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The cross section [tex]\frac{d\sigma}{d(cos \theta )} (e^+e^- \rightarrow \mu^+ \mu^-) \propto 1 + cos^2 \theta [/tex] for a purely vectorial (electromagnetic) interaction. Hence [tex]\sigma[/tex] is expected to be symetric in [tex]cos \theta[/tex].

The axial vector (weak) coupling of the Z boson violates parity and give an asymmetric contribution to the [tex]\sigma[/tex] distribution.

So obviously the asymmetry give a measure of the Z exchange contribution.

OK, so Q: I don't understand why this reasoning doesn't apply when you consider [tex]e^+ e^- \rightarrow e^+ e^- [/tex]

Surely this behaves the same, since the photon and Z exchange is equally probable?