Questions tagged [quantum-chromodynamics]
Quantum-ChromoDynamics (QCD) is the quantum field theory believed to describe the strong nuclear force.
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Sum rules for parton distribution functions
In Schwarz, the parton distribution functions for proton has the following sum rules,
$$
\int dx f_u(x)=2\quad,\quad\int dxf_d(x)=1
$$
where $x$ is the fraction of momenta of proton carried by the ...
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Evidence of more generations in the QCD beta function
We know that the beta function for QCD is
$$
\beta = -\left(11 - \dfrac{2N_f}{3}\right),
$$
where $N_f$ is the number of fermions in the theory. We have $\beta_{\text{SM}} = -7$.
Now, my question is, ...
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Is color charge internal symmetry or global symmetry?
I was told the color charge in the standard model could not be observed directly. This sounded like the gauge field $\vec A$ in the electromagnetism. However, it is a discrete charge and does have ...
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Is quark and gluon orbital angular momentum predicted to have the rest of the proton's spin?
The Phys.org news item How did the proton get its spin? outlines various experiments that probe proton structure and are sensitive to spin, and the current news is that CEBAF's major energy upgrade ...
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How is the mass distributed in ordinary matter?
How is the mass distributed in ordinary matter?
In the ordinary things around us, we know that most of the mass is in the cores of the atoms, the electrons around it contributing only a very small ...
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Visualization of a gauge field with non-null winding number
In QCD you may add the term $\mathcal{L}_{\theta} = \theta\dfrac{g^2}{16\pi^2} \text{Tr}F\tilde{F}$, which turns out to be a total derivative. Now, it can be proven that the action of this lagrangian ...
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Spinor-helicity formalism: relationship between 1 and 2 reference vector setups
The spinor-helicity formalism is usually set up so that for a massless vector boson (photon or gluon) with momentum $k$ an arbitrary reference momentum $p$ is introduced and the corresponding ...
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$u$-channel in $gg \rightarrow u\bar{u}$
I've seen that for the QCD process $gg \rightarrow u\bar{u}$, where $g$ is a gluon and $u, \bar{u}$ are the up quark and the corresponding antiquark, there is $s$, $t$ and $u$ channels.
I perfectly ...
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Proton-proton collisions cross section plot by Stirling
I am struggling to understand some details of the cross section plot by Stirling that is very often shown when talking about LHC physics. See e.g. here: http://www.hep.ph.ic.ac.uk/~wstirlin/plots/...
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$SU(3)$ color charge
Is the color charge the Noether charge due to the "global" $SU(3)$ symmetry of the QCD Lagrangian, or is it due to the "local" symmetry of the QCD Lagrangian?
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Can an electron be produced inside a proton?
We know that inside a proton there is a sea of quarks, antiquarks and gluons. This happens as the valence quarks emit gluons which then split into a quark-antiquark pairs. These pairs become gluons ...
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Photon-Gluon annihilation in QCD
I am starting to learn about QCD, and I wanted to calculate the squared matrix elements for photon-gluon annihilation into a quark and an anti-quark. However, I am having trouble writing down the ...
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What makes electric and magnetic fields in Yang-Mills theories gauge co-variant?
Specifically in QCD, why is it so?
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Peccei-Quinn-symmetry and effective Lagrangian for the Axion field
To solve the strong CP-problem Peccei and Quinn suggested the use of a new $U(1)$-symmetry called the PQ-symmetry. For this symmetry they constructed an effective Lagrangian involving the Nambu-...
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Wilson loop is not an element of $\mathrm{SU}(3)$ in color deconfinement
The center symmetry in QCD comes from the
$$a\ \mathcal{P}\mathrm{exp}\left(ig_s \int_C dx^\mu \ A_\mu(x)\right) a^{-1} = \mathcal{P}\mathrm{exp}\left(ig_s \int_C dx^\mu \ A_\mu(x)\right),$$
where $C$ ...
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