Why does S-matrix unitarity imply the cross section σ ∝ 1s?

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manpreet Tuteehub forum best answer Best Answer 2 years ago

I'm currently learning for an oral s://forum.tuteehub.com/tag/exam">exam in theoretical physics and as a learning s://forum.tuteehub.com/tag/aid">aid protocols of older s://forum.tuteehub.com/tag/exam">exams exist. In one protocol the question was asked:

Why is the scattering cross section σσ at least proportional to 1s1s. The correct answer was that this is the case because the S-Matrix is unitary. Furthermore in the answer it was mentioned, that MM, defined as usual by S=1+i(2π)4δ(...)MS=1+i(2π)4δ(...)M is unrestricted because SS is unitary.

Does anyone have an idea where I can read about this? I searched in quite a few books, but wasn't able to find anything related. (I do understand that S needs to be unitary and know the relationship between the cross section and the S-matrix)

Any idea or explanation would be awesome!

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manpreet 2 years ago

I wonder if this may be of some m.tuteehub.com/tag/help">help.

The optical theorem relates the total cross-section of a scattering with the forward scattering amplitude. For example for a 222→2 scattering you get that:

n|S|mδmn+i(2π)4δ4(pμmpμn)n|T|m⟨n|S|m⟩≡δmn+i(2π)4δ4(pmμ−pnμ)⟨n|T|m

σ12t=12|p1|sImn|T|nσt12=12|p1|sIm⟨n|T|n⟩

What is called amplitude is m" style="margin: 0px; padding: 0px; border: 0px; font-style: inherit; font-variant: inherit; font-weight: inherit; font-stretch: inherit;


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