Core-corona separation in ultrarelativistic heavy ion collisions

Simple geometrical considerations show that the collision zone in high energy nuclear collisions may be divided into a central part ("core''), with high energy densities, and a peripheral part ("corona''), with smaller energy densities, more like in pp or pA collisions. We present calculations that allow us to separate these two contributions, and which show that the corona contribution is quite small ( but not negligible) for central collisions, but gets increasingly important with decreasing centrality. We will discuss consequences concerning results obtained in heavy ion collisions at the BNL Relativistic Heavy Ion Collider and CERN Super Proton Synchrotron.