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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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How to figure out whether a force mediator is $W^+$ or $W^-$ in Feynman diagram?

How to figure out whether a force mediator is w+ or w- in feynman diagram? I always make sure each vertex is zero but sometimes I get my w+ or w- worng.
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Feynman loop diagrams

I am learning about Feynman diagrams, and I have trouble interpreting loop diagrams. The picture below shows a loop-induced decay of the neutral pion. $\pi^0 \rightarrow e^+ + e^-$. I understand that ...
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Manybody theory: cancellation contributions to proper polarization

I don't understand why the first order diagrams (c) e (d) give null contribute to the correlation energy. We are considering systems of fermions uniform in space and time (we are in momentum space) ...
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Spinless $e^{-}\gamma\rightarrow e^{-}$ Cross section

I was trying to figure out the cross section $\frac{d\sigma}{d\Omega}$ for spinless $e^{-}\gamma\rightarrow e^{-}$ scattering. First I wrote the terms associated with each component. Vertex: $$ie(...
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Feynman diagrams of phi-meson decay

I am learning how to make Feynman diagrams, and I have a couple of questions about two possible decays of the phi-meson. The first question relates to the reaction $\phi \rightarrow\text{K}^+ + \...
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How is the charge conserved at the up to anti-down vertex?

I can see how the charge is conserved in the overall diagram but I can not understand how it is conserved at the up to anti-down vertex.
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Literature result for generic one-loop triangle integral

I've never really asked a literature based question here before, but was wondering if anyone knows where I may find a reliable source for symbolic expressions for generic one-loop triangle diagrams ...
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LSZ reduction, momentum diagram, QFT

I was initially confused about which way to choose the sign of the momentum, since it gives rise to different exponential momentum combinations and thus different deltas for momentum conservation. I ...
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Why does a triangle anomaly appear in a gauge theory?

I have read that when we construct a theory with abelian gauge symmetry there will appear some anomalies when we do the quantum correction to the theory. In 4D space such anomalies are explained by ...
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What parameters determine the probability of virtual photon emission/absorption?

Suppose an electron is producing an electric field by emission of virtual photons and interacting with other particles. What parameters determine the probability that it will emit at least one virtual ...
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Higher point anomalies cancellation from trace

I had asked this question within this one before, but having also made other 2 independent enough questions there, decided to ask this one by itself here. So, it is a well known fact that the ...
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B violation and electric charge

Within SM you can prove that despite we have baryon number conservation respect to Noether theorem, at quantum level baryon (and lepton) number is violated as $$ \Delta B = 3·\Delta n_{CS}, \quad n_{...
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Orbital angular momentum for single particle in particle physics

I am taking an introductory course in particle physics and am quite confused about conservation of angular momentum at a vertex. This trouble arose specifically when considering the decay $\pi^0 \...
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Arrow convention for neutral particles in Feynman diagram

In a Feynman diagram, the arrows on the charged particles are along the direction of negative charge flow. As a result, the arrow is not always directed from the vertex of creation and not always ...
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Why do the diagrams in $\Gamma[\Phi]$ differ from those in $\Phi\Gamma^{\rm int}_{\Phi}[\Phi]$ only by numerical prefactors?

Suppose $W$ is the generator of connected Feynman diagrams in $\Phi^4$ theory. We define $$\Gamma[\Phi]=W[j]-W_jJ,\tag{13.37}$$ where $$W_jJ=\int{dxW_j(x)j(x)}\tag{13.38}$$ and $$ \Phi\equiv\frac{\...
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Substitution of propagator to a product in Zinn-Justin

I'm reading Quantum Field Theory and Critical Phenomena, 4th ed., by Zinn-Justin and on page 127 he defines an action $$S_{\epsilon}[\phi]=\int{dxdy} \phi(x)\phi(y)[K(x,y) + \epsilon ] + V(\phi).$$ ...
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How does absorption of virtual photons “smear” a particle?

I have read that the electron self-energy correction, a process in which the electron absorbs a virtual photon, changes the position of the electron (which leads to the lamb shift in Hydrogen-like ...
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What is the Quantum Mechanical analogue of the Bethe-Salpeter equation?

