Electron-positron annihilation into a quark pair
= Electron-positron annihilation into a quark pair
{title2=$\sigma_q=4\pi\alpha^2Q_q^2/s$}
For massless quarks at photon-exchange tree level, the initial-spin-averaged, final-color-summed amplitude is $\overline{|\mathcal M|^2}=8N_c e^4Q_q^2[(p_1\cdot q_1)(p_2\cdot q_2)+(p_1\cdot q_2)(p_2\cdot q_1)]/s^2$. In the centre-of-mass frame this is $N_c e^4Q_q^2(1+\cos^2\theta)$. With $N_c=3$ and the standard two-body flux, its integrated cross-section is the displayed result.