Qcd Definition and 169 Threads

Hi everyone, I am trying to understand some things about confinement in lattice QCD. It has been very difficult so far to find a clear book chapter or review article, so I had to resort to the original literature in many cases, and I came across some apparently incompatible statements about...

The QCD vacuum condensate which breaks chiral symmetry in qcd, also break the SU(2) electoweak gauge symmtery, right? The operator which gets a vacuum expectation value isn't SU(2) invariant since it is built out of left handed quarks and right handed quarks which are in different...

How do (offshell) QCD ward identities look like in the axial gauge? How to derive them? The standard treatment of ward identities uses BRST symmetry in the covariant gauge. I don't know where I can read about the axial gauge version of the ward identities.

Please teach me this: Why must strong interaction theory be QCD(SU(3)) theory?Because asymptotic freedom of strong force is the characteristic of any non-Abelian gauges theories.E.g why must it not be SU(2)? Thank you very much for your kind helping.

Hello I am looking to learn about Lattice Gauge Theory. I am already understand the general theory, (that is, defining fermions and gauge fields on lattice sites and links, respectively), but am having trouble intuiting the results of such calculations. As an example, the following is a...

I don't understand why technicolor interactions are unobservable at low energies due to the fact that they are strong and confining. I thought color confinement means that quarks cannot be separated but rather new quark-antiquark pairs form and result in Hadron jets. Isn't this a feature of...

I encountered a paper in which the authors presented parton-level cross sections as a function of these variables: incoming particle momenta, factorization scale, renormalization scale, and strong coupling constant at the renormalization scale. I used to think that QCD factorization scale should...

I'm reading about factorization in QCD. The terms "soft gluons" and "ultrasoft gluons" are frequently referred to. I know how they are defined in terms of power counting, but don't understand their physical significance. In which QCD processes are soft gluons important, and in which are...

Please teach me this: Can we deduce Ward Identity in QCD from U(1) symmetry of QED?Because QCD is a theory of quarks and quarks have electric charge.So we need not deduce the Ward Identity from SU(3) symmetry,but we can be able to demontrate the Ward Identity( considering gluons)with U(1)...

Hi everyone, I am finishing my second year of masters in France on theoretical physics and I am really really motivated in pursuing into a PhD. The topics that interest me the most are lattice QCD and numerical relativity (I want to focus on numerical methods). Also, I would like to work on one...

Please teach me this: It seem to me that when we consider high momentum,then the strong interaction become asymtotic week force,so we can use perturbative QCD.But at the same time,when considering large momentum,the ''electric charge'' became large,then the ''electric force'' between quarks...

In the parton model for deep inelastic scattering, the momenta of the partons are assumed to be longitudinal and on-shell. What's peculiar (at least rhetorically) is that even the "virtual" sea quarks are assumed to be on-shell. How does QCD justify these assumptions? Are there systematic...

Hello Everyone, I'm doing some research on meson decays, I was going to be attempting to use the 't Hooft Model. It's a model in which we use only 1 spatial and 1 temporal dimension, and we have the number of colors in QCD go towards infinity (or just very large). It's my understanding...

Hi folks, I was wondering if the two Casimir operators of the SU(3) color gauge group were of any physical significance, or corresponded to any familiar physical properties. For example, I know that in the Poincare group the two Casimirs correspond to mass and spin: is there a similarly...

i just wonder why we call it "sum rule"? waht do we sum, and what sum for?

Hello, In the lagrangian of QCD, there is q which is the quark field and it is the fundamental representation of SU(3). This q is multiplied by a gamma matrix and a q bar. So, how can we have a 4x4 matrix multiplying 1x3 matrix? Thanks

Hello, May you please provide me with a good reference that explains supersymmetric QCD, and dicusses the association of the superfields with the particles. Thanks

Hi! I have a process with multiple feynman diagrams where gluon propagators occur. When I use an axial gauge for the gluon propagator, do I have to use the same n-vector for every propagator? Following this I wonder whether I can use the same n-vector for every polarization sum in axial gauge...

Hey guys, does any of you know where I could find the Feynman rules for scalar QCD? If they where colour ordered, even better! Cheers, earth2

Two main questions here that google can't really help me with. Right now I do research in computational astrophysics (n-body simulations) and I think I like doing numerical simulations (I'm a pretty good programmer and I like to apply that to physics). I'm somewhat interested in particle...

Can you please specify a reference to help me understand how QCD explains the fact that all particles observed in nature transforms under SU(3).

How QCD explains the fact that all particles observed in nature transforms under singlet representation of SU(3)-color group. What does it mean that all other particles have zero color charge? What the color-charge operator looks like?

Homework Statement Calculate the symmetry factor of a 4-point, 1-loop diagram in QCD. Two legs are external, two are not Homework Equations I don't know how to include pictures here, but let's try to describe it: -It basically consists of 2 four-vertices connected by two propagators...

I've read that the mass of a proton has been calculated using QCD. But I assume some measured values are inputs to this calculation? If so can anyone provide a list of all of the values used (speed of light, Plank's constant, mass of other particles?)

Effective theory of bound states from QCD?? Do you know any work that actually succeeds in producing the action of an effective field theory for nucleons and mesons, starting from the QCD action?

Is QCD/modern university nuclear physics a worthwhile topic for someone aspiring to work with nuclear energy, reactor safety and the like? That person is me and I am interested in the answer as it pertains to a physics Ph.D. rather than a degree in nuclear engineering or something. Thanks!

