(adsbygoogle = window.adsbygoogle || []).push({}); how many tree-level diagrams in various QED processes

Why do Compton scattering, pair annihilation, Moller scattering and Bhabba scattering havetwo

tree-level Feynman graphs to sum over, whereas electron-positron to muon-antimuon scattering and electron-muon to electron-muon scattering have onlyonetree-level Feynman graph?

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

any help appreciated

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# How many tree-levels in various QED processes

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