Take the tour ×  The task is to draw the two 4th-order Feynman
diagrams of:  $$e^- + \mu^+ \to \nu_e + \bar{\nu}_\mu $$  I drew
the first one as (time left->right):     mu^+    \bar{\nu}_\mu
\nu_\mu          /         /         /W^-      /Z^0        /
/       /         / e^-    \nu_e        \nu_e  which is correct.
I drew the second one with a $W^+$ instead of a $W^-$, with the
$W^+$ going the other direction (from the $mu^+$ to the $e^-$).
However, the solutions gives the second diagram differently:
rather, the order (in time) of the $W^-$ and the $Z^0$ is
interchanged. I agree that this should be a valid feynman
diagram as well for the process, but I don't understand why my
alternative with the $W^+$ would be illegal. As far as I can
see, it obeys all conservation laws.   share|improve this
question Welcome, Carl! We have the MathJax rendering engine
active on the site which means that you can write equations as
LaTeX. I've marked up you question that way, ad you can see how
I did it my clicking