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
0
votes
0answers
4 views

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 ...
0
votes
2answers
45 views

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.
0
votes
1answer
46 views

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 $\...
0
votes
2answers
26 views

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: $$\...
0
votes
0answers
15 views

How can I simplify the amplitude of the following Feynman diagram as to obtain equation (v)?

I am trying to find the amplitude for the following pair production $$\gamma(p_1) + \gamma (p_2) \to e^- (p_3) + e^+ (p_4)\tag{1}$$ In an old exercise I am given the possible answers but I am not ...
1
vote
0answers
37 views

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 $|\mathcal{M}_{a}|^{2}$...
1
vote
1answer
18 views

(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 ...
3
votes
2answers
48 views

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} ...
3
votes
2answers
137 views

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 ...
0
votes
1answer
48 views

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 ...
2
votes
2answers
87 views

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}{\...
0
votes
0answers
27 views

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 ...
0
votes
1answer
36 views

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, ...
2
votes
1answer
62 views

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\...
0
votes
2answers
59 views

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-...
1
vote
1answer
32 views

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(\...
0
votes
1answer
51 views

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 two pions
1
vote
0answers
35 views

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 ...
0
votes
0answers
19 views

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 ...
0
votes
1answer
47 views

Perturbative expansion of the S-matrix in QFT

Beginner to QFT - While Taylor expanding the exponential term of the S-matrix, why is the 2nd order term (quadratic time integration term) written with two different dummy variables for time? (...
1
vote
1answer
91 views

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'\...
1
vote
1answer
53 views

Decay of particle question

How would you draw the diagram for the $D$ meson decay mode: $$ D^- \rightarrow K^+ + \pi^- + \pi^- $$
1
vote
0answers
28 views

Feynman's Diagram contribution in Green's Function

I was reading Many Particle Physics by G. Mahan and they calculated the Green's function for electron-phonon interaction using Feynman's Diagram. It was written that Green's function contribution by ...
4
votes
2answers
115 views

How to know if a Feynman diagram is planar?

A planar diagram is defined as being one of the leading diagrams for $N \to \infty$ (large $N$ expansion), and, as I understand it, it should have the lowest genus when compared to a non-planar ...
0
votes
0answers
18 views

What is the smallest virtual photon wavelength created from the repulsion of two charged particles (i.e two protons)?

Since the repulsion of two charged particles is mediated through virtual photons. What is the theoretical smallest wavelength these two particles create or is it a range of wavelengths. Is it around ...
1
vote
0answers
50 views

Derive Feynman rules for interacting Proca theory

Is there a smooth way to derive the Feynman rule for the interaction term $$\mathcal{L}_{int}=gA_{\mu}A^{\mu}A_{\nu}A^{\nu}?$$ $A_{\mu}$ denotes a massive vector field.
0
votes
1answer
41 views

Outgoing $Z$-bosons in $s$-channel

I'm working on the process $e^-+e^+\rightarrow ZZ$, taking into account Yukawa interactions, at tree level. Below is the diagram I got for the interaction with the Higgs. I want to write its ...
2
votes
2answers
37 views

Symmetry factor of a loop diagram with gauge-bosons

I have learnt in the lecture that the symmetry factor of the following diagram is 1/2: where the line corresponds to a gluon. But why is this the case? We get 1/2 from 2nd order in pertubation ...
1
vote
1answer
32 views

$u$-channel in Feynman diagrams

I have two questions regarding the $u$-channel in Feynman diagrams. $\textbf{Question 1:}$ Suppose I have a $\gamma\gamma\to\bar{\nu}_{\mu}\nu_{\mu}.$ One of the diagrams will look as one of the ...
4
votes
2answers
137 views

Is it possible that a series of Feynman diagrams converge?

A bit of maybe unnecessary context I'm reading "Lecture notes on Diagrammatic Monte Carlo for the Frohlich polaron". It says It is usually unknown whether a series converges or not. The series is ...
1
vote
1answer
63 views

About Feynman rules and symmetry factor

I have two simple questions about Feynman rules for Lagrangian that have a derivative of fields. For example for Lagrangian in this link part a with derivative couplings. Is the vertex depend on ...
3
votes
1answer
40 views

Why internal lines in a Feynamann diagram represent virtual particles

I am trying to understand the argument for why internal lines in a Feynman diagram are virtual particles that do not obey the energy-momentum relation. For the specific case of the following Feynman ...
1
vote
1answer
44 views

How do we know how many Feynman diagrams to draw for a particular order?

