The Stack Overflow podcast is back! Listen to an interview with our new CEO.

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.

Filter by
Sorted by
Tagged with
5
votes
1answer
65 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, ...
0
votes
1answer
55 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?
0
votes
1answer
63 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 ...
1
vote
0answers
85 views

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 ...
0
votes
0answers
29 views

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 [ \...
0
votes
0answers
12 views

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 ...
3
votes
1answer
202 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 ...
0
votes
1answer
46 views

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 ...
1
vote
1answer
81 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} =...
0
votes
1answer
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} \...
0
votes
2answers
39 views

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 ...
0
votes
1answer
39 views

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 ...
0
votes
0answers
39 views

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^...
2
votes
1answer
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 ...
0
votes
0answers
13 views

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 ...
0
votes
0answers
43 views

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 ...
2
votes
1answer
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} \...
1
vote
0answers
56 views

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 ...
3
votes
1answer
82 views

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 ...
5
votes
1answer
79 views

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 ...
1
vote
1answer
58 views

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 $e^+e^-$ ?
0
votes
1answer
123 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 ...
0
votes
1answer
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 ...
0
votes
1answer
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 ...
3
votes
1answer
151 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 ...
9
votes
2answers
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 ...
2
votes
2answers
48 views

Do virtual mesons exchanged between nucleons in the nuclear force ever decay before reaching the recipient nucleon?

So my understanding of the nuclear force so far is this (please correct anything I have wrong): Being a residual of the strong force, the nuclear force is mediated (in part) by the emission of ...
0
votes
1answer
60 views

Diagrammatic Representation of non-Gaussian perturbation expansions

I have no experience in graph theory and am a little confused with how Hugh Osborn represents a perturbation expansion with diagrams on page 15 of these notes. We have a perturbation expansion My ...
4
votes
1answer
156 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 ...
2
votes
1answer
62 views

The “Hartree-Fock energy” in the Feynman formalism vs the Hartree-Fock method

This question has been previously asked, but I do not understand the answer. When calculating the ground state energy of an interacting system by a perturbative expansion in terms of Feynman diagrams,...
0
votes
1answer
43 views

Contribution of a second-order Feynman diagram for the one-particle Green function

I am studyng how to construct Feynman diagams for the perturbative expansion of the one-particle Green function (or propagator) using the book "A Guide to Feynman Diagrams in the Many-Body Problem". ...
1
vote
2answers
59 views

Are decay Feynman diagrams really Feynman diagrams too? Or just vertices?

So I was wondering, the vertex diagrams for the standard Model, can they also be feynman diagrams with on shell particles? For instance here is the W - boson vertex which decays into electron and ...
0
votes
0answers
48 views

Feynman rules for space-dependent coupling

Let's say I have an effective action which looks like (I got this action from large $N$ method for $\varphi^4$ theory): $$\int \frac{d^4x}{2g}\phi^2(x)+\int d^4x \ \log(-\nabla^2+\mu^2+i\phi(x)). $$ ...
0
votes
1answer
38 views

Interaction - pion to tau and antineutrino

$$ \pi^{-} \to \tau^{-}+\overline{\nu}_{\tau} $$ So, this interaction (reaction) is supposed to be forbidden. But, can not see what should be wrong here. Charge is conserved. Baryon number is fine. ...
1
vote
1answer
36 views

Feynman diagram possible interaction

$$ p+\Lambda^{0} \to n+\Sigma^{0} $$ So, I am trying to draw a Feynman diagram for the written interaction. Baryon number is conserved, charge is conserved, so that works. When I look at the quarks, ...
1
vote
1answer
77 views

Antiproton proton reaction

So, I am doing some exercises in Feynman diagrams, and when I have proton and anti-proton reacting to produce only one photon $$ p + \overline{p} \longrightarrow \gamma $$ Spin, baryon number, ...
0
votes
1answer
38 views

Self-energy in two scalar Yukawa interaction

Considering the Lagrangian of two scalar fields in $d=4$: $$\mathcal{L}=\frac{1}{2}(\partial\phi)^2-\frac{1}{2}m^2\phi^2+\frac{1}{2}(\partial\chi)^2-\frac{1}{2}M^2\chi^2-g\phi^2\chi$$ What would be ...
2
votes
0answers
47 views

Exotic perturbative anomaly captured only by higher-loop Feynman graphs, but not by any 1-loop Feynman graph?

