Bhabha Scattering: Why Only Two Leading Order Diagrams?

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Discussion Overview

The discussion centers on Bhabha scattering, specifically the number of leading order Feynman diagrams involved in the process (e+ e- -> e+ e-). Participants explore the implications of different channels (s, t, and u-channel) and their contributions to the scattering event.

Discussion Character

  • Debate/contested

Main Points Raised

  • One participant questions why there are only two leading order diagrams in Bhabha scattering, suggesting that there should be contributions from s, t, and u-channel diagrams.
  • Another participant asserts that there is only one diagram, clarifying that the s-channel and t-channel contribute to Bhabha scattering, while the u-channel is associated with Moller scattering.
  • A different participant expresses confusion, presenting a diagram they believe to be a u-channel diagram for the same reaction.
  • One participant clarifies the concept of "crossing" in relation to Feynman diagrams, indicating that it involves a rotation of the diagram rather than a literal crossing of lines.
  • A later reply acknowledges a misunderstanding regarding the distinctiveness of the diagram they drew, expressing gratitude for the clarification.

Areas of Agreement / Disagreement

Participants exhibit disagreement regarding the number of leading order diagrams and the role of the u-channel in Bhabha scattering, with no consensus reached on the matter.

Contextual Notes

Some participants may be operating under different assumptions about the definitions and contributions of the various channels in the scattering process, leading to confusion and differing interpretations.

gamma5772
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I don't understand why in Bhabha scattering (e+ e- -> e+ e-) why there are only two leading order Feynman diagrams. It seems to me like there should be s, t, and u-channel diagrams. Could someone explain why I am wrong?
 
Last edited:
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Actually there is only one diagram. If the reaction in the s-channel is AB → CD (e+ e- → e+ e-) then the reaction in the t-channel is ACBD (e+ e- → e+ e-) and both of these contribute to Bhabha scattering. But the reaction in the u-channel is Moller scattering: AD → CB (e+ e+ → e+ e+).
 
Sorry if I'm being slow, but the attached Feynman diagram seems to be a u-channel diagram for the reaction e+ e- -> e+ e-.

(Apologies -- I don't know how to make Feynman diagrams in latex)
 

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  • uchannel.png
    uchannel.png
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! "Crossing" does not mean that the lines literally cross over one another. The diagram is just rotated, so that one or more legs move from a past direction to a future direction, or vice versa.
 
Oh my. This is embarrassing -- I thought for a minute the diagram I just drew was distinct from the other two!

Thank you!
 
Last edited:

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