Quark-gluon plasma

A quark-gluon plasma (QGP) is a phase of quantum chromodynamics (QCD) which exists at extremely high temperature and density. It is believed to have existed during the first 20 or 30 microseconds after the universe came into existence in the Big Bang. Experiments at CERN's Super Proton Synchrotron first tried to create the QGP in the 1980s and 1990s, and may have been partially successful. Currently, experiments at Brookhaven National Laboratory's Relativistic Heavy Ion Collider (RHIC) are continuing this effort CERN's new experiment, ALICE [http://aliceinfo.cern.ch/index.html, will start soon (around 2007-2008) at the Large Hadron Collider (LHC).

General Introduction

The QGP 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 make a much larger mass of quarks and gluons ^*. 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; in other words, Coulomb's Law is modified to yield a distance-dependent charge. In a QGP, the colour charge of the quarks and gluons are screened. The QGP has other analogies with a normal plasma. There are also dissimilarities due to the fact that the colour charge is non-Abelian, whereas the electric charge is Abelian.

How is this studied theoretically?

One consequence of this difference is that the colour 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 and the AdS/CFT correspondence. The transition temperature (approximately 170 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.

How is this created in the lab?

The QGP can be created by heating matter up to a temperature of 170 MeV(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. Gold and lead nuclei have been used to do this at CERN and BNL. The resulting hot volume is called a fireball. 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 to the QGP phase is about 170 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 is 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 found recently that some mesons built from heavy quarks (such as the charm quark) 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.

The Experimental Situation

Unfortunately, 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

Single particle spectra
Strangeness production
Photon and muon rates (and J/ψ melting)
Elliptic flow
Jet quenching
Fluctuations
Hanbury-Brown and Twiss effect and correlations

For more details, see the web pages of the RHIC experiments ^*.

News

April 2005: Formation of quark matter has been tentatively confirmed by results obtained at Brookhaven National Laboratory's Relativistic Heavy Ion Collider (RHIC). The consensus of the four RHIC research groups is that they have created a quark-gluon liquid of very low viscosity. However, contrary to 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 fluid.

References and external links

The Relativistic Heavy Ion Collider at Brookhaven National Laboratory
The Alice Experiment at CERN
The Indian Lattice Gauge Theory Initiative
Quark matter reviews: 2004 theory, 2004 experiment
Lattice reviews: 2003, 2005
BBC article mentioning Brookhaven results
Physics News Update article on the quark-gluon liquid, with links to preprints

Quark matter | Plasma physics | phases of matter

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