A quark–gluon plasma (QGP) or quark soup
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 only be produced in 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. Liquid or even near-perfect liquid
flow with almost no frictional resistance
or viscosity was claimed by research teams
at RHIC and LHC's Compact Muon Solenoid detector.
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"
in 2000. In 2010, 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. 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, by colliding relativistically
accelerated gold and other ion species (at
RHIC) or lead (at LHC) with each other or
with protons. Three experiments running on
CERN's Large Hadron Collider (LHC), on the
spectrometers ALICE, 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. A new record breaking
temperature was set by ALICE: A Large Ion
Collider Experiment at CERN on August, 2012
in the ranges of 5.5 trillion (5.5×1012)
kelvin as claimed in their Nature PR.
== 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. 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.
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.
=== 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,
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.
=== 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 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.
=== 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. 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.The important classes of experimental
observations are
Single particle spectra (photons and dileptons)
Strangeness production
Photon and muon rates (and J/ψ melting)
Elliptic flow
Jet quenching
Fluctuations
Hanbury Brown and Twiss effect and Bose–Einstein
correlationsIn 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 ==
In April 2005, formation of quark matter was
tentatively confirmed by results obtained
at Brookhaven National Laboratory's Relativistic
Heavy Ion Collider (RHIC). The consensus of
the four RHIC research groups was that they
had created a quark–gluon liquid of very
low viscosity. However, contrary to what was
at that time still the widespread assumption,
it is yet unknown from theoretical predictions
whether the QCD "plasma", especially close
to the transition temperature, should behave
like a gas or liquid. Authors favoring the
weakly interacting interpretation derive their
assumptions from the lattice QCD calculation,
where the entropy density of quark–gluon
plasma approaches the weakly interacting limit.
However, since both energy density and correlation
shows significant deviation from the weakly
interacting limit, it has been pointed out
by many authors that there is in fact no reason
to assume a QCD "plasma" close to the transition
point should be weakly interacting, like electromagnetic
plasma (see, e.g.,). That being said, systematically
improvable perturbative QCD quasiparticle
models do a very good job of reproducing the
lattice data for thermodynamical observables
(pressure, entropy, quark susceptibility),
including the aforementioned "significant
deviation from the weakly interacting limit",
down to temperatures on the order of 2 to
3 times the critical temperature for the transition.
== See also