For studying the bound states of quantum fields theories (e.g. studying excitons or mesons), the Bethe-Salpeter equation is often used as the starting point. Quoting Wikipedia the equation is: $$\...
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Any connected diagram is a tree of full propagators

In P. Etingof, Geometry & QFT, MIT 2002 online lecture notes; Lemma 3.11 (https://physics.stackexchange.com/users/7266/abdelmalek-abdesselam).) He says that any connected diagram is a tree of ...
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Feynman diagram and vertices in a hadron decay

This question is about Feynman diagram and vertices in a hadron decay, which comes from Problem 89.5 in Srednicki's textbook Quantum Field Theory. The reaction involved is \begin{equation} d\...
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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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Euclidean Feynman Rules derivation using Wick Theorem

When studying perturbation theory and Feynman rules, the standard derivation seems to start from the S-Matrix element in the interaction picture, and expands it into some series, after which Wick's ...
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Feynman diagram combinator and multiplier value

Dear experts, I try to learn by self feynman diagram from examples of x^3 and x^4 for first and second order consolidated from different reading material. For x^3, combinator values of 3 * 3 for n=1 ...
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Neutrino scattering cross-section in high energy limit

I've come across a problem in out particle physics lecture notes that I have been unable to resolve on my own. I've paraphrased the bits of the notes that are giving me so much trouble: The notes ...
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How to evaluate the Feynman gauge photon propogator in coordinate space?

In coordinate space the Feynman gauge looks like: $$\frac{g_{\mu\nu}}{\partial^2} .$$ Without fourier transforming, how does this operator act on currents? In general for inverse derivative operators ...
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How to calculate $n$-point functions of interacting fields in curved spacetime (Schwarzschild metric)?

How to renormalize quantum field theory in curved spacetime? (or in Schwarzschild spacetime?)  I want to calculate n-point functions $$<0|Tφ(x_1)...φ(x_n)|0>$$ in massless $φ^4$ theory in ...
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Why do we demand that the counterterms in $\varphi^3$ theory be $O(g^2)$?

In Srednicki's QFT book, section 9, he introduces the $\varphi^3$ lagrangian: $$\mathcal{L}= -\frac{1}{2}Z_\varphi(\partial_\mu\varphi)(\partial^\mu\varphi) -\frac{1}{2}Z_mm^2\varphi^2 +\frac{1}{6}...
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Failing to show $\xi$-gauge-independence in an abelian Spontaneously Broken Gauge Theories (SBGT)

I am studying the following paper: Appelquist, Carazzone, Goldman & Quinn, Renormalization and Gauge Independence in Spontaneously Broken Gauge Theories, https://doi.org/10.1103/PhysRevD.8.1747 ...
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Lorentz decomposition of electromagnetic current Cheng&Li $\mu\rightarrow e\gamma$ p.421

I've been struggeling with this for a while, should be very easy probably.... In Cheng and Li p. 421 they consider the transition $$T(\mu \rightarrow e\gamma) = \epsilon^\lambda<e|J^{em}_\lambda|\...
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$\phi^3$ 2D 1-loop diagram disambiguation

I would like to calculate the 1-loop 1-PI correction to the propagator for $\phi^3$ scalar theory in 2 dimensions, where the integral is finite. Performing the usual procedure (Feynman trick, Wick ...
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Ward identity for 'general' operator and current diagrams

This is actually about two related doubts and I hope is appropriate for a single question (if not, I will happily divide it). So, my problems are related to the analysis and calculation of chiral ...
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Drawing lowest-order Feynman diagrams for $\rho^0 \rightarrow \pi^- \pi^+$ decay [duplicate]

I am trying to draw the lowest order Feynman diagrams for $\rho^0 \rightarrow \pi^- \pi^+$ for a homework question. I came up with the following answers: In the answer key given, only the diagram ...
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Error in proof of Wick's theorem

I am having a little trouble proving wick's theorem. I'll start from the last step that I know is correct. We define $$\left\langle \prod_{j=1}^{2m}x_{i_j}\right\rangle:=\left.\frac{\partial^{2m}}{\...
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1answer
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Feynman diagrams in Gaussian integrals

I am looking for suggestions for material regarding Feynman diagrams for gaussian integrals. I am looking for something of the sort of: Pedro Vieira, Statistical Physics Applied to Quantum Field ...
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Is there any Feynman diagram for Baryon/Hyperon decay with missing energy?

