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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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74 views

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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62 views

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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121 views

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 ...
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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 ...
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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 ...
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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 ...
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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 ...
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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 ...
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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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Why does calculating $\langle J^{\alpha 5}(x) J^{\mu}(y) J^{\nu}(z)\rangle$ involve certain triangle diagrams?

In deriving the chiral anomaly, one wishes to compute the correlation of the axial-vector-vector currents. One writes this explicitly as $$\int d^4xd^4yd^4z \, e^{-ipx} e^{iq_1y}e^{iq_2z} \langle [ \...
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620 views

Calculation of electron vertex correction in Peskin and Schroeder

I am trying two days now to simplify the numerator of the electron vertex correction in the one-loop contribution. My problem is to prove that $$\bar{u}(p')\left[-\frac{1}{2}\gamma^\mu l^2+(-y \gamma ^...
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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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83 views

First-order Contribution to the Self-energy Operator

In Altand and Simons' book 'Condensed Matter Field Theory,' on page 225 they claim that the first-order contribution to the self-energy (effective mass) operator reads $$\big[\Sigma_p^{(1)}\big]^{ab} =...
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Symmetry factor of gluon self-energy

In Peskin & Schroeder, p.523, they give the diagram contributing to the gluon self-energy that arises from the 3-gluon vertex, and they claim that the $1/2$ factor is a symmetry factor: How can ...
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268 views

Charge of $W$-bosons in Feynman diagrams

When looking at Feynman diagrams of particle decays, how would I be able to find out the charges of the $W$-bosons involved in the decay?
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Feynman Lectures on Physics: Vol 1, 11-6: acceleration vector

I’m trying to get through 11-6 section of Feynman’s Lectures on Physics, Vol 1, particularly explanation of acceleration vector calculation in his example: It’s clear that acceleration ...
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1answer
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Neutrino-Neutron Interaction Feynman Diagram (W Boson Direction)

I am currently studying differential cross sections for my Nuclear Physics module. I'm looking an experiment where muon-neutrinos are interacting with nucleons in a scintillator producing muons (which ...
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60 views

Feynman diagram literature for antiproton production via proton-proton collisions

I'm looking for literature, or anywhere I can find a Feynman diagram, which describes the proton-proton collision where antiprotons are produced in the following reaction: $$\rm{p} + \rm{p} \...
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132 views

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 ...
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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 ...
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Question about Gluon self-energy diagram/symmetry factor

I am a qft beginner and have a question: In Peskin & Schroeder chapter 16.5 there is given an expression for a gluon self energy diagram: According to the feynman rules we get $$\frac{1}{2} \int ...
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Local fields vs particles

I have heard it said that Richard Feynman was a proponent of a particle approach to QFT while Julian Schwinger preferred a local fields description. What is meant by “local fields”? Surely when one ...
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Feynman rules for scalar field with second order derivatives in the interaction term

Given the interaction term with $N$ scalars $\phi_i$, each massless, what would be the Feynman rules for an interaction term in the action as $$ \int d^dx (\partial^2 \phi^i)\phi_i(\partial_\mu \phi^...
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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 ...
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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 ...
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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 ...
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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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378 views

Cutoff-Scheme Renormalization and Order of Integration in QFT

The following is the result of Fubini's Theorem, describing when you can replace a double integral with an iterated integral safely: For a set $X \times Y \subset \mathbb{R}^2$, if $\iint |f(x,y)| d(...
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63 views

Is there a program or a website able to perform all Wick contractions for a given expression?

Imagine I have an expression of the type: $$\langle \phi_{x_1} \phi_{x_1} \phi_{x_2} \phi_{x_2} \phi_{z_1} \phi_{z_1} A_{z_1} \phi_{z_2} \phi_{z_2} A_{z_2} \phi_{z_3} \phi_{z_3} A_{z_3} \phi_{z_4} \...
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SUSY Loop diagrams from a categorical viewpoint

In the paper "A Prehistory of $n$-Categorical Physics" J. Baez and A. Lauda give an account of the use of category theory throughout physics. In section “Penrose (1971)” starting from page 25 they ...
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125 views

Simple explanation of a particular diagrams of Feynman [closed]

In relation to this question posed on the website TeX.SE. I am curious to know the use in Physics of green functions about the signs of feynman diagrams with fermionic fields. I have not understood ...
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How does the introduction of the charm quark suppress FCNC?

I did some reading on the GIM mechanism today, and simply fail to understand how it works. I understand how the CKM-matrix can be used to do the basic calculation of the probability of, say, observing ...
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268 views

Definition of one-particle irreducible diagrams

Text books often defines one-Particle Irreducible diagram (1PI diagram) as a connected diagram which does not fall into two pieces if you cut one internal line. Is this internal line the full ...
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Intuitive explanation of superficial degree of divergence

Consider $\varphi^p$ theory in dimension $D$. For a Feyman diagram $\Gamma$ one can introduce the superficial degree of divergence $deg(\Gamma)$. It is defined as $DL-2I$ where $I$ is the number of ...
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608 views

Analytical expression for Hartree-Fock diagram

The picture above is taken from "introduction to many-body physics" by Coleman. When I try to write down the momentum space expression for the two diagrams, I got the same expression for the first one,...
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1answer
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Why are there two photons in pair production Feynman diagram? [duplicate]

Given I wonder why are there two photons entering in a) pair production? Isn't it one photon resulting in $e^+e^-$ ?
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97 views

Feynman Parameters vs Passarino-Veltman reduction

I have computed the following one-loop integral: $$\int \frac{d^dp}{(2\pi)^d} \frac{p^{\mu}p^{\nu}}{(p+k)^2p^2}.$$ Using both Feynman Parameters and the Passarino-Veltman reduction. However, while I ...
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175 views

Higgs Feynman Diagrams

We have been told to compute the amplitude of these diagrams: I am struggeling with some basic points( I've only computed QED diagrams such as Bhabha scattering). Do you use $\epsilon$ for the ...
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124 views

Feynman diagram for Hawking radiation?

I'm starting to wrap my head around Feynman diagrams, and the idea of "real" vs. "virtual" particles, but one area where this distinction seems to break down is in describing Hawking radiation, where ...
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87 views

Why can we use the equation of motion to calculate the amplitude in “Quantum Field Theory”?

I am reading the chapter on electron-proton scattering from "Quantum Field Theory in a Nutshell". The author calculates the amplitude of the electron-proton scattering (up to the second order). The ...
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62 views

How to find theoretically the decay width of a particle-decay process? [closed]

The problem is devided into two parts: First part: There is an interaction Lagrangian for real scalar fields given by L=$\lambda_1 \phi_1 \phi_2\phi_3+\lambda_2\phi_1^2\phi_3$. I need to know how to ...
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152 views

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 ...
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238 views

Are unstable particles ever “real”?

It's always stressed that internal lines of Feynman diagrams have no physical meaning, that is, they don't actually correspond to anything physical. Where this confuses me is in the case of unstable ...
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115 views

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 ...