of matter and mass: the
matter and antimatter is created when the quark–gluon plasma ‘hadronizes’ and the
mass of matter originates in the confining vacuum structure.[19]

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.[23] 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).

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 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.

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 }}{\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).

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.[25][26]
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 of QGP in the laboratory is achieved by colliding heavy atomic nuclei
(called heavy ions as in an accelerator atoms are ionized) at relativistic energy
in which matter is heated well above the Hagedorn temperature TH= 150 MeV per particle,
which amounts to a temperature exceeding 1.66×1012 K. 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.

There is an overwhelming evidence for production of quark–gluon plasma in relativistic
heavy ion collisions.[27][28][29][30][31]

The important classes of experimental observations are

Strangeness production
Elliptic flow
Jet quenching
J/ψ melting
Hanbury Brown and Twiss effect and Bose–Einstein correlations
Single particle spectra (photons and dileptons)

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 1 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.

The discovery of the perfect liquid was a turning point in physics. Experiments at
RHIC have revealed a wealth of information about this remarkable substance, which
we now know to be a QGP.[32] Nuclear matter at "room temperature" is known to behave
like a superfluid. When heated the nuclear fluid evaporates and turns into a dilute
gas of nucleons and, upon further heating, a gas of baryons and mesons (hadrons).
At the critical temperature, TH, the hadrons melt and the gas turns back into a liquid.
RHIC experiments have shown that this is the most perfect liquid ever observed in
any laboratory experiment at any scale. The new phase of matter, consisting of
dissolved hadrons, exhibits less resistance to flow than any other known substance.
The experiments at RHIC have, already in 2005, shown that the Universe at its
beginning was uniformly filled with this type of material—a super-liquid—which once
the Universe cooled below TH evaporated into a gas of hadrons. Detailed measurements
show that this liquid is a quark–gluon plasma where quarks, antiquarks and gluons
flow independently.[33]

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 so 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 non-equilibrium mixture containing even
heavier quarks such as charm quarks or bottom quarks.[34][35]

The equation of state is an important input into the flow equations. The speed
of sound (speed of QGP-density oscillations) is currently under investigation
in lattice computations.[36][37][38] 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 been concluded.[39][40] 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 active research area.[41][42][43]