Some Questions about QGP

July 2004

Basic Question

• Has Quark-Gluon Plasma been observed?

or

• When and How will it be observed?

The answer depends on the definition.

A proper definition should catch the essential points.

Question 1: What is Plasma ?

2 kinds of plasma

• EM Plasma

• QG Plasma

T - ρplot

NormalMatter

Nor-malMat-ter

Plasma

Plasma

In normal matter Charge is bounded Colour is confined

In Plasma Charge and Colour can move freely

EM Plasma

Definition:

Consists of a collection of free-moving electrons and ions

Ions and Electrons move independently.

Some Examples of EM Plasma

Tokamak plasma in actionPrinceton Plasma Physics Laboratory

Nebula M1-67 a massive stellar wind NASA

High-intensity plasma arc lamp Courtesy of OSRAM Sylvania, Danvers, MA

Atmospheric PlasmaCourtesy of J. ElstonLitmas

No (local) equilibrium is demandedfor a plasma.

Some of these plasmas are in or near thermal equilibrium, but some are not.

Question 2: Why RHIC ?

Workshop on BeV/n collisions of heavy ions – how and why,

Bear Mountain, 1974

T.D.Lee & G. C. Wick

Lee emphasized, whether the vacuum is a medium whose properties one could change; "we should investigate," he pointed out, “ ... by distributing high energy or high nucleon density over a relatively large volume."

If in this way one could restore broken symmetries of the vacuum, then it might be possible to create abnormal dense states of nuclear matter

Our ultimate goal in doing Relativ. Heavy Ion Coll. is to

• Change the QCD vacuum• Librate the confined quark-gluon• Restore the chiral symmetry

No (local) equilibrium is demanded

too

The Definition of QGP

A (locally) thermally equilibrated state of matter in which quarks and gluons are deconfined from hadrons, so that color degrees of freedom become manifest over nuclear, rather than merely nucleonic, volumes.

The Prevalent DefinitionShould be changed into:

hot and dense

• X-N Wang’s Criteria:• High density: є >> єC• Large volume: V >> λ (mean-free-path)

• Long life-time: t >> λ

• Parton degrees of freedom

• Local thermal equilibration (interaction)

approximately

• Debye screening of strong interaction:

deconfinement

Should be changed into:

Multiple scattering (interaction)between partons (collective behavior)

Question 3: Has Quark DoF been observed yet?

Has Color Deconfinement been observed yet?

or

Direct Evidence v2 scaling

Parton recombination Partonic degrees of freedom

The n’s here count the number of quarks that flowed.

V2 scaling tells us that they are the constituent quarks that started to flow at the early stage of collision.

Constituent Quark Degree of Freedom

If this were the case, then all the flowing quarks would contributeto v2 and the numbers of quarks in baryon and meson would not be 3 and 2 any more. The scaling breaks down.

n >>3

Some model says that originally the current quarks flow, and they recombine into constituent quarks before hadronization.

Questions• Are they really the constituent

quarks that flow ? • Is it the constituent quark DoF

that shows up at the early stage of collision ?

• What kind of vacuum is that?

We still cannot say for sure that Quark DoF has been observed.

Conclusion

Never forget that our ultimate goal in doing Relativ. Heavy Ion Coll. is to:• Change the QCD vacuum• Librate the confined quark-gluon• Restore the chiral symmetry

Our main effort should be concentrated in finding out direct, model-independent evidences for these 3 points.

The other properties of the producedsystem, e.g. thermalized or notare worthwhile investigating. But the results of this investigation do not affect the discovery or non-discovery of QGP.

Thanks!

Thanks!

Thanks!

A model-independent evidence of such a state of matter would be a great discovery. It would mean:

• The QCD vacuum has been successfully changed

• The Color Confinement has been successfully broken

This discovery will become a milestone in the development of physics

Other Questions like

• Whether or not there is (local)equilibrium?

• Why there seems to be an expansion, but at the same time the volume measured by HBT is unchanged?

• • • • • • • • ?

can be left for further study.