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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20 views

Does this is an onshell propagator?

This image is from here (page 6-7). My question refers to Eq. 26 and 27 The denominator changed from $(p_1 -p_3)^2 - m^2$ to $p_3^2 - 2p_3p_1$. So they used $p_1^2 = m^2$. But I thought, this relation ...
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Tadpole diagrams in $\phi^3$ theory

In "Quantum Field Theory" by Mark Srednicki, Chapter 9 page 67, after he proves that $\langle 0|\phi(x)|0 \rangle$ vanishes (meaning sum of all connected diagrams with a single source is zero), he ...
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Detailed calculation of the scattering amplitude between two particles in $\phi^4$-perturbation theory using the path integral formalism

I'm looking for detailed example/calculation of the scattering amplitude between two particles (mesons) in $\phi^4$-perturbation theory using the path integral formalism. Do anyone of you guys have a ...
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Correction to the scalar propagator - derivative coupling

Given the scalar field Lagrangian $$\mathscr{L}=\frac{1}{2}e^{-\lambda\phi}\partial_\mu\phi\partial^\mu\phi,$$ evaluate the order $\lambda^2$ correction to the propagator. At that order in $\...
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Taylor Expansion of Feynman Propagator with regulator masses and gamma matrices

I am currently trying to understand a really old paper of Jackiw and Coleman: "Why dilatation generators do not generate dilatations". There, at some point they arrive at the following ...
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Scattering of electron anti-neutrino on hydrogen (Feyman diagram)

Does anyone have a reference how the (lowest-order) Feynman diagrams for the scattering of an electron anti-neutrino $\overline\nu_e$ on a hydrogen atom would look like? I'm a bit confused about the ...
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Confusion among channel $u$ and channel $t$ in exclusive reactions

In the this paper (https://arxiv.org/abs/1203.4392), on page 2 it is said that Since DVCS amplitude is symmetric under $s \leftrightarrow u$ channel crossing, the CFFs and $^{S}C^i_{\cdots}$ ...
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Summing over disconnected diagrams - Peskin and Schroeder

In Peskin & Schroeder, page 97, the following expression is given as part of the demonstration of how the $n$-point correlation function is calculated using connected diagrams: $$\sum_{\text{...
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Are there software for drawing Feynman diagrams easily? [duplicate]

I want to know if there softwares to draw Feynman diagrams easily.
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QFT and coordinate transformations

I was going through this lecture, where Leonard Susskind explains that these two Feynman diagrams are considered to be equivalent in QFT: One diagram explains the same phenomena in one possible way ...
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Existence of bound state in (higher dimensional) conformal field theory

I'm wondering why correlation function is a more fundamental object in conformal field theory than the notion of particle. By conformal symmetry, I think we can still discuss about massless particle. ...
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Physical Interpretation of Partition Function with Background Field

Integrals of the form $$\langle{\phi(x_1)\cdots\phi(x_n)}\rangle=\frac{1}{Z}\int\mathcal{D}\phi\,e^{-\frac{1}{\hbar}S(\phi)}\phi(x_1)\cdots\phi(x_n)$$ can be evaluated by considering Feynman diagrams ...
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Pair production through massless gauge mediator

I am wondering if it is possible for a massless boson to change the particle interacting with it, into another type of particle. In other words, is it possible to have a pair production process (in S ...
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How to determine correct spinor in Feynman diagram

Setup Consider Moller scattering, that is $$e^-(\vec p_1, \alpha)+e^-(\vec p_2, \beta) \quad\longrightarrow\quad e^-(\vec q_1, \gamma)+e^-(\vec q_2, \delta),$$ where the $\vec{p}_i,\vec q_i$ label the ...
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Feynman diagram of $\Xi^{-}$ decay

How would one go about drawing the Feynman diagram for the following reaction? $$\Xi^{-} \rightarrow n\space+\pi^{-}$$ In terms of quarks: $$dss\rightarrow udd\space + d\overline{u} $$ Could the ...
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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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Cutkosky rules for a two-loop diagram

