Collision location for hard spheres in stationary regime

Abstract

Consider two balls with radius $r>0$ whose centers are at a distance $2$, positioned symmetrically with respect to the origin in ${\mathbb{R}}^d$. Suppose that the initial velocities are independent standard normal vectors. When $r\to0$, the collision probability goes to 0 as $r^{d-1}$, and the asymptotic collision location distribution is a (defective) $t$-distribution. This distribution is rotationally symmetric about the origin for no apparent reason.