inally, use Eq. (14) to calculate the differential cross section for muon-
antimuon production in electron-positron scattering associated with the
chosen spinor helicities.
\sum_r ,r^(')\sum_s ,s^(')|M|^(2).
$$ \sum_{r,r'} \sum_{s,s'} |\mathcal{M}|^2. \tag{14} $$