  Koszul type Coxeter simplex tilings exist in hyperbolic $n$-space
$\mathbb{H}^n$ up to $ n = 9$, and their horoball packings achieve the highest
known regular ball packing densities for $n = 3, 4, 5$. In this paper we
determine the optimal horoball packing densities of Koszul simplex tilings in
dimensions $6 \leq n \leq 9$, which give new lower bounds for optimal packing
density in each dimension. The symmetries of the packings are given by Coxeter
simplex groups, and a parameter related to the Busemann function gives an
isometry invariant description of different optimal horoball packing
configurations.
