If $\nu \mathrm{e} \rightarrow \nu$ e scattering proceeds by $\mathrm{Z}$ exchange, show that (13.46) is obtained from the Feynman rules using (13.41) as the vertex factor. In particular, use the expression for the boson propagator (see Section 6.17 ) to verify that $(13.46)$ is valid provided the four-momentum transfer $q$ is such that $\left|q^2\right| \ll M_Z^2$.