How many tree-levels in various QED processes

  • Context: Graduate 
  • Thread starter Thread starter Lapidus
  • Start date Start date
  • Tags Tags
    Qed
Click For Summary

Discussion Overview

The discussion revolves around the number of tree-level Feynman diagrams in various Quantum Electrodynamics (QED) processes, specifically comparing processes like Compton scattering, pair annihilation, Moller scattering, Bhabha scattering, and others involving different particle types. The focus is on understanding why some processes have multiple diagrams while others have only one.

Discussion Character

  • Technical explanation, Conceptual clarification, Debate/contested

Main Points Raised

  • One participant questions why certain QED processes have two tree-level diagrams while others have only one, suggesting a possible connection to identical particles.
  • Another participant proposes that when the final particles are the same as the initial particles, a simple photon exchange leads to multiple diagrams, which is not the case when the particles differ.
  • A follow-up question is raised regarding the difference in the number of diagrams for Compton scattering compared to electron-muon scattering, despite both involving similar initial and final states.
  • It is noted that electrons and photons can interact directly, allowing for two ways to connect incoming and outgoing photons in Compton scattering, whereas electron-muon scattering only allows for a single photon exchange.

Areas of Agreement / Disagreement

Participants express differing views on the reasons behind the number of diagrams, with some agreeing on the role of identical particles while others provide alternative explanations. The discussion remains unresolved regarding the specific mechanics of each process.

Contextual Notes

Some assumptions about particle interactions and the nature of identical particles are not fully explored, and the discussion does not clarify the implications of these assumptions on the number of diagrams.

Lapidus
Messages
344
Reaction score
12
how many tree-level diagrams in various QED processes

Why do Compton scattering, pair annihilation, Moller scattering and Bhabba scattering have two
tree-level Feynman graphs to sum over, whereas electron-positron to muon-antimuon scattering and electron-muon to electron-muon scattering have only one tree-level Feynman graph?

I suspect that it has something to do with identical particles, but I do not see how.

any help appreciated
 
Last edited:
Physics news on Phys.org
Just look at the individual graphs - if the final particles are the same as the initial particles, you always have a simple photon exchange. Something you cannot have if the particles are different.
 
thanks for answering mfb!

But then why does "electron + photon --> electron + photon" (Compton scattering) have two Feynman diagrams at the tree level, but "electron + muon --> electron + muon" only one!
 
Electrons and photons interact directly with each other, and you have two possible ways to connect incoming + outgoing photons to the electron.
In electron/muon scattering, the single photon exchange is the only option.
 

Similar threads

Undergrad Confusion about Scattering in Quantum Electrodynamics
  • · Replies 134 ·
5
Replies
134
Views
12K
Undergrad QED - ordering of Feynman diagram rules
  • · Replies 6 ·
Replies
6
Views
5K
Undergrad One vertex interactions in QED - error in the book?
  • · Replies 4 ·
Replies
4
Views
2K
Tree-level diagram Moller scattering
  • · Replies 3 ·
Replies
3
Views
2K
Graduate How, and in what atoms does the Lamb shift occur?
  • · Replies 8 ·
Replies
8
Views
3K
Undergrad How to determine Spinor in Feynman diagram
  • · Replies 6 ·
Replies
6
Views
2K
Undergrad Progress In Calculating The HLbL Contribution To Muon g-2
  • · Replies 0 ·
Replies
0
Views
3K
Graduate Questions concerning Bhabha scattering
  • · Replies 14 ·
Replies
14
Views
6K
Graduate On tree level Feynman diagrams
  • · Replies 2 ·
Replies
2
Views
6K
Undergrad Gamma absorption at increasing energy levels
  • · Replies 5 ·
Replies
5
Views
2K