Is my understanding correct:(adsbygoogle = window.adsbygoogle || []).push({});

Charge is conserved at a vertex. So the charges of particles pointing in towards a vertex and the charges of particles pointing out must be equal. Alternatively: The sum of the charges at a vertex must be zero.

So if this is correct, can someone explain the attached diagram to me?

This is for the decay process:

[tex]\pi^- -> \mu^- + \overline{\nu_{\mu}}[/tex]

To me, it should be a W^+ particle rather than W^-, since the charge at the first vertex is d = -1/3, anti-u = -2/3, and therefore if it's a W^- then the charge will be -2 rather than zero. If we put a W+ that solves the problem. Similarly on the other side, we have -1 from the muon and zero from the anti-muon-neutrino so we need a +1 from a W+ to conserve charge at the vertex.

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# Conservation Laws at a Vertex

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