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Questions tagged [quantum-chromodynamics]

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

The charge of QCD is called "color" and comes in three fundamental values called "red", "green", and "blue", to which are coupled eight distinct states of a single vector boson called a "gluon".

Unlike the photon in QED, gluons are themselves color charged objects, which means the theory is complicated by the existence of three- and four-gluon vertexes.

A important feature of the strong interaction is confinement, which demands that all all objects able to exist at large distance separation must be color-neutral: either adding equal amounts of red, green, and blue (as in baryons); or by adding color and anti-color (as in mesons).

1087 questions
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What is a mass gap?

I read that Quantum Chromodynamics is a theory with a mass gap. What is a mass gap in laymen terms? Why some theories have it? Which theories does not have it? Note: I searched for mass gap before asking, all topics about it assume that the reader…
user09876
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Why are baryons equivalent to Skyrmions?

Baryons in quantum chromodynamics can be described as a bound state of three quarks. But they can also be described as a topological soliton of the pion field. How can both descriptions be equivalent?
Dough
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Cusp anomalous dimension in QCD

What is the "cusp anomalous dimension", starting from the basics? I came across this term while reading some QCD papers.
felix
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Why are the Higgs phase and the confinement phase identical in Yang-Mills-Higgs systems?

If we couple a Yang-Mills theory with a Higgs field and some quarks in the fundamental representation, we can have a Higgs phase and a confining phase. However, they are indistinguishable. The Wilson loops scale according to the perimeter law, not…
Dough
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Why are Wilson lines the effective degrees of freedom for high energy partons?

I've read that Wilson lines are the effective degrees of freedom for high-energy partons, when you consider collinear gluon emission. But I'm struggling to find a readable account of this topic. Does anyone have suggestions?
felix
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Generalization of relation between charge and magnetic moment

In classical theory, for electrons or protons, charge and angular momentum combine to give the magnetic moment. Does a similar consequence hold for the generalization of charge to other forces, like the strong force or weak force?
user48801
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Why hasn't QCD fed back to nuclear engineering?

A recent commenter on one of my questions noted that Fermi could take a tour of a modern nuclear powerplant and not be suprised by anything, and I believe that. I also think this shouldn't be the case, there have been many advances in high energy…
Mike Flynn
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What is the exact reason quark-gluon plasma behaved like liquid instead of gas as predicted?

I have searched all over the net (including here) but most references and answer were either old like written in 2006 (or around 2010) or they didn't directly address it why the QGP behaved like liquid. Anyone know what is the exact reason or…
Jtl
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Why are the 'color-neutral' gluons confined?

What makes the two 'color-neutral' gluons $(r\bar r−b\bar b)/\sqrt2$ and $(r\bar r+b\bar b −2g\bar g )/\sqrt6$ different from the pure $r\bar r +b\bar b +g\bar g $ ? Why don't they result in long range (photon-like) interactions?
NoEscape
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Experimental evidence for color-neutral gluons $( (r\bar{r}−b\bar{b})$ and $(r\bar{r}+b\bar{b}−2g\bar{g}) )$

Is there any experiment/measurement that would have a different outcome if one of the following scenarios is applied: The two color-neutral** gluons would not exist Those gluons would have very large masses ** we are talking about the 2 gluons not…
NoEscape
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Colour counting in $\mathrm{e}^++\mathrm{e}^-\to \mathrm{q}+\bar{\mathrm{q}}+\mathrm{g}$ final states

I want to calculate the cross section associated to the $\mathrm{e}^++\mathrm{e}^-\to \mathrm{q}+\bar{\mathrm{q}}+\mathrm{g}$ process as in the Feynman diagram (source) and the other one in which the gluon is "emitted" from the antiquark leg. I'm…
yellon
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How to interpret $\langle\pi|J|\pi\rangle$?

The decay constant $f$ of a hadron with momentum $P$ or its in-hadron condensate $\kappa$ (See this paper, Eq. (8)) are given by terms like $ \begin{align*} f_\pi P^\mu &\sim \langle 0|J_\text{axial}^\mu (0) |\pi(P)\rangle,\tag{1}\\ \kappa_\pi &\sim…
ersbygre1
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QCD: In some phase diagrams, the axis are temperature and baryon chemical potential - what does it have to do with densities?

In this figure one axis is the temperature (this one is no problem), the other one is the baryon chemical potential which causes my confusion. Figure from here: Phase diagram of simplified QCD. The text I'm referring to (with a similar figure):…
Cancara
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Amplitude for $q\bar{q} \to Z$

In ELectroweak theory, the amplitude for the incoming quark and anti quark to $Z$ boson production in the following diagram is given zero. The outgoing particle $Z$ is given here as blue boson. I am confused what is the reason behind it. One reason…
Tanmoy Pati
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Electon-positron annihilation ratio and vacuum polarization in QCD

It is commonly defined the ratio $$R(s)= \frac{\sigma(e^+e^-\longrightarrow \textrm{hadrons}; s)}{\sigma(e^+e^-\longrightarrow \mu^+\mu^-;s)}$$ where $s$ is the center-of-mass energy squared. I found in many references that $$ R(s)= \frac{1}{2i\pi}…
ar em
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