How many tree-levels in various QED processes

[Lapidus]

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

 

[mfb]

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.

 

[Lapidus]

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!

 

[mfb]

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.