Hi I have been asked to produce the lowest order Feynman diagram for the following scattering process:
$$a.~~~ \mu^-+\mu^-\rightarrow \mu^-+\mu^-$$
$$b.~~~ \mu^-+\mu^+\rightarrow \mu^-+\mu^+$$
The Muon can interact via the electromagnetic and weak forces (and gravity). So how do I know whether the virtual particle in each elastic scattering process is a photon or Z boson? My first guess would be the photon, but I'm unsure why.