1. The problem statement, all variables and given/known data a,b is constant and s is impact parameter, θ is scattering angle. i know that ψ in the picture is <ψ=(π-θ)/2> 2. Relevant equations differential scattering crossection dσ/dΩ = (s/sinΘ) I ds/dθ I and σ(θ)=∫(dσ/dΩ)dΩ 3. The attempt at a solution i guessed, first step is that finding a relation of impact parameter s and scattering angle θ. but i couldn't. This picture is my attempt at a solution. i think for getting a relation of <s and θ> to use tan(θ/2) but i do not find. How can i get impact parameter s(θ)?