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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 $e^+e^-$ ?
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 ...
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 ...
39 views

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 ...
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 ...
170 views

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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 ...
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 ...
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 $B$ ...
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 ...
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 ...
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....
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: ...
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 ...
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 ...
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 ...
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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.
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...