'Special' fourth order Feynman diagram Compton Scattering - Why is it allowed?

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Discussion Overview

The discussion revolves around the validity of a specific fourth order Feynman diagram related to Compton scattering, represented by the process ##e^- + \gamma \rightarrow e^- + \gamma##. Participants are examining the diagram's compliance with fundamental electromagnetic vertex rules and its interpretation in the context of particle physics.

Discussion Character

  • Debate/contested

Main Points Raised

  • One participant questions the validity of the Feynman diagram, asserting it is not allowed based on fundamental principles.
  • Another participant agrees, reinforcing that the diagram is not permissible, referencing Griffith's book on elementary particles.
  • Some participants suggest that there may be a printing error in the representation of the diagram, specifically regarding the depiction of propagators.
  • There is a clarification that a curly line in the diagram indicates a photon propagator, which does not necessarily imply a virtual particle.
  • One participant points out that the diagram in question may correspond to another diagram in a referenced solution, suggesting that the issue lies in the interpretation of the diagram rather than its inherent validity.
  • Another participant notes that the sum of certain diagrams, rather than an individual diagram, results in a cancellation, indicating a more complex interaction than initially presented.

Areas of Agreement / Disagreement

Participants generally disagree on the validity of the Feynman diagram, with some asserting it is not allowed while others propose that it may be misrepresented or misinterpreted. The discussion remains unresolved regarding the diagram's acceptance in the context of Compton scattering.

Contextual Notes

There are indications of potential misinterpretations of the diagram's elements, particularly regarding the types of lines representing different propagators. The discussion also highlights the complexity of interactions in Feynman diagrams, where the cancellation of contributions from multiple diagrams is noted.

unscientific
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I thought the fundamental electromagnetic vertex is

fundamentalem1.png


Why is the following diagram below allowed? The 'special' feynman diagram for compton scattering ##e^- + \gamma \rightarrow e^- + \gamma## is

fundamentalem2.png
 
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It is not.
 
Orodruin said:
It is not.

This is taken straight from Griffith's book: "Introduction to Elementary Particles".
 
It is still not allowed, assuming the solid lines actually represent electron propagators.
 
Orodruin said:
It is still not allowed, assuming the solid lines actually represent electron propagators.

True. I think there is a printing error. The long vertical line is supposed to be a virtual propagator (curly line) I think.

Check http://hep.uchicago.edu/~pilcher/p237-06/hw8_sol.pdf
 
Curly line means photon propagator, not necessarily a virtual particle.
 
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unscientific said:
True. I think there is a printing error. The long vertical line is supposed to be a virtual propagator (curly line) I think.

Check http://hep.uchicago.edu/~pilcher/p237-06/hw8_sol.pdf
I don't see your diagram there. The last one on page 1 (rotated by 90 degrees) has a curly line = photon at the place where your diagram has an error.

It is certainly virtual independent of its type because it has two ends in the diagram.
 
Assuming its a photon connecting the bottom to the triangle loop, its the last diagram in that list. This diagram and the one with the loop's arrows in the opposite direction exactly cancel. (Write it out, take the trace, add it up). So it is not the diagram that is zero, but the sum of the two.
 
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