All Questions

Filter by
Sorted by
Tagged with
1
vote
1answer
46 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 $e^+e^-$ ?
3
votes
0answers
61 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
23 views

How to put the terms in correct order when deriving amplitudes in QED from the S-Matrix?

I am trying to derive the amplitude for the Møller scattering process $e^-+e^-\rightarrow e^-+e^-$ from "first principles", that is, by working out the calculation with the relevant S-matrix terms ...
0
votes
0answers
39 views

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(...
0
votes
1answer
34 views

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 ...
0
votes
1answer
57 views

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 ...
0
votes
0answers
34 views

The different limits of a scattering process

Let's assume I have a process of some sort ($e^-e^+\rightarrow e^-e^+$ / $e^-e^-\rightarrow e^-e^-$ / etc) and I calculated its unpolarized differential cross section using no assumptions at all (...
1
vote
1answer
103 views

Physics Meaning of Trace Technology in QED [closed]

As it pointed out on page 133 of Peskin and Schroeder, any QED amplitude involving external fermions, when squared and summed or averaged over spins, can be converted to traces of products of Dirac ...
1
vote
1answer
52 views

u-channel in $gg \rightarrow u\bar{u}$

I've seen that for the QCD process $gg \rightarrow u\bar{u}$, where $g$ is a gluon and $u, \bar{u}$ are the up quark and the corresponding antiquark, there is s, t and u channels. I perfectly ...
15
votes
3answers
315 views

Why don't very high order Feynman diagrams contribute significantly?

In a particle physics lecture I had today it was stated that the magnetic moment, $g$, is not quite equal to 2, and the difference is accounted for by QED. Later it was stated that we can see this ...
0
votes
1answer
59 views

Can the Hermitian conjugate of a column vector still be column vector?

From what I understand, contravariant vectors are represented by column vectors, and covariant vectors are row vectors. So for a QED current, say $j ^ { \mu } = \overline { \psi } \gamma ^ { \mu } \...
2
votes
1answer
186 views

How many counterterms does QED have?

I have read the statement that QED has four counterterms to cancel divergences. However, I have learnt that there are only three counterterms (vertex, electron propagator, photon propagator), which is ...
-1
votes
2answers
71 views

How can I prove the annihilation electron positron can't generate positron + muon? [closed]

How can I prove the process $e^{+}+e^{-}\rightarrow e^{+}+\mu ^{-}$ doesn't happen?
3
votes
1answer
812 views

Feynman rule for trace over fermion loop

Where does the Feynman rule for "taking the trace over the matrix product arising from a fermion loop" come from? I can not derive it in the "usual way", that is writing the correlator all the way ...
1
vote
1answer
378 views

Diagrams involved in 1-loop electron self-energy in QED

I'm following the derivation of electron self-energy at 1-loop in QED on Peskin-Schroeder, page 216. To second order in the coupling the considered diagram (7.15) is The 2-point correlator at second ...
2
votes
0answers
73 views

Diagrams Contributing to Electric Charge Correction (M. Schwartz)

In M. Schwartz's Quantum Field Theory and the Standard Model, he talks about the correction to the $g$-factor in Chapter 17. In Section 17.2 (Pg. 318), he is evaluating the diagram that gives the ...
2
votes
1answer
320 views

Why can there be only 3-particle vertices in Feynman diagrams? [closed]

Why can there be only 3-particle vertices in Feynman diagrams? For example, when a particle and its anti-particle annihilate to form two photons, why is its Feynmann drawn the way it is in diagram 2 (...
5
votes
2answers
412 views

Simplify quantum electrodynamics expression

I have a quantum electrodynamics exercise on the one-loop electron self-energy correction in which I need to show that $$ \tag{1} ie^{2}\Sigma(p)=\frac{\left(-ie\right)^{2}}{\left(2\pi\right)^{4}}\...
1
vote
2answers
648 views

What do the arrows on this Feynman diagram represent? [duplicate]

The picture follows the convention of the time axis being on the vertical direction. I think this Feynman diagram represents Coulomb repulsion between two electrons with a virtual photon being ...
1
vote
1answer
102 views