For the purposes of this question, I'll limit the question to quantum electrodynamics, although this question could just as well be asked of electroweak theory and QCD. For a particular order of ...
2
votes
1answer
47 views

Simplest “physical” photon generating Feynman diagram

I'm half way through the excellent "Student Friendly Quantum Field Theory" and I read that single vertex Feynman diagrams in QED are "not physical" because their corresponding amplitudes are zero. ...
2
votes
2answers
32 views

Mismatch in the subscripts of two-body operators in occupation number formalism

I'm reading Prof. Mattuck's "A Guide to Feynman Diagram" and came across this rather simple deviation of the "two-body" operator in the occupation number formalism. How the author labelled the ...
1
vote
1answer
54 views

Resources for One-Loop Calculation in QED

I am recently trying to study the calculation on one-loop diagrams in QED. Since the most resources i found where rather cryptic since they where very very general i wanted to ask whether or not there ...
2
votes
1answer
36 views

Why does the chances of particular Feynman diagram occurring reduces by 1% at each photon-electron interaction?

I saw a youtube video regarding Quantum Electrodynamics which explained how one can eliminate the Feynman diagrams with complex photon-electron interactions or loops. The guy explained that each time ...
3
votes
0answers
101 views

Feynman rules for Gross-Neveu (four fermion) model

I'm having hard times in finding the Feynman rules for the Gross-Neveu model in two dimensions: $$ \mathcal L = \bar \psi^i (i \gamma^{\mu} \partial_{\mu} ) \psi^i + g^2 (\bar \psi^i \psi^i )^2 \; ,$$ ...
2
votes
0answers
49 views

The form of QED Vertex Correction

In chapter 6 of Peskin-Schroeder's text Introduction to Quantum Field Theory it is argued that the form of vertex correction for QED can only have the following form (since we have only the constants, ...
2
votes
0answers
16 views

Feynman rules of electroweak interactions applied to the muon decay

We know that the muon can decay as follows: $$\mu^{-}(p){\longrightarrow}e^-(p') + \bar{\nu}_e(k')+\nu_{\mu}(k)$$ I want to write down the invariant matrix element for this process at tree level, ...
7
votes
2answers
261 views

Path integrals vs. Diagrammatics

The question is about the approximation techniques available in the path integral formulation and their equivalents in the context of the traditional Feynman-Dyson expansion (aka diagrammatic ...
1
vote
1answer
80 views

How to draw quark flow diagram when no flavour change occurs?

I've been asked to draw the quark flow diagram for $K^-p\rightarrow \pi^-\Sigma^+$, I did it like this: (Apologies for the handwriting) As you can see, I basically assumed that since the initial and ...
4
votes
0answers
77 views

Verify Furry theorem in scalar QED

I want to verify Furry theorem in scalar QED. Consider a process with N photons. Is it correct to say that at 1 loop the two classes of Feynman diagram that contribute are the following? with $n+2m = ...
0
votes
0answers
36 views

Reference on QCD Feynman diagrams calculations

I need to compute some amplitudes from simple QCD Feynman diagrams, like the process $q\bar{q} \rightarrow q\bar{q}$ or $q\bar{q} \rightarrow gg$. I have been looking for a book or paper that provides ...
5
votes
1answer
104 views

Scalar QED photon vacuum polarization

Consider a calculation of photon vacuum polarization in scalar QED. The 1PI diagram which people calculate is this one, But in scalar QED the seagull vertex is also possible Then why not include a ...
1
vote
1answer
71 views

Why does this amplitude not vanish by the Ward identity?

Consider the process $e^-\rightarrow e^-\gamma$ depicted in the following Feynman diagram. The spin-averaged amplitude with linearly polarised photons is $$\overline{|M|^2}=8\pi\alpha\left(-g^{\mu\nu}...
1
vote
1answer
51 views

Factors for topologically similar Feynman Diagrams

I am reading Dr. Mattuck's A Guide to Feynman Diagrams in the Many-body Problem and found a rule I didn't see in my other QFT textbooks. It states as: If we are given a diagram, and form a new ...
1
vote
1answer
37 views

Calculation of second-order diagram

Can't figure out how to calculate Feynman diagrams of orders higher than one. One of the simplest diagram of second order is: Integrating over $q_1$ is not a problem because there is $e^{i\omega\...
0
votes
0answers
39 views

Tadpoles and photon self-energy in QED

Tadpoles diagrams are zero in QED for many reasons, one of them is the Lorentz invariance of the vacuum expectation value of the current. Without considering the external photon line (as Peskin &...
3
votes
2answers
94 views

Why do all Feynman diagrams with same number of external legs have the same mass dimension?

In the Ch.18, book of QFT by Mark Srednicki (p.118), it says the diagram have the same mass dimension with tree diagram with the same external lines, because both of them contribute to the same ...

1
2 3 4 5
19