Past exam of the mathematics course of the University of Cambridge / 2015 / ia / Paper 4 / 4C / Solution

Use the Minkowski metric convention $+---$. A particle's four-momentum is $P=(E/c,\mathbf{p})$, with $P\cdot P=m^2{c}^2$ for rest mass $m$. For a photon, its energy and wavelength obey $E=hc/\lambda$, and its four-momentum is null. Here $h$ is the Planck constant.

The initially stationary electron has $P=(mc,0,0,0)$. Choose the scattering plane as the $xy$ plane. The incoming and outgoing photon four-momenta areBy four-momentum conservation, the final electron has $P^{\prime }=P+K_1-K_2$. Its rest mass is unchanged, so the relativistic energy-momentum relation gives $P^{\prime 2}=P^2=m^2{c}^2$. Since $K_1^2=K_2^2=0$, expansion yieldsorMultiplying through gives the Compton scattering shift:It is nonnegative and vanishes for forward scattering, consistent with the recoil energy being supplied by the photon.