My question: Are there any perturbative anomaly captured by higher-loop but not by captured at the 1-loop Feynman graph (say, not enough)? We are familiar with the text book example of a ...
2
votes
1answer
68 views

One-loop corrections to vacuum polarization with a specific Lagrangian

I'm having some difficulties regarding this problem in QFT I'm doing to prepare for an exam. For the following problem I consider the theory described by the Lagrangian: $$\mathcal{L}=-\frac{1}{4}F_{\...
1
vote
2answers
89 views

Why only loop effects be quantum corrections when the full theory is quantum?

Feynman diagrams with one or more loops in an interacting QFT are diagrammatic representation of corrections to the Green's functions and amplitudes beyond the lowest order in perturbation theory (...
0
votes
1answer
61 views

Does the W boson necessarily change an anti-fermion's flavor to its anti-neutrino counterpart?

I'm writing the diagrams for the following process in Standard Model: $$\nu_e + e^+\rightarrow \mu^++\nu_\mu+\gamma$$ I want to know if the W boson changes the flavor of $\mu^+$, for instance, ...
3
votes
0answers
69 views

An equivalent computation of a Feynman diagram

A typical second-order diagram for the self-energy gives integrals such as: $$\int \int d \omega^\prime \omega^{\prime \prime} g(\omega-\omega^{\prime})g(\omega^{\prime \prime})g(\omega^{\prime}+\...
0
votes
2answers
53 views

Question on the $1/N$ expansion

My question is from Coleman's Aspect of Symmetry, on the large $N$ approximation. We will consider the $O(N)$ version of the $\phi^4$ theory. Its Lagrangian density is given by: $$ \mathcal{L}=\...
0
votes
0answers
53 views

Question about Bhabha scattering Feynman Diagram

I'm a uni student going into his third year, and thought I might try to read through Griffith's particle physics textbook for the heck of it. I've been trying to become familiar with Feynman diagrams, ...
1
vote
1answer
80 views

Tadpole diagrams in 1-loop massive scalar amplitudes?

Consider a massive scalar diagram such as or The loop momentum enters and exits the tadpole vertex, so that in the first diagram the momentum in the propagator connecting the two vertices is zero ...
1
vote
1answer
92 views

Pion decay as a point-particle

The $\pi^-$ meson is a composite particle of $\bar{u}d$ quarks, but for many practical purposes it can be treated as a point particle with an effective interaction. The vertex responsible for the $\pi^...
0
votes
1answer
77 views

Interpretation of Feynman diagrams

I'm currently taking the second part of a two semester course in quantum mechanics and while discussing the interaction of matter and radiation some Feynman diagrams came up. The context is "...
3
votes
0answers
72 views

Scalar electrodynamics “seagull” vertex factor

By expanding the covariant derivative of the Scalar QED lagrangian one gets the following term, sometimes called "seagull" vertex. $$\mathcal{L}_{seagull} = -q^2A_\mu A ^\mu \phi^\dagger \phi$$ Most ...
0
votes
0answers
31 views

Four-Divergence of a massive vector field in the interaction term

In an exercise is given the following Lagrangian $$\mathcal{L} = -\frac{1}{4}F^{\mu\nu}F_{\mu\nu} + \frac{1}{2}MA^\mu A_\mu + \bar\psi(i\gamma^\mu\partial_\mu - m)\psi + \mathcal{L}_{int}$$ with the ...
0
votes
0answers
23 views

Computation of the self-energy term of the exact propagator for $\varphi^3$ theory in Srednicki

In M. Srednicki "Quantum field theory", Section 14 -Loop corrections to the propagator-, the exact propagator $\mathbf {\tilde \Delta} (k^2)$ is stated as $$\frac{1}{i} \mathbf {\tilde \Delta} (k^2)...