Currently reading through Robert Klauber's Student Friendly Quantum Field Theory and he got me wondering about electron-positron annihilation.
In Chapter 8 he goes over the expansion of the S-Operator. He uses examples like Bhabha scattering, which includes a second-order diagram for electron/positron annihilation, showing an incoming electron/positron pair annihilating and producing a virtual photon followed by creation of another pair of outgoing electron/positron pair.
Here's my current hang-up: In the expansion of the S-operator, he demonstrates that 1-vertex interactions are NOT physical, which seems to mean an electron/positron pair CANNOT annihilate to produce a real photon.
Does that mean all physically realizable electron/positron pair annihilation MUST be followed by another pair being created?
Otherwise it's a 1-vertex interaction, right?