Thanks! I was hoping to get an answer from you, as you always put me in the right direction. Do you also happen to know a reference that discusses this topic in detail rather than books only highlighting this mechanism?
I was reading Diagrammatica by Veltman and he treats the photon field as a massive vector boson in which gauge invariance is disappeared and the propagator has a different expression than in massless photon. After some googling, I found that this is one way to formulate QED which has the...
Can you perhaps say a bit more about vacuum QFT and adiabatic switching. Recently I saw these terms more frequently and would like to know more about them. Do you know a good reference to read about this subject?