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I have a very naive question concerning Feynman diagrams: If we consider the process $\pi^{+}\rightarrow \mu^{+}\nu_{\mu}$ via the weak interaction, then we get this Feynman Diagram according to Wikipedia:

enter image description here

Now, my question is the following: If I drew this diagram, is this also correct?

enter image description here

Or if I only exchanged the positions of $\mu^{+}$ and $\nu_{\mu}$? Personally, I'd say yes, but I just wanted to double-check.

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  • $\begingroup$ What have you learnt that a Feynman diagram means? Also note that your drawn diagram doesn't make sense $\endgroup$ Commented Nov 3, 2020 at 8:02
  • $\begingroup$ Hi @NiharKarve, why doesn't my Feynman diagram make sense? I learned that a Feynman diagram is helpful tool for calculating things like cross-sections etc., but we haven't done these things yet. $\endgroup$
    – user248824
    Commented Nov 3, 2020 at 8:08
  • $\begingroup$ An antimuon is an antiparticle, while a neutrino is a particle, so the arrows are the wrong way round $\endgroup$ Commented Nov 3, 2020 at 8:10
  • $\begingroup$ Ah, I see! But apart from the arrows - which are of course the wrong way around - it should be okay? Also, would the diagram still be valid if I exchanged only the positions of $\mu^{+}$ and $\nu_{\mu}$ in my own Feynman diagram? $\endgroup$
    – user248824
    Commented Nov 3, 2020 at 8:14
  • $\begingroup$ Yeah, I thought that was what you want ask - yes, it would be valid: only the topology of the Feynman diagram is important $\endgroup$ Commented Nov 3, 2020 at 8:16

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The current diagram, as drawn, is incorrect as the antimuon $\mu^{+}$ and muon neutrino $\nu_{\mu}$ should have opposite arrows. However, if you were to reverse the arrows, the diagram would be correct as only the geometric topological properties of the Feynman diagram are relevant, and not the relative position of vertices.

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