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Questions tagged [feynman-diagrams]

A diagrammatic technique introduced by Richard Feynman to describe the quantum behaviour of subatomic particles and their interactions. Do not use for general questions on diagrams that are not of the Feynman kind.

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Is there an interaction vertex that have an initial state of particle and final state an anti-particle?

I have a home work problem stated as follows: I am only required to draw out the Feynman diagram of the examples for three cases and calculate the charge of initial and final state respectively to ...
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Example using Cutkosky cutting rules where a cut is not allowed by violation of conservation of momentum

Using $(+,-,-,-)$ the Cutkosky cutting rules are stated as Cut through the diagram in any way that can put all of the cut propagators on-shell without violating momentum conservation. For each cut, ...
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Software for calculating Feynman Diagrams

Is there a software (open source preferred) where I would input something like "Ingoing: a fermion $(p1, s1)$ and a photon $(p2, s2)$. Output: A fermion $(k1, r1)$ and a photon $(k2, r2)$" and I would ...
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Feynman diagrams as topology

When we talk about Feynman diagrams we know they are tools to make calculations easier and more intuitive. Moreover, it's said that they are "topological" representations of the interactions. But, ...
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Why does $\zeta(3)$ appear in the expression for the anomalous magnetic moment?

The anomalous magnetic moment, to fourth order in $\alpha$, contains $\zeta(3)$. Is there a simple explanation for the appearance of this value of the Riemann zeta function?
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Optical theorem in $\phi^4$: which poles contribute to discontinuity in Feynman amplitude?

Section 7.3 ("The Optical Theorem") in Peskin and Schroeder's QFT text contains a leading order verification of the optical theorem in $\phi^4$ theory by calculating the (discontinuity across the ...
66 views

Schrodinger equation vs Feynman diagrams

If one wants to assess how an electron orbits in a hydrogen atom one uses the Schrodinger equation. Ditto for an electron in a magnetic well. However if one wants to assess how particles interact or ...
223 views

Renormalization group and summation of diagrams

Currently I'm studying renormalization group, and I'm having trouble understanding the following statement which I see almost everywhere in books on QFT: renormalization group sums a series of ...
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How to solve integrals with $3$ Feynman parameters? [migrated]

I would like to evaluate integrals of the following type (in position space): $$\int \frac{d^{2\omega}z}{\left[(x_1-z)^2 (x_2-z)^2 (x_3-z)^2 \right]^A} \tag{1}$$ I can introduce three Feynman ...
189 views

Feynman diagrams: from QFT to condensed matter

I studied Feynman diagrams in quantum field theories and I'm going to study them in the context of condensed matter physics. In this post Books for Condensed Matter after Ashcroft/Mermin, two books ...
156 views

Combinatorics geometric series two-point function

In this answer Proof of geometric series two-point function it is said: Now what about the coefficients in front of each Feynman diagram? Due to the combinatorics/factorization involved it ...
600 views

Use my example to explain why loop diagram will not occur in classical equation of motion?

We always say that tree levels are classical but loop diagrams are quantum. Let's talk about a concrete example： $$\mathcal{L}=\partial_a \phi\partial^a \phi-\frac{g}{4}\phi^4+\phi J$$ where $J$ is ...
173 views

Chirality in Feynman diagrams

In quantum mechanics, chirality is preserved for massless particles. But in Standard Model, is there any reason for chirality to be preserved for massless particles in annihilation diagrams? (unless ...
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A bilinear form or a dot product $X \cdot Y$ of emission X and absorpion Y of a vector boson W in the context of transformation of quark flavors

The reason for this question is related to the fact that I think something is missing when an absorpion and emission of a (intermediate) vector boson $W$ in the context of transformation of quark ...
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Feynman rules of $\mathcal{N}=4$ supersymmetric Yang-Mills in Euclidean space

I am trying to derive the Feynman rules for $\mathcal{N}=4$ supersymmetric Yang-Mills. The (Euclidean) action that I start with comes from this paper (Wilson Loops in $\mathcal{N}=4$ Supersymmetric ...
558 views

Feynman rules from Lagrangian

I know there are already questions about determining the Feynman rules given a particular Lagrangian, but this question is more about finding out whether I understand correctly how to do this. I'm ...
132 views

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What is a Witten diagram?

Recently I heard the terminology of Witten diagram. Studying QFT, I frequently see Feynman diagrams and use them to compute scattering amplitudes, one-loop corrections and so on. In string theory ...
199 views

Faster ways of computing feynman diagrams

Obviously the machinery of QFT allows us to calculate processes, such as QED diagrams, to great precision, and whilst it is effective, it seems there are many processes that make calculations (say by ...
263 views

Pauli- Villars regularization in the Electron Vertex Function: Evaluation

I'm studying one loop contribution for electron vertex function form Peskin and Schroeder's book " An introduction to quantum field theory " Section: 6.3. I have some troubles with Pauli- Villars ...
158 views

Feynman diagrams for gravity

Feynman rules is the basic tool to compute amplitudes in perturbation theory for a QFT. Here, I am trying to understand perturbation theory in GR around the flat space metric, in terms of Feynman ...
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What is the relationship between the cross section of two different mechanism that produce identical outcome?

I want to calculate the Compton collision cross section for a pair of entangled photons produced by two different mechanisms. Let mechanism A and B be different but both produce an identical outcome ...
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Feynman diagram with polarized particles

While evaluating Feynman diagrams it's common to deal with unpolarized particles, meaning that we have to sum over the final particles and average over the initial ones: this is quite straightforward ...
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Calculating the numerical factor from Feynman diagram

I kind of understood the symmetry factor quite well. However, I just do not understand how one can relate the Feynman diagram to the term (especially the numerical factor in front of it) in the ...