Questions tagged [quantum-chromodynamics]
Quantum-ChromoDynamics (QCD) is the quantum field theory believed to describe the strong nuclear force.
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Euclidean LQCD not on a lattice?
How much the idea of calculating Euclidean path integrals in LQCD is fundamentally tied to using formulations based on the discretized spacetime lattice?
In computational approaches to quantum many-...
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How does the on-shell (OS) scheme work if we assume mass to be zero?
When calculating the self-energy correction of a massless quark up to one loop, I get
$$i\Sigma(p)=i\frac{\alpha_s}{4\pi}C_F/\!\!\!{p}\left[\frac{1}{\varepsilon_{\text{UV}}}-\gamma+\ln(4\pi)+1+\ln(\...
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Can $\gamma^5$ matrices be ignored in $q\bar{q}\to ZZ$ processes?
In the $q\bar{q}\to ZZ$ process, the following Feynman diagram in LO appears:
This means for each vertex, the Feynman amplitude contains a term proportional to $(g_V-g_A\gamma^5)$, which makes $D$-...
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Why is the fine structure constant the probability for photon emission by a charged particle?
I see in page 31 of Martin, Hanzel "Quarks and Leptons", that the fine structure constant is the probability for photon emission by a charged particle. Also I read from Lubos Motl's answer on this ...
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In a gauge theory, are two states related by a global phase transformation identified?
In a gauge theory (non-abelian for this question), I am told that two states $|\psi\rangle$ and $|\phi\rangle$ are to be identified if they are related by a gauge transformation $U(x)$
$$|\psi\rangle\...
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Why are two gluons needed for Feynman diagram?
Why do we need two gluons for the decay $$\pi^-+ p\rightarrow\pi^-+n+\pi^+\:\:?$$
If we have always the gluon $$\frac{1}{\sqrt{2}}(r\bar{r}-g\bar{g})$$ it should be possible with only one gluon
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Quantum Chromodynamics (QCD) Prediction of Proton Mass
I’m trying to understand the prediction of the proton mass according to Quantum Chromodynamics (QCD). I know that the proton is composed of quarks and gluons, and that the mass of the proton is not ...
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Uniquness of the vacuum in a theory with/without mass gap
Context
I read the note Light Cone Quantization and Perturbationwritten by Guillance Beuf. He gives a argument in section 3.3.2, p17, 2nd paragraph :
In particular, in a theory with a mass gap, ...
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Solving for gluon propagator in axial gauge
I know the two-point function is given by:
$$
\Gamma^{A_\mu^a A_\nu^b}(p) = -i \delta^{ab} (g_{\mu \nu} p^2 - p_\mu p_\nu + \frac{1}{\zeta}n^\mu n^\nu)
$$
and I am looking to solve for the inverse of ...
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What is the meaning of twist in OPE?
In Operator Product Expansion (such as explained in Peaking) there appear a quantity for an operator called twist, defined to be $d-s$ where $d$ is the scaling dimension of the operator and $s$ is it'...
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Deriving gluon propagator in axial gauge
I am currently checking my work against an answer and I understand most of it except I am having difficulty understanding the signs in a particular part. The question is as follows:
(a) Derive the ...
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Is there any physical reason behind the choice of Lie group in a Yang-Mills theory?
A Yang-Mills theory can be constructed for any Lie group that is compact and semisimple. The motivation behind this is discussed in this question. Is there any physical reason we choose $SU(3)$ or $U(...
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When can colour charge indices be equated?
I'm currently studying QFT and QCD for the first time and I have a question about the colour charge indices given below. I was asked the following question:
(a) Derive the Feynman rule for the 3-...
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What is the temperature of an atomic nucleus?
I conjecture that atomic nucleus with $N\approx 250$ particles could be interpreted as a system in equilibrium (maybe the number is too low to consider the use of statistical mechanics as a reasonable ...
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What is vector meson dominance and why is that related to QCD having a repulsive potential?
At long distances QCD is said to give rise to attractive force between nucleons by pion exchange and to give rise to repulsive force at short distance mediated by vector mesons. Is there a ...
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