Grazing collisions of gravitational shock waves and entropy production in heavy ion collisions

AdS/CFT correspondence is now widely used for the study of strongly coupled plasmas, such as those produced in ultrarelativistic heavy ion collisions at the Relativistic Heavy Ion Collider. While properties of equilibrated plasma and small deviations from equilibrium are by now reasonably well understood, the plasma's initial formation and thermal equilibration is a much more challenging issue which remains to be studied. In the dual gravity language, these problems are related to the formation of bulk black holes, and studying trapped surfaces, as we do in this work, is a way to estimate the properties (temperature and entropy) of such black holes. Extending the work by Gubser et al. for central collisions, we find numerically trapped surfaces for noncentral collisions of ultrarelativistic black holes (gravitational shock waves) with different energies. We observe that beyond a certain critical impact parameter, the trapped surface does not exist, and we argue that there are some experimental indications for a similar jump in entropy production as a function of the impact parameter in real heavy ion collisions. We also present a simple solvable example of the so-called wall-on-wall collision, for colliding objects that depend on the holographic coordinate only. Finally, we critically discuss the applicability of the AdS/CFT approach to real-world heavy ion collisions.