= Solution
Count a reference process whose fiducial <cross section> is accurately calculable. At an <electron-positron collider>, small-angle <Bhabha scattering>, dominated by $t$-channel <photon> exchange, is abundant and provides a standard choice. Require coincident high-<energy> <electron> and <Positron> deposits on opposite sides of the interaction point, with consistent back-to-back directions, and impose precisely specified angular and <energy> cuts. Then
$$
\boxed{\mathcal L_{\rm int}=\frac{N_{\rm Bh}-N_{\rm Bh,bg}}{\epsilon_{\rm Bh}\,\sigma_{\rm Bh}^{\rm fid}}.}
$$
The theoretical prediction must include radiation and other relevant electroweak effects under the same event definition. Finite angular resolution, alignment, beam position, live time, trigger efficiency and background contamination enter the uncertainty budget. For small <scattering angle> $\theta$, $t\simeq-s\theta^2/4$ and the leading <differential scattering cross-section> behaves approximately as $d\sigma/d\theta\propto\theta^{-3}$; the inner fiducial angle must therefore be particularly accurately known. Binning these counts in time can also measure the instantaneous <collider luminosity>.
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