I read there are two definitions about S-operator:
The first one (e.g (8.49) in Greiner's Field Quantization) is: Sfi≡⟨Ψ−p|Ψ+k⟩
SoSfi≡⟨Ψ−p|Ψ+k⟩=⟨p|(Ω−)†Ω+|k⟩
In this case the S-operator ˆS=(Ω−)†Ω+, where Møller operator Ω+=limt→−∞U†(t)U0(t)
Another definition (e.g (9.14) (9.17) (9.99) in Greiner's Field Quantization) is : Sfi≡⟨Ψ−p|Ψ+k⟩≡⟨Ψ−p|ˆS′|Ψ−k⟩=⟨Ψ+p|ˆS′|Ψ+k⟩
It seems that these two definitions are differnt, but many textbook can derive the same dyson formula for these two S-operators. https://en.wikipedia.org/wiki/S-matrix#The_S-matrix
How to prove: Ω+(Ω−)†=eiα(Ω−)†Ω+
related to this question: There are two definitions of S operator (or S matrix) in quantum field theory. Are they equivalent?
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