Let's say we want to calculate the imaginary part of the following scalar diagram in $\varphi^3$ theory: This amplitude is given by the expression $$i \mathcal{M} = i^5 \int \frac{d^4 \ell_1}{(2\pi)^4}...
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Confusions on effective action and 1PI 3-point Green's function [duplicate]

In Peskin's QFT book chapter 11.5, the author gives a graph(see below) and claims that the third functional derivative for effective action $\Gamma$ gives the 1PI (one-particle-irreducible) 3-point ...
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Finding the amplitude for the pair production $\gamma(p_1) + \gamma (p_2) \to e^- (p_3) + e^+ (p_4)$

I'm trying to find the amplitude for: $$\gamma(p_1) + \gamma (p_2) \to e^- (p_3) + e^+ (p_4)\tag{1}$$ (My questions are stated in the end) The possible answers are: My take on it: and so $$\tag{2} \...
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Decay of the Phi Meson: Error in Question?

I'm trying to draw Feynman diagrams for 'The decay of the pseudoscalar meson $\phi = |s \bar{s}\rangle $ into these final states: i) $\phi \rightarrow \pi^{+} \pi^{-},$ where $\pi^{+}=|u \bar{d}\...
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1answer
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Calculating traces for triangle diagrams with massless fermions

I am following Schwarz Quantum Field Theory textbook. In particular, I am looking at triangle diagrams with massless fermions. On pg. 623 - 624 Schwarz attempts to calculate $q_\mu^1 M_{5}^{\alpha\mu\...
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One-loop Feynman integral over Euclidean momenta

I am trying to perform the following one-loop computation $$ \int \frac{d^Dq}{(2\pi)^D} \frac{(k+q)^2 q^2}{((k+q)^2+m^2)(q^2+m^2)} $$ where $k$ is fixed and everything is on the Euclidean setting, so ...
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Feynman diagrams and pair creation/annihilation

I'm trying to make sense of QFT (I am a mathematician). I have absorbed "from the ether" the following physical interpretation of Feynman diagrams in the Hamiltonian picture, which I really ...
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2answers
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Symmetry factor of certain 1-loop diagrams in $\phi^4$-theory

I have to derive a formula for the symmetry factor of the diagrams of the form in $\phi^4$-theory, where $\phi$ is a real scalar field. By symmetry factor I mean only the number of possible ...
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Is the following Feynman diagram (given in Schwartz) wrong?

For the process $e^+e^-\rightarrow \gamma \gamma$ Schwartz gave following Feynman diagram in section 9.4 I think the diagram is wrong since it didn't conserve charge at both the vertices. The correct ...
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280 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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How is the interaction between two strings (in string theory) computed or quantified?

What properties or quantities of two strings are considered when computing the interaction between them? Is there a conventional way to represent interaction of two strings diagramatically? Or even ...
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About the decay of Higgs into 2 $Z$-bosons

I need to know if I understand this point. The factor of $HZZ$ vertex in the SM Lagrangian is half the value of the Feynman rule which appears in Diagrammatica. When I calculate the decay of Higgs ...
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1answer
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How does one calculate the absolute value of a Feynman diagram's amplitude?

How do I obtain the absolute value of a Feynman diagram's amplitude if I do not have values for the components of this amplitude? If the amplitude of a process such as $e^+(p_1) + e^- (p_2) \to \phi (...
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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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How to derive Goldstone-Wilczek current?

I am reading this Goldstone-Wilczek celebrated's paper on fractional quantum number. In particular they derived for the following Dirac Lagrangian ($\phi_1$ and $\phi_2$ are scalar fields) $$\mathscr{...
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Can I ignore the integration function in the amplitude of a scattering process for the scalar Yukawa theory?

When calculating the amplitude of a scattering process for the scalar Yukawa theory involving two scalar ($\phi$) and two complex scalar particles ($\varphi$ and $\bar{\varphi}$) I have noticed the ...
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Which of these Feynman diagrams are correct?

