# Feynman Diagrams, are these allowed?

Homework Statement:
I have drawn a couple of diagrams for each of the two following equations. Are any of them wrong? Are either preferred? I don't have much practice with drawing these...
Relevant Equations:
electron + positron -> tau+ anti-tau
electron + positron -> tau-neutrino + anti-tau-neutrino

These are my attempts, I have found two possibilities for each, but have no idea if they're 'legal'...

Last edited:

mfb
Mentor
As a rule of thumb: strong > electromagnetic > weak, and tree-level > loops
While the diagrams are possible (except for the signs in the second one, check these) there is a much more important diagram for tau+antitau unless you are close to the Z energy in the collision.
The fourth diagram has additional particles that shouldn't be there.

NumberBucket
As a rule of thumb: strong > electromagnetic > weak, and tree-level > loops
While the diagrams are possible (except for the signs in the second one, check these) there is a much more important diagram for tau+antitau unless you are close to the Z energy in the collision.
The fourth diagram has additional particles that shouldn't be there.
Ah ok, thanks! So would the first one be better with a photon then? And would flipping the RHS of the second one fix the charge issues?

mfb
Mentor
"Better" is subjective. "Stronger" at low energies: Certainly.

Yes, flipping the RHS will fix it.

NumberBucket