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The S-matrix (scattering matrix) relates the initial state and the final state of a physical system undergoing a scattering process in quantum mechanics and quantum field theory. It is the unitary matrix connecting asymptotic particle states in the Hilbert space of physical states (scattering channels).

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On-shell propagator to an off-shell propagator

I am learning the Ward-Takahashi Identity part of Peskin and Schroeder's textbook of quantum field theory. In the prove process, it involves a diagram 7.66. Then it says that I can understand physic …
  • 375
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Proof of unitarity of gauge-invariant S-matrix in Peskin and Schroeder

I'm reading chapter 9.4 "Quantization of the electromagnetic field" of Peskin's and Schroeder's book. When proving the unitarity of the gauge-invariant S-matrix, a trick is used. $$ SS^\dagger=P_0S_\t …
  • 375