1. The problem statement, all variables and given/known I have to prove an equation for the differential cross section of compton scattering of an electron and a photon (electron (P) + photon(K) ⇒ electron(P') + photon (K') ) where P and so on are the inital and final four momenta. Given is that the square of the matrix element for the process, summed over all spin states and divided by 4 is given by: i have to prove: Where E and p are the initial energy and the size of the three momentum of the electron. We can also use the following formula: Where i believe we assume the factor 1/va-vb to be 1/2. Can someone help me solve this? I also have no idea where the squares of the cosines and sines in the expression for the cross section come from .