  We numerically simulate planar shock wave collisions in anti-de Sitter space
as a model for heavy ion collisions of large nuclei. We uncover a cross-over
between two different dynamical regimes as a function of the collision energy.
At low energies the shocks first stop and then explode in a manner
approximately described by hydrodynamics, in close similarity with the Landau
model. At high energies the receding fragments move outwards at the speed of
light, with a region of negative energy density and negative longitudinal
pressure trailing behind them. The rapidity distribution of the energy density
at late times around mid-rapidity is not approximately boost-invariant but
Gaussian, albeit with a width that increases with the collision energy.