The first is from a textbook, the second is from Wikipedia. Intuitively, I think the textbook one is correct.
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How do I know which Feynman diagram is right? ($s,t,u$-channel)

Following the Lagrangian: $$\tag{1} \mathcal{L} = \mathcal{L}_{initial}-M^2 \varphi^* \varphi - g\varphi^*\varphi -g\phi^3$$ where $\phi$ is a particle associated with a real scalar field, and $\...
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1answer
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Expansion of trace in photon self-energy

I am studying through the photon self-energy $$ i\Pi_{\mu\nu}(q) = \int\frac{d^4 k}{(2\pi)^4}Tr\left[(-ie\gamma_\mu)\frac{i(\require{cancel}\cancel k+m)}{k^2-m^2+i\epsilon}(-ie\gamma_\nu)\frac{i(\...
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w do I draw the tree-level Feynman diagram if the interaction term only represents the scalar particles?

Consider the process $$e^+(p_1)+e^−(p_2) \to S(p_3)S^∗(p_4)\tag{1}$$ $S/S^*$ is scalar particle/antiparticle described by the complex scalar field $\phi$ coupled to QED through the Lagrangian: $$\...
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Problem with the conservation law in Feynman diagram

I have some problem with the virtual particle process as discussed below. We have some interactions that have real photon as produced particle. We consider a tree level Feynman diagram for it and the ...
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Deriving the number of loops and number of lines meeting at a vertex in a diagram

When reading about Superficial Degrees of Divergence (SDOD) I have seen in An Introduction to Field Theory (Chapter 10.1) that for Lagrangians with a $\phi^n$ interaction term we know that: $$\tag{1} ...
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How to average over colour factor for process gluon-quark scattering?

My question is how to average the color factors when calculating the magnitude of amplitude for gluon-quark scattering process, for example, how to average over color factor for $|\mathcal{M}_{a}|^{2}$...
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1answer
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(Coleman many-body Chapter 8) Validity of near-Fermi-surface approximation

In the Chapter 8 of Coleman's many-body physics book, he argues as follows. In the impurity problem, the approximate self-energy can be written as (8.89). I have no problem until this part. However, I ...
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1answer
165 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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How does the transformation of 4-derivative into a 4-momentum actually happen in a derivative coupling?

Consider a derivative coupling with $$\mathcal{L}_{int} = \lambda \phi_1 (\partial_\mu \phi_2) (\partial_\mu \phi_3),\tag{7.101}$$ and a scalar field $$ \phi(x) = \int \frac{d^4p}{(2\pi)^3} \frac{1}{\...
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How to calculate the Feynman diagram under $SU(N_c)$ group?

I give my own answer below. I use the feynman rules for QCD directly to calculate the process: $qq'\rightarrow qq'$, where $q$ and $q'$ have different flavor. But I don't how to give my answer ...
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Physical interpretation of propagator pole

In QFT the propagator is divergent for the on-shell momentum of the particles. When e.g. calculating the amplitude for a box loop, the propagator diverges for on-shell particles running in the box-...
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Question about gluons and the W-boson regarding Feynman diagrams

My questions are: 1) Does a gluon always have no charge? 2) Is the W-boson always positive or negatively charged? I thought these two statements were correct, but I came across an exercise that ...
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1answer
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Question about Feynman diagrams

In the picture there are feynman diagrams for two reactions. My question is: Why aren't 6.3 and 6.4 possible for the first reaction? And as a sidenote, I don't really have a good source for the rules, ...
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1answer
57 views

Decay of particle question

How would you draw the diagram for the $D$ meson decay mode: $$ D^- \rightarrow K^+ + \pi^- + \pi^- $$
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Making feynman diagram [closed]

I have interaction hamiltonian as $${\cal H}_I = -\frac{1}{2}\sum\limits_{k\neq k',q}\sum\limits_{\sigma,\sigma'}\frac{\omega_{k-k'}M_{k-k'}^2}{\omega_{k-k'}^2-(\epsilon_k-\epsilon_{k'})^2}\hat c_{k'\...
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Question about Renormalization of Ghosts in QCD in MS scheme

During a calculation of the Renormalization constant of the ghosts in QCD I stumbled over the following question: When I calculate the self-energy of the Faddeev-Popov ghosts in $SU(N)$ non-abelian ...
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How to deal with integral operators in the action, in the path integral of a field theory?

One could imagine adding to the free action of a scalar field theory some non-local operators given as integrals over the base manifold (or over the boundary) of some smooth function of the scalar ...

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