New answers tagged feynman-diagrams
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Non-crossing approximation (Altland & Simons)
Thank you for the post! The Fradkin's note you posted really helped me a lot.
I also ran into this problem recently and was really confused about it. But I think the problem is in how you define the ...
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How to interpret virtual electron neutrino in $e^+e^-\to W^+W^-$?
This is really a comment to Riley's answer but it got a bit long for a comment.
Feynman diagrams are widely misunderstood outside of the small community that works on quantum field theory. They do not ...
- 360k
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Diagrammatic computations in the theory with spontaneous symmetry breaking
Popov's book on functional integral method develops the diagrammatic perturbation theory for non-relativistic superfluid in some detail. You can easily adopt the result for K-G theory, only ...
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Accepted
How to interpret virtual electron neutrino in $e^+e^-\to W^+W^-$?
Remember that these diagrams are computational tools, and the relevant information is encoded in their topology. It does not matter whether you choose to identify the vertical line with an electron ...
- 4,106
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Accepted
What are the 4 lowest-order Feynman Diagrams for 𝑒+ 𝑒− → 𝑒+ 𝑒− scattering?
In QED, these are the only two. Under the electroweak theory, there is an identical set which exchange $Z$'s instead of $\gamma$'s.
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Understand "Quantum effective action" in Weinberg's book "The quantum theory of fields"
In the background field method the partition function is $$Z[J;\overline{\phi}]~:=~\int \! {\cal D}\frac{\phi}{\sqrt{\hbar}}~\exp\left\{ \frac{i}{\hbar} \left(S[\phi+\overline{\phi}]+J_k \phi^k\right)\...
- 209k
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Can off-shell particles have imaginary four-momentum components?
Those lines are really propagators.
If you look it up, the propagators are always integrated over all real valued 3-momenta. They were originally only well-defined for the 3-momenta, but 3-momenta are ...
- 10.3k
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$T$-matrix for unstable particle, and the sign of the imaginary part of $T$-matrix
OP asks a valid question.
Usually the transfer matrix is [1]
$$ {\cal T}~=~(2\pi)^4\delta^4(\sum p){\cal M}, \tag{5.17}$$
where ${\cal M}$ is an amputated$^1$ Feynman diagram. The imaginary part ${\...
- 209k
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