Consider Compton scattering off an electron, (k1) + e(p1) (k2) + e(p2), where k1 (k2) represents the 4-momentum of the incoming (outgoing) photon, and p1 (p2) the 4- momentum of the incoming (outgoing) electron.

By using the Mandelstam variable t = (k1 k2)^ 2, show that the scattering angle of the photon (angle between both photons) in the lab frame (electron at rest) and in the center-of-mass frame are related through cos = (cos )/ (1 cos ) with = (s me^2 )/(s + me^2 ), s = (k1 + p1)^ 2 , and me denoting the electron mass.