Quantum-ChromoDynamics (QCD) is the quantum field theory believed to describe the strong nuclear force.

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Isospin and Energy

I would to know if there is a correlation between isospin and energy. Consider for example the $\eta$ meson ($I =0$) and the pions($I=1$). The $\eta$ turns out to be much heavier then the pions ...
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Could quarks be free in higher-dimensional space than 3D?

Reading this answer, I now wonder: if quarks are confined by $r^2$ potential, could their potential allow infinite motion in higher-dimensional space? To understand why I thought this might be ...
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No non-trivial UV asymptotically free and IR free

How it could be proven that a non-trivial theory cannot be both asymptotically free and IR free (g=0 both in the UV and IR with some interpolating function in between)? This is of course contrary to ...
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Gluon Fusion in Top Pair Production

In my textbook, there is a feynman diagram showing top + anti-top pair production with a highly energetic gluon resulting from the fusion of two gluons. I know that all vector bosons have spin 1, so ...
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Are mesons color polarized?

The binding of quarks in mesons baffles me. It's an Occam's Razor thing. Since a meson is a colorless, the simplest way to bind its two quarks together is to use a $U(1)$ Cartan subalgebra of ...
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Is quark electric charge also confined?

Since we will (maybe) never observe free quarks which has fraction charge, is it logically correct to say that the fraction (1/2,2/3 etc) electric charge is also confined. I.e. the charges that we do ...
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What is the importance of the Odderon?

From hep-ph/0001149v1: (1) an Odderon contribution is absolutely necessary to reproduce quantitatively well the data; while its presence is not explicitly needed at $t = 0$, its inclusion is ...
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324 views

Is forward scattering = no scattering?

What is forward scattering? If it is equivalent to no scattering, then why not call it "no scattering"?
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What is quark transverse momentum?

When you google my question you get something on the order of 400 000 results but none of them explains how it is defined (No I didn't check them all). I know what the words quarks, transverse and ...
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81 views

What is the nucleon axial charge?

Can someone point me to a short definition of what the nucleon axial charge is?
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Why Cronin Effect Happens?

I'm looking for explanation on Cronin effect but unfortunately there's no Wikipedia entry or self-contained paper to start from. The statement of this effect is that: "At leading order, multiple ...
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How to describe hadron-hadron strong interactions from QCD?

I'm trying to set up the problem of deriving the thermodynamics of hadronic matter. I know how to proceed in the case of an effective description such as mean field (Walecka/linear sigma model) but ...
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Proof that QCD is the theory describing strong interactions?

I would like to ask what are the experimental evidences that led to the conclusion that QCD is the right theory to describe strong interactions. I know that some of the key point are the decay of ...
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Quark space tensor product Vs Angular momentum space tensor product

For two triplet angular momenta states, say $J=1$ and $I=1$, if we wanna look at it in the coupled basis $F=I+J$, we use the regular Angular Momentum rules: $$|I-J|\leq F\leq I+J,$$ and from that ...
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Yan - Drell process

I'm looking for more information about Yan - Drell process but didn't manage to find a good place to learn from (still looking for one). I understood that this process can help us to understand the ...
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126 views

Fermion field structure in non-abelian gauge theories

I am trying to understand the structure of the fermions in non-abelian gauge theories. Disclaimer: my question might be very trivial (I suspect the answer could simply be "a change of basis"), but I ...
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245 views

How to understand the QED, QCD and standard model Lagrangians? [closed]

How do you read the QED, QCD and standard model Lagrangians? What do all the symbols and tensors represent? And, how can you derive them by yourselves?
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123 views

QCD mass gap finite temperature

QCD has a mass gap. Does heating up QCD to a finite temperature change this fact, or is it a property which is independent of temperature?
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How do you derive the Lagrangian for the Standard Model?

Is there a way to derive the Lagrangian for the Standard Model just like one would for Einstein's field equations for instance? Also, how do you derive the Laganrigans for QCD and QED? Is it possible ...
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What's the heuristic reasoning for Chiral Symmetry breaking

In Peskin and Schroeder (page 669), and other references, the heuristic reasoning for why one would expect chiral symmetry breaking at low energies is that quark masses are small and hence it's not ...
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momentum conservation and gluons

The process is the following: $$e^-e^+ \rightarrow photon \rightarrow quark + antiquark$$ Regarding the momentum conservation law, how come we have a photon of spin 1 and at the end some meson with ...
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Baryon operators for $N_f<N_c$

Simple question. The answer is probably obvious, but I just don't see it. In section 3 of this paper http://arxiv.org/abs/hep-th/9509066 it is stated that in SUSY QCD if $N_f<N_c$ the classical ...
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283 views

What is the precise statement of the OZI Rule?

What is the precise statement of the OZI Rule? I've heard that a diagram is OZI suppressed if it can be "cut in two by cutting only gluon lines", but I don't really understand. For example, consider ...
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How does flux tubes between quarks bind them together?

If you have, say, a proton it has gluon field fluctuations around it. Those flux tubes between the quarks suppresses the gluon field fluctuations and create a true vacuum between them(correct me if ...
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223 views

Is Higgs mechanism necessary in QCD?

As we known, we introduce Higgs mechanism into Standard model to explain mass of Gauge boson, lepton and neutrino(maybe?). Higgs mechanism is necessary for electroweak theory because gauge symmetry ...
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Tool for Born-level Cross Section Generation with Color Decomposition

I'm looking to find a tool, like CalcHEP, that calculates basic cross sections and allows for symbolic output which is decomposed by color. I understand that MADGRAPH 5 will decompose amplitudes by ...
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195 views

Dimensional transmutation in QED?

