Q&A: Understanding Quark Gluon Plasma

[Ene Dene]

I'm having a problem understanding QGP. Hadrons are bound states of quarks which interact by interchanging colour. Now, as I understood we haven't observed free quarks because the force rises by distance and that force is really strong. As I read in QGP quarks and gluons are not bound, you can't tell the difference between nuclei. But how come you can't observe a free quark? For example, what stops quarks just from flying away from that plasma? Why when you cool things down you don't get some pentha quarks states, since it was all mixed up?

 

[malawi_glenn]

You can't observere free quarks since

i. the magnitude of the force grows with distance, as you said.

ii. No color less objects can exist, and quakrs MUST carry color.

 

[Ene Dene]

You can't observere free quarks since
ii. No color less objects can exist, and quakrs MUST carry color.

I don't understand this one. Why wouldn't be, in principal, possible to produce force strong enough to break the bond, how does colour prevent that? Quark would carry colour anyway, as do free or bound electrons carry electrical charge, isn't that right?

i. the magnitude of the force grows with distance, as you said.

Yes, I had some wrong understanding about gluon exchange.

 

[malawi_glenn]

The magnitude of the force goes to infinity when the distance between quarks increase.

So when you move quarks away from a certain distance, the color-field becomes so dense that new quarks is created (flux tube, colour confinement, asymptotic freedom are key words) and bounds to the quarks that you tried to separate.

Only colourless particles exists, you can.t draw the analogy that free electrons carries charge.. Quarks are particles that interact both via EMforce, colour force and the weak force. Electrons are leptons..

 

[humanino]

I can't help to give my opinion

The magnitude of the force goes to infinity when the distance between quarks increase.

You may say the force increases, but it certainly does not go to infinity.

So when you move quarks away from a certain distance, the color-field becomes so dense that new quarks is created (flux tube, colour confinement, asymptotic freedom are key words) and bounds to the quarks that you tried to separate.

This is the dual-superconductor model of confinement. Although it is very popular, it is not established as the true mechanism.

In fact, the string picture works remarkably well for heavy quarks. For light quarks however, the situation is far more complicated, and to my understanding, more interesting as well. One should notice, for instance, that contrary to pQCD the coupling constant most probably does not grow infinitely at low momentum scales, as expected from full dressed propagators in Dyson-Schwinger models. This behavior is now confirmed both from lattice and data. On the data side, the plateau in alpha_s reached at the pion mass is hinted by analysis of the generalized (at non-vanishing Q^2) Gerasimov-Drell-Hearn sum-rule (see attached plot from http://arxiv.org/abs/hep-ph/0509113" [Broken]).

So my first comment is that confinement is still an experimental fact but a theoretical hypothesis. I do not claim that the dual-superconductor picture is wrong, but that one must be careful with analogies. The condensation of charges in the vacuum phase of this model is certainly a great idea with regards to chiral symetry breaking, but the confinement phase might be more complicated than in the QED case. Besides, non-trivial genuine quantum effects such as instantons can play a crucial role.

So why would people expect to have such a thing as a "Quark-Gluon Plasma" in the first place anyway ? It would be hard to summarize all the history, and I am not an expert anyway. Let me say that it has long been believed that there is an interplay between high-energy and low-energy phenomena in QCD, a sort of duality between quark-gluon and hadron-meson degrees of freedom, corresponding to two ways to look at the nuclear phase diagram. So even if the naive "plasma" picture, with free quarks and gluons, is wrong, we still need to understand nuclear matter at high temperature and pressure.

Contemporary interpretation is that the new state of matter observed is indeed strongly interacting, more "liquid" than plasma. There is currently little consensus between experts (as far as I discuss with them, they do not seem to agree with each other ) In fact, those results made a lot of noise, with some people trying to make far-speculative interpretations such as "Dual-black hole" picture (for instance). Much progress is required, even the initial state before collision, a possible "Color Glass Condensate", must be clarified. See http://www.physicstoday.org/vol-56/iss-10/p48.html" [Broken] and references therein.

My second comment is thus that the QGP might be different from what we expected. One could therefore conclude that our losses of understanding, at high and low energy, might be dual to each other

 

[malawi_glenn]

well yes, my misstake. The thing I learned was that it doesent matter how the force behaves on large distances, since hadronisation will occur anyway.