In Peskin's textbook section 7.3 The Optical Theorem for Feynman Diagrams(Page233), he said it is easy to check that the corresponding t- and u-channel diagrams have no branch cut singularities for s above threshold.(adsbygoogle = window.adsbygoogle || []).push({});

But I can't figure out how to prove it. Can angone help me? Thanks!

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# A The Optical Theorem for Feynman Diagrams

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