Everything about Quark-gluon Plasma totally explained
A
quark-gluon plasma (QGP) is a
phase of
quantum chromodynamics (QCD) which exists at extremely high
temperature and/or
density. This phase consists of (almost) free
quarks and
gluons which are the basic building blocks of matter. 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 the discovery of a "new state of matter" in 2000. Currently, experiments at
Brookhaven National Laboratory's
Relativistic Heavy Ion Collider (RHIC) are continuing this effort. Three new experiments running on
CERN's
Large Hadron Collider (LHC),
ALICE,
ATLAS and
CMS, will continue studying properties of QGP.
General Introduction
The quark-gluon plasma contains
quarks and
gluons, just as normal (
hadronic) matter does. The difference between these two phases of
QCD is the following: in normal matter each quark either pairs up with an anti-
quark to form a
meson or joins with two other quarks to form a
baryon (such as the
proton and the
neutron). In the QGP, by contrast, these mesons and baryons lose their identities and dissolve into a fluid of quarks and gluons
(External Link
). In normal matter quarks are
confined; in the QGP quarks are
deconfined.
Why is this a plasma?
A
plasma is matter in which
charges are
screened due to the presence of other mobile charges; for example:
Coulomb's Law is modified to yield a distance-dependent charge. 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 due to the fact that the
color charge is non-
abelian, whereas the
electric charge is
abelian. Note that outside a finite volume of QGP the color electric field isn't screened, so that volume of QGP must still be color-neutral. It will therefore, like a nucleus, have integer electric charge.
How is this studied theoretically?
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 is a new interesting conjecture allowing insights in QGP.
How is this created in the lab?
The QGP can be created by heating matter up to a
temperature of 2×10
12 kelvins, which amounts to 175
MeV per particle (note that this isn't the colliding beam's energy). This can be done in the lab by colliding two large nuclei at high energy.
Lead and
gold nuclei have been used to do this at
CERN SPS and
BNL RHIC, respectively. The nuclei are accelerated to ultrarelativistic speeds and slammed into each other while Lorentz contracted. They largely pass through each other, but a resulting hot volume called a
fireball is created after the collision. Once created, this fireball is expected to expand under its own
pressure, and cool while expanding. By carefully studying this flow, experimentalists hope to put the theory to test.
How does this fit 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 trillion. The
theory of weak interactions has been tested and found correct to a few parts in a thousand. Perturbative aspects of QCD have been tested to a few percents. In contrast, non-perturbative aspects of QCD have barely been tested. The study of the QGP 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. While this may seem esoteric, this is crucial to the physics goals of a new generation of observations of the universe (
WMAP and its successors).
Expected Properties
Thermodynamics
The cross-over temperature from the normal hadronic to the QGP phase is about 175
MeV,
corresponding 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's still in an active stage.
Excitation spectrum
Does the QGP really contain (almost) free quarks and gluons? The study of thermodynamic and flow properties would indicate that this is an over-simplification. Many ideas are currently being evolved and will be put to test in the near future.
It has been hypothesized recently that some mesons built from heavy quarks (such as the
charm quark) don't 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.
The Experimental Situation
Those aspects of the QGP which are easiest to compute are not the ones which are the easiest to probe in experiments. While the balance of evidence points towards the QGP being the origin of the detailed properties of the fireball produced in the
RHIC, this is the main barrier which prevents experimentalists from declaring a sighting of the QGP.
The important classes of experimental observations are
For more details, see the web pages of the
RHIC experiments
(External Link).
Further Information
Get more info on 'Quark-gluon Plasma'.
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