# Scattering amplitude

1. Sep 30, 2009

### Unkraut

1. The problem statement, all variables and given/known data
I'm supposed to calculate the scattering amplitudes of some spherically symmetric potentials in the Born approximation and just trying to figure out how that works in general and what a scattering amplitude is actually.

2. Relevant equations
1+1=2

3. The attempt at a solution
With an incoming plane wave $$\psi_0(\vec r)=e^{ikz}$$ and a potential $$V(r)=\frac{\hbar^2}{2m}U(r)$$ the solution can be written as $$\psi(\vec r)=\psi_0(\vec r)+\int G(\vec r-\vec r')U(r')\psi(r')d^3r$$ where the Green's function G takes the form $$G(\vec r)=G(r)=-\frac{1}{4\pi}\frac{e^{ikr}}{r}$$ and for big r we have $$G(\vec r-\vec r')=-\frac{1}{4\pi}\frac{e^{ikr}}{r}e^{-ik\vec r \cdot \vec r'}$$.
Now $$\psi(\vec r)=e^{ikz}-\frac{1}{4\pi}\frac{e^{ikr}}{r}\int e^{-ik\vec r \cdot \vec r'}U(r')\phi(\vec r')d^3r'=e^{ikz}+f(\theta, \phi)e^{ikr}{r}$$ ???
That means $$f(\theta, \phi)=-\frac{1}{4\pi}\int e^{-ik\vec r \cdot \vec r'}U(r')\phi(\vec r')d^3r'$$. But this is dependent on r, isn't it? In other sources I find rougly the same, with no explanation why this is considered independent of r. In my eyes that looks just wrong. Physics is strange.

2. Sep 30, 2009

### Unkraut

Ah, I see. It's not $$\vec r$$ but $$\hat r$$, i.e. the unit vector in radial direction. I misinterpreted the notation. Right?