# Peskin and Schroeder eq. 18.84

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1. Aug 5, 2016

### qqbar

1. The problem statement, all variables and given/known data
So I am self-studying the book of Peskin&Schroeder, and there is something I don't understand on page 616.

In eq. 18.80, there is a numerical factor of ½ and going from e2 to α will introduce a factor 4π when proceeding to eq. 18.84. But then there should be a numerical factor of 2π in eq 18.84. What gives an extra factor of 2?

2. Relevant equations

(this is the first time I post here, please do tell if uploading a page in this format is not acceptable)

3. The attempt at a solution

I believe I understand the rest of the calculation on this page.

Last edited: Aug 5, 2016
2. Aug 5, 2016

### Dr.AbeNikIanEdL

Well, equation (18.80) is for $e^+e^-\rightarrow e^+e^-$, equation (18.84) is for $e^+e^-\rightarrow \text{hadrons}$. As the text says, the two are related by the optical theorem.