All Questions
Tagged with quantum-chromodynamics quantum-field-theory
402
questions
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255
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What are all the 1PI gluon 3-point function Feynman diagrams at 1-loop?
As an exercise in renormalization, I want to calculate the divergent part of the gluon 3-point function and gluon 4-point function matrix element. What are all the 1PI one-loop Feynman diagrams?
- 2,599
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106
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Do quarks have a non-zero electric dipole moment?
It is written here, in this PSE link that an electron has a measured perfect spherical charge density therefore a zero electric dipole moment (i.e. perfect monopole charge).
My question is, are quarks ...
- 4,170
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109
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Quark mass dependence of Glueball masses
In pure QCD, we have glueballs. Pure QCD can also be thought of as QCD where the fermion masses have been sent to infinity. If we vary the fermion masses (say, for simplicity, we deal with a single ...
- 6,896
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185
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How does the Nambu-Goldstone mode explain the absence of parity doubling?
I've been doing some reading about chiral symmetry breaking since it was not touched in my particle physics course
I found these slides
As explained in the above link, if we take $|\psi \rangle$ as ...
- 3,464
2
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92
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Why does a gluon soft factor depend only on the adjacent legs?
The gluon soft factor can be obtained by taking the soft gluon limit in the $n$-point MHV amplitude $$A(1,2,\cdots, n)=\frac{\langle i,j\rangle^4}{\langle 1,2\rangle\cdots\langle n, 1\rangle},$$ to be ...
- 302
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68
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Laplace transformation with mass
I am doing some QFT calculations with the fermion mass term, and it is making the calculation much more challenging.
Say I have
$$F(t,m) = \int_0^1 dx \frac{1}{1-x}\frac{1}{(t ~(1-x)+m(1-x)^2)^\...
- 857
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96
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Is the pion valence PDF symmetric?
The positive-pion $\pi^+$ is comprised of 1 valence up-quark and 1 valence anti-down-quark. Assuming isospin symmetry, the parton distribution functions (PDFs) for these two valence quarks should be ...
- 5,371
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88
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Is QCD parity conserving also non-perturbatively?
Since QCD is fundamentally non-perturbative at low energies one may ask if QCD is still Parity conserving. In the path-integral formalism using the Faddeev–Popov ghosts as gauge fixing terms the ...
- 183
3
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304
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Renormalization group applied to a simple QCD problem
Consider page 551 of Peskin & Schroeder's book on QFT, where he is treating the process $e^{+}e^{-} \to \text{Hadrons}$. I have 3 misunderstandings regarding some lines of thought effected there:
...
- 1,742
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122
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What means Higgs branches in 5d?
I've been reading advanced string theory text but I came across with a paper that I definitely don't understand at all, what it means?, this text is Higgs branches of 5d rank-zero theories from
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How to derive Collinear amplitude proportional to Born amplitude
In the collinear limit, the squared matrix element factorises into (for partons 4 and 5 going collinear)
\begin{eqnarray}
\overline{\sum}|M_3(1+2 \to 3+4+5)|^2 \approx \overline{\sum}|M_2(1+2 \to 3+4')...
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120
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Spinor index contractions for baryons
For an $SU(N)$ QCD with $N_f$ flavours one writes baryons as
\begin{equation}
B^{{a_1}…a_{{N}_f}}=\epsilon^{i_1…i_N}\psi_{i_1}^{a_1}… \psi_{i_N}^{a_{Ν_f}}
\end{equation}
by contracting the colour ...
- 143
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506
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What does the Pontryagin index do in BPST instanton (solution to Yang-Mills theory)?
$$
\mathcal L = -\frac12\mathrm{Tr}\ F_{\mu\nu}F^{\mu\nu}+i\bar\psi\gamma^\mu D_\mu\psi
$$
We take this Lagrangian for QCD, after this I need to calculate BPST instanton with topological Pontryagin ...
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575
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Why is the QCD $\theta$-term aware of the topology of the space?
The QCD Lagrangian without the $\theta$-term $$\mathcal{L}_{QCD}=-\frac{1}{4}G_{\mu\nu}^aG^{\mu\nu a}\tag{1}$$ is not topological. However, the $\theta$-term $$\mathcal{L}_\theta=\frac{\theta}{32\pi^...
- 26.8k
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How to calculate the branching fraction of hadrons? Or good resources on chiral perturbation theory
I am wondering how I could calculate branching ratios (or any effective diagram) for various hadrons. In particular I am interested in a rare pion decay via a virtual photon:$\pi^0\to \gamma \gamma^* \...