The tree-level invariant amplitude for the equivalent of Compton scattering in scalar QED was calculated in Eq. (7.6.54). Use this to calculate the unpolarized differential cross-section \(d \sigma / d \Omega\) in the fixed-target frame (initial \(\phi\) at rest). (Hint: Use \(\sum_{\kappa} \epsilon^{\mu}(k, \kappa) \epsilon^{\mu}(k, \kappa)^{*} \rightarrow-g^{\mu u}\) as in Eq. (7.6.83).)