Feynman amplitude $\mathcal{M}$ for non-physical processes in QED

I know $e^-e^+\rightarrow\gamma$ is an example of a non physical process in QED because it violates four-momentum conservation. For that reason, the Feynman amplitude $\mathcal{M}$ is never written or ...
2
votes
1answer
170 views

The photon propagator term in Peskin & Schroeder Eq. 6.38

In Peskin and Shcroeder, when calculating the one-loop vertex correction, the line above Eq. (6.38) reads $$ \rightarrow \int \frac{d^4 k}{(2\pi)^4} \frac{-ig_{\nu\rho}}{(k-p)^2 + iϵ} \bar{u}(p') (-...
3
votes
0answers
151 views

QED parity invariance and gamma 5 [closed]

Why does the invariance of parity of QED indicate that gamma-5 can not appear in Feynman diagrams?
0
votes
1answer
61 views

Which channel processes are allowed in the interaction $e^-+e^+\rightarrow 2\gamma$

s-, t- or u-channel? How can you tell?
3
votes
0answers
73 views

Identifying diagrams for optical processes

I was reading some papers on the study of the optical properties of some metals and came upon these conference proceedings by Hopfield from 1972. They are on the study of the infrared properties of ...
0
votes
2answers
198 views

Can all light-matter interactions be reduced to the photoelectric effect, the Compton scattering, and pair production?

My book says that all interaction of light and matter can be reduced to the photoelectric effect, the Compton scattering and pair production. How true is this? What about reflection and absorption ...
0
votes
1answer
745 views

Feynman Diagrams Pair Annihilation

In Griffith's Introduction to Particle Physics it says that there are two processes mainly contributing to pair annihilation (i.e. $e^++e^-\to\gamma+\gamma$) How does the second process (the one with ...
2
votes
2answers
425 views

Determining Feynman rules from Lagrangian

To preface, I'm not a physics major so I've never taken a QFT class before. But I'm working on something that requires me to find the amplitude of that following Lagrangian: $$ \mathcal{L} = -\frac{1}...
3
votes
1answer
504 views

Guessing of the vertex factor of derivative interactions

I want to ask if my "empirical" method of guessing the vertex factor is correct. As far as I understand in canonical QFT the Feynman rules are only a way to rearrange the important parts of the ...
2
votes
2answers
109 views

QFT: Range of 'collision'

If two particles approach each other, they can [provided that their properties add to those of other particle(s)] interact and go from, say, $$e + \bar e \to \gamma + \gamma$$ My question is how ...
1
vote
1answer
97 views

How to describe the phase change for a charged matter wave in an external magnetic field using virtual photons?

In the case of an electron travelling through a region with an external magnetic field, experiments show that the field changes the phase of the electron wave function. The phase change depends on $B$ ...
1
vote
2answers
178 views

What exactly happens in the basic QED Feynman diagram?

When a photon is absorbed by an electron, I think that the following things happen: The electron changes in momentum, angular momentum and energy. The phase of the electron wave function changes by a ...
1
vote
1answer
298 views

Decay of K+ Meson

In Griffiths Introduction to Elementary Particles we are asked to draw the Feynman diagram for The decay of The $K^{+}$ Meson into a photon, antimuon and muon-neutrino. I am confused by the Diagram in ...
0
votes
1answer
533 views

Is Compton Scattering the same as Pair anihilation?

In Griffith's Elementary Particles we are asked to draw all four vertex Feynman Diagrams for Compton Scattering. Most are straightforward but one puzzles me. As has been said here: https://www....
3
votes
2answers
2k views

How to correctly understand these “1-particle-irreducible insertions”?

In QED, when dealing with the vacuum polarization and the photon propagator, some authors like Peskin & Schroeder introduce the so-called "1-particle irreducible" diagrams. These are defined as: ...
4
votes
3answers
430 views

How does a perturbation theory make sense in quantum field theory?

The idea of a perturbation series in powers of a coupling $\alpha\ll1$ (for example, the fine structure constant in QED) make sense if the contribution of $(n+1)^{th}$ term in the series is smaller ...
0
votes
1answer
346 views

Is the non-perturbative approach to QFT a path integral approach? If so then how, given we don't have simple path integral formula for Dirac equation?

