What is "impact parameter"

The impact parameter b is defined as the perpendicular distance between the path of a projectile and the center of a potential field U(r) created by an object that the projectile is approaching (see diagram). It is often referred to in nuclear physics (see Rutherford Scattering), as well as in Classical Mechanics.

The impact parameter is related to the scattering angle θ by

$$\theta=\pi-2b\int_{r_\mathrm{min}}^\infty \frac{dr}{r^2\sqrt{1-(b/r)^2-2U/mv_\infty^2}}$$
where v is the velocity of the projectile when it is far from the center, and r is its closest distance from the center.