Hi! I'm currently performing lots of research into the Higgs Boson. In many papers, when talking specifically about production and decay mechanisms, they talk about the "high QCD background". I can't find a specific definition for what this means and refers to. Any clarification would be...

Hi guys, So I'm working on this project to simulate QCD using a computer using (hybrid) monte carlo. I follow the majority of what I've read thus far, though there are a few things I'm uncertain about. Firstly, the Wilson loop is often written as U_{P} and the invariant gauge action is...

I've been looking at how people calculate basic things on the lattice. How would you go about calculating a single number? By this I mean decay rate, cross section, amplitude or contribution to an amplitude. The reason I ask is that I've been looking at some lattice QCD code up close. The code...

Thanks guys for your responses on my last thread...It was very informative for me...This time I also have some unsolved problem on which I want to have your opinions and discussion I've heard that The equations of QCD remain unsolved at energy scales relevant for describing atomic nuclei, why so?

Can someone explain to me how the concept of particle is defined in lattice QCD? Here are the reasons why it seems problematic to me: 1) Lattice QCD is based on functional-integral formulation of QFT, which does not contain any operators in the Hilbert space. In particular, it does not contain...

Hi, I was wondering about the G\tilde{G} term that can be added to the QCD Lagrangian. It seems clear that the analogous QED term can be left out because it is (just as the G\tilde{G} term) a total divergence and thus has no influence on the physics. But in the case of a non-abelian gauge...

I'm trying my best to learn particle physics and cosmology for the last year or so. One of the concepts I'm trying to understand is how the 95% figure humanino quotes above is arrived at. I've seen this figure (or something close to it) a few times here. I read the paper...

Dear all I have done some studies trying to understand the relation between the QCD theta angle and neutron electric dipole moment. General, the QCD vacuum produces the term L_{\theta} = \theta g_s^2\: G_a^{\mu\nu} \, G^a_{\mu\nu} this I can derive! I have studied Srednicki ch 93, Ramond...

What are Hard QCD processes ?

I'm asking this question as someone who has not studied this topic technically. By radius of convergence, I mean exact results, not just approximations, can be obtained by summing sufficiently many terms. (does this ever happen?) I don't mind if you need 1000 terms, as long as the series is...

In e+e- annihilation, are the jets that are produced quarks and gluons, or hadrons? What is the basic idea behind a QCD calculation for such a process? For example, do I take the two initial states to be e+e-, and the two final states to be q+q- (quark and antiquark), but also add the...

Has the physics community come to an agreement on the form an approximate potential function would take for the strong force in a meson? I am just starting out as a physics major, but I have been reading a few books on basic quantum theory and QFT. I am currently reading a book on particle...

Have you seen this paper, Federico Urban and Ariel Zhitnitsky, "The QCD Nature of Dark Energy", http://lanl.arxiv.org/abs/0909.2684", They claim can generate approximately the right amount of dark energy in the universe, just from the effect of the normal Hubble expansion upon the QCD vacuum...

Hi @ all, during tmy exam preparation I stumbled upon some facts that I couldn't explain to myself. 1. Strong coupling constant g_s is universal -> colour charge is quantised 2. The MS_bar quark mass \bar{m} = m_0 + \delta \bar{m} can be given as a function of the quark pole mass m =...

In theoretical physics talks involving QED and QCD, people speak of the 'Eikonal Factor?' Generically, what's an Eikonal factor? Thanks

Ed Witten was essential in organizing the 1 million dollar prize for clarification of the mass gap in QCD that is offered by the Clay institute. But why is this issue so important? Wouldn't all the successes of QCD stay also if no glueballs existed? Can somebody explain why Witten wants the...

I have a simple sounding question about QCD and its scale parameter \Lambda: The general consensus amongst field theorists is that QCD is not a well defined theory by its Lagrangian and the quantization procedure alone. One needs an additional parameter \Lambda, which sets the scale of...

I am reading a paper right now on lattice QCD that presents a "method that improves the cooling method and constructs an improved topological charge operator based on the product of link variables forming rectangular Wilson loops." Unfortunately I...

I've been playing around with the QCD Lagrangian to get a better understanding of how it works. I can derive some classical, Maxwell-like equations; the inhomogenous ones are \nabla \cdot \vec E^a = -gf^{a}_{bc} \vec A^b \cdot \vec E^c + \rho^a \nabla \times \vec B^a -...

I'm interested to find out a bit more about current work that is going on in QCD.What are the main areas of research? What are the main problems that people are working on? Where could I find some further information about current research in QCD? Thanks

Hello: I've heard that Yukawa's interaction could explain the nuclear force, but I also heard that the strong nuclear force is explained by QCD, Could you please explain me more about it ? Thanks a Lot

Hi, I was wondering what the connection between the colour charges; Y^C , colour hypercharge and I^C_3 , colour isospin charge and the SU(3) symmetry of the colour interaction? Has it anything to do with Noethers theorem to do? Does anyone have good source? Thanx in advance!

“Teaching aids” The moderators and Gurus might want some recent “teaching aids”. Perhaps you will want to put some of these into a "sticky" The CERN site is producing some very good “teaching aids”. If you can, look at the ppt presentation, since it has animations. There are 3 pdf files at...

Hi! Has anybody seen the perturbative expansion of the generating functional of QCD Z[J,\xi,\xi*,\eta,\eta*] expressed with Feynman diagrams? I mean, there should be an expansion, containing external sources denoted by something like -------o abbreviation for i \int d^4 x J -------#...