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I am trying to solve equation 4.77 (about cross-sections) in peskin/schroeder's book.

They state that:

[tex] \int d\bar{k}^{z}_{A} \delta \left( \sqrt{\bar{k}^{z}_{A}+m^{2}_{A}} + \sqrt{\bar{k}^{z}_{B}+ m^{2}_{B}} - \Sigma E_{f} \right) \Big{|}_{\bar{k}^{z}_{B}=\Sigma p^{z}_{f}-\bar{k}^{z}_{A}} = \frac{1}{\lvert \frac{\bar{k}^{z}_{A}}{ \bar{E}_{A}} - \frac{\bar{k}^{z}_{B}}{\bar{E}_{B}} \rvert} [/tex]

With A and B the initial protons that collide, z the longitudinal direction, k the momentum, E the energy, and the subscript f denotes the final state (i.e., after collision).

I don't understand where does this come from. I tried to use some definitions of the delta function, but I'm not even getting close. Does someone have an idea?

Thank you.

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# A Peskin/Schroeder 4.77 page (about cross-sections)

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