I am reading the hyperon decays with missing energy this paper tells for the prospects of baryon decays with missing energy I want to know some of the Feynman diagrams for hyperon decays. Can anyone ...
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Feynman diagram for semileptonic decay of neutral kaon

I am unsure how to draw a feynman diagram for a reaction that occurs as follows $$ K^0 --> l^+\nu_l\pi^- $$ Any tips would be helpful.
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IR divergence Feynman diagram topology query

I am trying to calculate the superficial degree of Infrared divergence. To do this I am reading section 12 of this source. It seems you can calculate it by a method involving the 'shrinking' of ...
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Reference for Feynman diagram technique(position space) in Thermal Field Theory

I am trying to study perturbative expansion of Sachdev-Ye-Kitaev model, where I know that the dominant terms are the Melonic diagrams. I am interested in seeing how perturbative corrections affect the ...
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Why do we always choose centre-of-mass frame to study particle physics problems (e.g. Feynman diagrams)? Even in non-relativistic regimes (v<0.3c)

Example: electron positron annihilation forbids a single photon production, and searching through this forum (Why can't a single photon produce an electron-positron pair?) and a few books, they ...
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Free energy: comparison between exact solution and perturbation theory

I'm studying a system in condensed matter for which there is an exact solution for the free energy $F(\lambda)$, where $\lambda$ is the parameter of the interaction. I can expand $F(\lambda)$ at some ...
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1answer
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Lepton flavor violating process on loop level $\mu \rightarrow e \gamma$ via Higgs, Divergent integral

I am stuck with calculating the proces $\mu \rightarrow e \gamma$ as in this diagram: I wrote down the matrix element like this: $$ \mathcal{M}=\bar{u}(p-q)\left[\int \frac{d^4k}{(2\pi)^4}A\frac{(k+...
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Computation of Feynman integrals in terms of Chen Iterated integrals

One of the techniques used to compute Feynman integrals in dim-reg is the so-called "differential equations method". Exploiting integration-by-parts identities (linear functional relations between ...
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Cross section for 2 particles that decay independently

This maybe is a naive basic question but I want to be sure. If I want to calculate the cross section of the process $$p\bar{p} \rightarrow W^+HX \rightarrow e^+\nu_e b\bar{b} X$$ ($H$ is a Higgs ...
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EW theory vertices

I'm trying to undertand the following vertex: Initial state of up and anti-down quarks with finalk state made of $W^+$ boson. Does it go with left or right projector? I think that from Lagrangian it ...
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Why are there infinitely many Feynman diagrams for any particular reaction?

In Griffith's Intro to Elementary Particles book, he wrote there are infinitely many Feynman diagrams for any particular reaction! Why is this true? Take for example Moller scattering that ...
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Perturbation theory development for the ground state of the QM particle in the box with a centered dirac-delta spike

In the course of a discussion in the chat there emerged an interesting problem, namely a particle in an infinite well with an additional Dirac-delta function spike of scalable hight: $$ H = -\frac{\...
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How to search Feynman diagrams for entanglement [closed]

This question is partly about physics, partly about love -- but I don't believe there's a Stack Exchange for the later :) My girlfriend is a PhD physicist who finds the notion of entanglement very ...
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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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Why this loop diagram is not included in Phi-4 theory of Peskin?

Consider a $2\rightarrow2$ scattering process in $\phi^4$ theory. In the book of Peskin's, he consider the 3 loop corrections in the parenthesis: My question is: Why doesn't he include this diagram? ...
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How do one find the first correction order of $\lambda$ for scattering cross section?

I have a lagrangian $\mathcal{L}=\mathcal{L_0+\mathcal{L_1}}$, where $\mathcal{L_1}$ is a perturbation given by:$$\mathcal{L_1}=-(1/3!) \lambda \phi^3-(1/4!) \lambda \phi^4 $$ and$$\mathcal{L_0}=-\...