Questions tagged [quantum-chromodynamics]
Quantum-ChromoDynamics (QCD) is the quantum field theory believed to describe the strong nuclear force.
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Is glueball stable and why or why not?
I heard that the glueball can be a candidate for dark matter. Glueball is electric neutrality, which is fine, but is it stable? Is its lifetime longer than the universe? If it is true, what is the ...
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Derivation of the BRST invariance in QCD
I am trying to follow the proof for the BRST invariance in QCD in the following pdf file: https://scipp.ucsc.edu/~haber/ph222/BRST.pdf (section 3, from end of page 6)
I can understand the derivations ...
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Confinement, Holographic QCD, Seiberg-Witten Theory
I have seen a bunch of articles that all try to show confinement using AdS/QCD or Holographic QCD method.
I pretty much know that the lastest attempt to prove confinement based on breaking SUSY even ...
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A list of failed attempts towards a proof of confinement [closed]
Can one give a list of failed or open attempts (not necessarily Supersymmetric) towards a proof of confinement in 4d regarding YM or QCD?
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How to avoid the ordinary Coulomb solution in QCD?
To see where QCD starts to differ from the behavior of EM fields, we might begin by looking at the classical field. A search brings up
[question 339978] and [question 360061] but no answer is found ...
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Superposition of charge states in the Neutral Pion
The quarks of neutral pions don't exist in a pure flavour state, and instead are described as a superposition of up-antiup with down-antidown:
$\frac{u\bar{u}-d\bar{d}}{\sqrt{2}}$
However up and down ...
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$ \pi^0\to \gamma\gamma$ parity conservation
Let's consider the decay process $\pi^0\to \gamma \gamma$. After we spontaneously broke the chiral symmetry of QCD coupled to an abelian gauge field $A^\mu$, we end up with the Goldstone boson ...
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How can dense quark matter stabilize wormholes?
I have read in several articles, such as
https://www.sciencedirect.com/science/article/abs/pii/S0577907323000266 and https://arxiv.org/abs/1403.0771 that dense quark matter can stabilize wormholes. ...
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Relative minus sign in radiation of gluon jets
I am trying to calculate the cross-section for electron-positron annihilation into a quark-antiquark pair and a gluon. I find that I need a relative minus sign between the two contributing diagrams in ...
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The commutation relations of photon and gluon?
In QED, the photon field has the following commutation relations:
\begin{equation}
[A^{\mu}(t,\vec{x}),A^{\nu}(t,\vec{y})]=0, \tag{1}
\end{equation}
where $A^{\mu}(t,\vec{x})$ is the photon filed. ...
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Constant in Regge trajectory
The Regge trajectory in QCD is given by
$$m=\sqrt{\frac{J}{\alpha}-\alpha_0},$$
where $m$ is the mass and $J$ is the angular momentum of the hadrons, $\alpha=(4\pi\sigma)^{-1}$ is the inverse QCD ...
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Asymptotic Freedom QCD
I'm trying to understand the derivation of asymptotic freedom with the renormalisation group equations. I'm reading Taizo Muta's book on QCD. What I don't understand is how he obtains the last ...
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QCD parton shower hard scale
Currently I'm studying parton showers from QCD and I'm having trouble with understanding how the hard scale $Q$ is related to the virtuality and energy of the parent parton. The Sudakov factor $\Delta(...
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Why does an all connected diagram contribute to two-point function?
I am recently reading E.Witten's review for $1/N$ expansion of QCD. In there, considering the main contribution of quark bilinears like $\bar{q}q$, then He mentions that in free field theory there is ...
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Why the kinetic term of the Hamiltonian has to be positive definite for well-posed time evolution?
I was going through this paper on QCD chaos, where in Appendix B (page 10), for equation B12:
$$\frac{\mathcal{S}}{\mathcal{T}}= \int \mathrm{d}t\sum _{n=0,1} \left(\dot{c}_n^2-c_n^2 \omega _n^2\right)...
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Sum rules for parton distribution functions
In Schwarz, the parton distribution functions for proton has the following sum rules,
$$
\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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