Here is my understanding of the scenario. Please correct me if i go wrong somewhere. Initially, the perturbative approach to QED (Feynman diagrams​) was very successful. But the same approach to QCD ...
0
votes
1answer
590 views

Is a Feynman diagram for an electron emitting and later absorbing the same photon possible with just 2 vertex in the diagram?

In a Feynman diagram, the particles behave like free particles everywhere except at one of the vertex of the diagram. The vertex is the site for an interaction to occur. So, if we have an electron ...
3
votes
1answer
177 views

Finding Feynman Rules for Reduced/Pseudo-QED

I am trying to guess/compute the Feynman rules for the following theory. $$ \mathcal{L} = \delta(x^d)\bar{\psi}\iota\gamma^\mu D_\mu \psi(x)-\frac{1}{4}F^{\hat{\mu}\hat{\nu}}F_{\hat{\mu}\hat{\nu}}(x) ...
3
votes
1answer
372 views

Select Feynman diagrams for 2-loop QED vertex correction

I am trying to calculate the 2-loop correction for a basic vertex in QED So I need to figure out what diagrams I need to consider. My literature says (without much explanation), that there are 7 ...
2
votes
1answer
346 views

Photon propagator counterterm in QED

The lagrangian for QED including counterterms is $$\mathcal{L} = -\frac{1}{4}F_{\mu \nu}^{2}+i\bar{\psi}\gamma^{\mu}{\partial_{\mu}} \psi-m_R \bar{\psi}\psi-e_R \bar{\psi}\gamma^{\mu}A_{\mu}\psi- \...
2
votes
1answer
756 views

Calculating Feynman amplitude for closed two loop diagrams

Could someone explain or post a link for how one calculates the Feynman amplitudes for closed two loop diagrams? This is in QED with the same fermion in both loops and a photon connecting them.
2
votes
0answers
500 views

Feynman rule for closed fermion loop in QED

One of the Feynman rules of QED is the following: For a closed fermionic loop, the Feynman rule is to start at an arbitrary vertex or propagator, follow the line until we get back to the starting ...
0
votes
0answers
106 views

1PI diagram of photon scattering

Consider the set of 1PI diagrams with three external photons. It is apparent that there are no tree-level diagrams for this process. What are the possible one-loop diagrams for this process? Why do ...
2
votes
2answers
518 views

Feynman diagram of electron- electron interaction and cool down of an electron gas

Reading this answer I wondered if during an electron-electron interaction real photons have to be emitted. Since the interaction of two electrons is accompanied by the change of direction and velocity ...
0
votes
1answer
129 views

Basic question regarding Feynman diagrams

I am considering a muon-electron scattering process: $$ \mu^{-} + e^{-} \rightarrow \mu^{-} + e^{-}$$ I need to draw all possible diagrams and discuss which diagrams that does not contribute to the ...
7
votes
1answer
511 views

Feynman diagrams: sum of square of diagrams or square of sum of diagrams?

I’m asking this question in the context of the discussion of the optical theorem in section 7.3 of Peskin and Schroeder. The optical theorem states that the imaginary part of a scattering amplitude ...
1
vote
0answers
159 views

Orthogonality of matrix elements, trace techniques

I am currently reading Thomson's "Modern particle physics", and I have trouble understanding a concept that he somewhat gives for granted. Using Feynman rules, I am able to compute the matrix element ...
1
vote
1answer
77 views

Understanding massive internal boson line and its virtual nature

Consider the scattering $$e^-(p_1)+e^+(p_2)\rightarrow e^-(p_1^\prime)+e^+(p_2^\prime)$$ at the tree-level via a internal photon line of four-momentum $q$. Using energy momentum conservation at the ...
1
vote
1answer
325 views

Virtual Photon in Electron Scattering Feynman diagram [duplicate]

If we know that the virtual photon emitted in an electron scattering Feynman diagram violates the energy and momentum conservation laws (though temporarily), why do we accept it as a feasible diagram ...
1
vote
3answers
984 views

Electron-positron annihilation: Feynman diagram

Maybe my question is just a blunder. Consider Compton interaction: $e^{-}+\gamma \rightarrow e^{-}+\gamma$. There are two Feynman diagrams related to this process to the lowest order of $\alpha$. Now ...