Is anyone familiar with Breit-Wigner Cross section? Say, for a reaction with 2 particles in the initial state, 1 intermediate and 2 final:(adsbygoogle = window.adsbygoogle || []).push({});

[tex] \sigma = \frac{g \pi \lambda^2 \Gamma_i \Gamma_f}{(E-E_0)^2 + \frac{\Gamma^2}{4}} [/tex]

I can't see why for the wavelength we should use the momentum of EITHER one particle in the initial state. Sure, I can choose either one because in the COM frame it doesn't matter which momentua of the initial particles I choose. But in the derivation of the above equation it is not obvious why the momentum cannot be, say, the TOTAL momentum in the lab frame. Afterall, isn't it true that working in the COM frame is simply a convention?

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# Breit-Wigner Cross section

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