# Relationship between plasma physics and quark gluon plasma

To what extent do the ideas common in modern plasma physics, such as magnetohydrodynamics, cold plasma models, common types of plasma waves, Maxwell's Equations, etc, relate to the study of quark gluon plasma? I have heard that quark gluon plasma is "not really a plasma", and I am under the impression that it is more usually studied with QFT and particle physics techniques as opposed to plasma physics. How true is this? Is there any way to study quark gluon plasma using plasma physics?

• As far as I know, no one can conclusively state how the QGP ought to behave (as a "classic" plasma or something else). For example, the speed of sound in the QGP has yet to be tied down. I believe QCD is the primary method of investigating the QGP, not QFT; but I'm not 100% sure on that note. – Kyle Kanos Sep 22 '14 at 21:59
• Then what is the motivation for calling it a plasma in the first place? – Physics_Plasma Sep 22 '14 at 23:41
• Actually, it was called a "quark soup" first. Given the high temperatures and densities required for it to exist, I think plasma is a more apt descriptor than liquid or gas; though none of them are truly correct (probably due to the fact of the other things involved, such as color charge). – Kyle Kanos Sep 23 '14 at 0:46
• @KyleKanos QCD IS a QFT as everybody knows ... – Dilaton Sep 23 '14 at 20:34
• @Dilaton: Yes, QCD is QFT, but QFT is not necessarily QCD, as everybody knows. It usually pays to be specific. – Kyle Kanos Sep 23 '14 at 20:36

Quark Gluon Plasma was originally referred as a "plasma" because some of the physics relating to "color charge" are very similar to the physics relating to a plasma of Coulomb charges.

Quarks have "color charge", which are referred as red, green, and blue. Three colors will yield "white" and thus colorlesness/color-neutrality, analogously to how a positive and negative coulumb charge will yield charge neutrality.

One of the important plasma-like properties of QGP is debye screening. Analogously to how charges can only influence other charges in the vicinity because there is global charge neutrality in the plasma, QGP has global color neutrality and will screen the influence of color charges. This is one of the reasons why QGP's description of quarks as free particles works: high momentum component of color is suppressed by asymptotic freedom, and low momentum component is screened by the a debye effect.

QGP also has something analogous to plasma frequency opacity, where gluons (the color charge virtual particles) cannot propagate through the QGP if their energy is too low.

Finally QGP is a coulumb plasma in itself as well. quarks have coulumb charges so the MHD equations apply to them too. This might prove important in the context of neutron stars, where QGP could exist in the cores. A neutron star has a magnetic field in the order of $10^{12}$ G, so the QGP in the center would experience magnetic pressure.