I am familiar with the QCD dimensional transmutation (QCD is dimensionless theory but via renormalisation we get dimensional parameter; we have to choose some referent scale...), but I'm not sure if ...
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What is a “reggized gluon”?

I'm reading a paper in which the author used these words many times assuming that the reader knows what he is talking about. Can someone please explain what it is? What is the difference between a ...
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Reduced graphs and pinch-singular surfaces

I am reading a book on perturbative QCD by John Collins. In Chapter 5, the terms reduced graph and pinch-singular surface are used for the analysis of mass singularities. However, their meanings are ...
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QCD in the non-perturbative regime

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 ...
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Nambu-Goldstone bosons from a quantum anomaly symmetry breaking?

We know that: Nambu-Goldstone bosons come from Goldstone theorem: a spontaneous (continuous)-symmetry breaking of the system leads to massless scalar modes. quantum anomaly: is the anomalous ...
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What kinds of contributions can be neglected in the leading logarithmic approximation?

I'm looking for some good explanation on leading logarithmic approximation (LLA) in QCD; in particular, what types of contributions can be neglected while assuming LLA?
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'Validity' of QED/QCD/Electroweak interaction

I am currently attending a course on Quantum Field Theory and I got into thinking how valid these theories are. As the theory attempts to describe reality only far above the Planck (length) scale, ...
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Multiquark states and molecular QCD

Due to some recent "discoveries" (yet to be completely confirmed), we have hinted the existence of tetraquark particles. Why QCD exotics like multiquark states beyond n=2,3 (mesons, ...
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The divergence in QCD Series— How many are they, and what do they mean?

I am referring to this question, and especially this answer. In addition, QCD has - like all field theories - only an asymptotic perturbation series, which means that the series itself will ...
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Vacuum polarization in QCD and gluon bubbles

In analogy to QED, the following Feynman diagram is a diagram contributing to the vacuum polarization effect, leading to anti-screening, asymptotic freedom and running of the strong coupling constant. ...
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Can massive fermions have zero modes?

I'm confused about fermion zero modes in relation to instantons. I understand that instantons can create fermion zero modes, but it's not clear to me when a fermion has a zero mode. For example, ...
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Possible implications of Quark Quartet

Today on Nature's website appeared a news about the discovery of a quark quartet (formed from two quarks and two antiquarks). They say that this particle containing four quarks is confirmed. This is ...
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Is there a non-technical way to understand why proving confinement is difficult?

As far as I understand, the basic reason why we are not able to solve QCD at low energies is because it is a strongly coupled quantum field theory, and a part from exceptional situations in simplified ...
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Neutral pions and chromodynamics

$\pi^0$ particles are either up-antiup or down-antidown (or strange-antistrange?) They must be opposite colors to preserve neutrality. Why don't the opposite quarks annihilate?
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Charge in terms of wavefunctions

For a charged quantum particle, say, an electron or a quark, how in the particle's wavefunction is the electric charge represented? Is it truly possible to represent electric charge using the wave ...
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What is the $t\bar{t}$ production supposed to bring up

The $t\bar{t}$ production, I've read, that will somehow confirm the QCD and might bring up new physics. Why are we studying $t\bar{t}$ production from $p-p$ collisions at the LHC? What are we trying ...
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What is the role of the color-anticolor gluons?

Ignoring for the moment the experimental necessity of eliminating the gluon singlet state, the nine "raw" gluon color components form two quite distinct sets. The first set consists of the six ...
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Topological Phases and Confinement

I recently attended a talk in which the speaker defined a topological phase as "A phase which has a gap above the ground state for bulk excitations in the thermodynamic limit." I am interested in what ...
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Non-existence of gluon singlets: Any recent theoretical progress?

An unanswered question from last year (2012) on gluon singlets asked whether there is any theoretical explanation for the experimental absence of the ninth or colorless (singlet) gluon. This is the ...
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Asymptotic Freedom - Qualitative Explanation

I am doing a (mostly qualitative) course on Particle Physics, and am confused about the concept of asymptotic freedom. The lecture notes basically say that a quark may experience no force/be "unbound" ...
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478 views

Theta parameter of QCD: Very small, periodic in $2\pi$ or both?

I've been learning about the theta parameter of QCD and I'm confused about the fact that it's supposed to be very small but at the same time some sources say that the Yang-Mills theory should be ...
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For mesons, or baryons, do sea quarks contribute to the angular momentum of the bound state?

The total angular momentum of a bound state of quarks, such as a meson say, can be done by studying the spin and orbital angular momentum of the 2 valence quarks. What about the sea quarks why they ...
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The status of $SU(3)_C$ symmetry in the Standard Model

In the Standard Model of Particle physics the $SU(2)_{EW}$ symmetry and the $SU(2)$ isospin symmetry are broken. What about $SU(3)_C$? Is it broken too? if YES, what breaks the symmetry? If NO, what ...
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Does the number of left handed chiral quark superfields always equal half the number of quark flavours?

In Weinberg's "The Quantum Theory of Fields Vol III" page 267 we're told that $n_f = 2N_f$. Where $n_f$ are the number of flavours and $N_f$ is the number of left chiral quark superfields (or the ...