Quantcast
  • Register
PhysicsOverflow is a next-generation academic platform for physicists and astronomers, including a community peer review system and a postgraduate-level discussion forum analogous to MathOverflow.

Welcome to PhysicsOverflow! PhysicsOverflow is an open platform for community peer review and graduate-level Physics discussion.

Please help promote PhysicsOverflow ads elsewhere if you like it.

News

PO is now at the Physics Department of Bielefeld University!

New printer friendly PO pages!

Migration to Bielefeld University was successful!

Please vote for this year's PhysicsOverflow ads!

Please do help out in categorising submissions. Submit a paper to PhysicsOverflow!

... see more

Tools for paper authors

Submit paper
Claim Paper Authorship

Tools for SE users

Search User
Reclaim SE Account
Request Account Merger
Nativise imported posts
Claim post (deleted users)
Import SE post

Users whose questions have been imported from Physics Stack Exchange, Theoretical Physics Stack Exchange, or any other Stack Exchange site are kindly requested to reclaim their account and not to register as a new user.

Public \(\beta\) tools

Report a bug with a feature
Request a new functionality
404 page design
Send feedback

Attributions

(propose a free ad)

Site Statistics

205 submissions , 163 unreviewed
5,082 questions , 2,232 unanswered
5,353 answers , 22,789 comments
1,470 users with positive rep
820 active unimported users
More ...

  QCD in the non-perturbative regime

+ 4 like - 0 dislike
1289 views

In the list of unsolved problems in physics.

  • Confinement: the equations of QCD remain unsolved at energy scales relevant for describing atomic nuclei. How does QCD give rise to the physics of nuclei and nuclear constituents?

  • Quark matter: the equations of QCD predict that a plasma (or soup) of quarks and gluons should be formed at high temperature and density. What are the properties of this phase of matter?

what is this means that "non-perturbative QCD are the exploration of phases of quark matter, including the quark-gluon plasma."?

This post imported from StackExchange Physics at 2014-08-29 16:49 (UCT), posted by SE-user Guest
asked Sep 2, 2013 in Theoretical Physics by Guest (20 points) [ no revision ]

2 Answers

+ 4 like - 0 dislike

Due to asymptotic freedom, the perturbation series expansion of QCD breaks down at low energy. The expansion parameter, the coupling constant, becomes too large and therefore we cannot rely on results anymore. The value of the coupling constant approaches the order of $1$ at an energy of several hundred MeV, a scale referred to as $\Lambda_{QCD}$ ("Lambda-QCD"). Perturbation theory becomes reliable several GeV above this scale. But what does that mean for the so-called "QCD phase diagram", which is a diagram with temperature as the vertical and baryon chemical potential (or density, which might be easier to imagine) as the horizontal axis?

As you can see in the image in the other answer, there is a variety of interesting phenomena at a relatively low energy scale. This means that a perturbative approach in the conventional sense (expansion in the coupling constant) is not sufficient to gain a complete picture of QCD. Phenomena like the transition to quark-gluon plasma, colour-superconductivity and colour-flavour-locking are out of reach. But what can be done in order to gain some insights?

This is where non-perturbative QCD comes into play. On the one hand, there are traditional approaches, for example lattice QCD, where you assume that dynamics takes place on a discrete lattice and solve problems numerically. On the other hand, there are novel approaches like the so-called AdS/QCD duality, which is a manifestation of the AdS/CFT correspondence. It applies the holographic principle and maps a strongly coupled field theory (which is similar to QCD) to a string theory in its weak-coupling limit which allows for the treatment of problems which would be otherwise inaccessible.

This post imported from StackExchange Physics at 2014-08-29 16:49 (UCT), posted by SE-user Frederic Brünner
answered Sep 2, 2013 by Frederic Brünner (1,130 points) [ no revision ]
+ 2 like - 0 dislike

I don't get what you're asking?

What it means that non-perturbative QCD is the exploration of phases of quark matter, including the quark-gluon plasma?

Because, at high energies, you're at the so called asymptotic freedom regime. At this regime, your quarks interact weekly and the perturbative calculations are possible. And quark-gluon plasma is theoretically assumed to have been existed microseconds after the universe was created (because of high temperatures at the beginning of the 'Big Bang').

Also experiments such as ALICE at LHC in Cern and RHIC are studying the collisions that should, theoretically, produce quark-gluon plasma. A state, theoretically, where quarks and gluons are decoupled: since in nature, quarks are never on their own, they are either in mezon 'state' - two coupled quarks, or baryon 'state' - three coupled quarks (both part of hadrons - family of coupled quarks).

But it is still not clear how this happens, the mechanism behind it, and is this what they are seeing ate LCH indeed Q-G plasma.

My friend worked on ALICE, and he had a thesis from this feild. They are studying the impulse correlations in Pb-Pb collisions and they trying to see if the Q-G plasma state emerges from this collisions using Hanbury Brown and Twiss effect. That way, from correlations of two body wave functions you could deduce things about hadronization of quarks - since Q-G plasma exist only for a tiny amount of time, and quarks start to hadronize very quickly, you can only see that effect, and try to see if that effect really came from Q-G plasma. But it's still not clear if the Q-G plasma was created, and they're working hard to find that out.

Oh, also you have this QCD diagram

enter image description here

Which describes why you need high energies to produce Q-G plasma.

I hope this clears things a bit...

This post imported from StackExchange Physics at 2014-08-29 16:49 (UCT), posted by SE-user dingo_d
answered Sep 2, 2013 by dingo_d (110 points) [ no revision ]

Your answer

Please use answers only to (at least partly) answer questions. To comment, discuss, or ask for clarification, leave a comment instead.
To mask links under text, please type your text, highlight it, and click the "link" button. You can then enter your link URL.
Please consult the FAQ for as to how to format your post.
This is the answer box; if you want to write a comment instead, please use the 'add comment' button.
Live preview (may slow down editor)   Preview
Your name to display (optional):
Privacy: Your email address will only be used for sending these notifications.
Anti-spam verification:
If you are a human please identify the position of the character covered by the symbol $\varnothing$ in the following word:
$\varnothing\hbar$ysicsOverflow
Then drag the red bullet below over the corresponding character of our banner. When you drop it there, the bullet changes to green (on slow internet connections after a few seconds).
Please complete the anti-spam verification




user contributions licensed under cc by-sa 3.0 with attribution required

Your rights
...