# Electron-Muon Scattering Cross Section

by boltzman1969
Tags: cross, electronmuon, scattering
There is only one diagram in the process, so the amplitude has two momentum-conserving delta functions: $\delta^{ 4 } ( k - k' - q )$ at the electron vertex and $\delta^{ 4 } ( p - p' + q )$ at the muon vertex. So when you do the integral $\int d^{ 4 }q$ you forced to put $p' - p= q = k' - k$. After that you can use the Casimir's trick the trace theorems to obtain the $| \langle \mathcal{ M } \rangle |^{ 2 }$.