ABSTRACT
QED is the field theory that describes the interaction between the photon and the charged fermions. Before calculating cross sections, this chapter discusses in more detail the helicity of the fermions and its relation to charge conjugation. That charge conjugation is really a symmetry of the quantum theory is most convincingly demonstrated by the fact that the Dirac Lagrangian is invariant under charge conjugation. An important consequence of the charge conjugation symmetry is Furry’s theorem, which states that a fermionic loop with an odd number of vertices will not contribute to the amplitude. This chapter discusses electron-photon scattering, also known as Compton scattering. The gauge invariance is again instrumental in decoupling the longitudinal component of the photon field.
