Time dependence of partition into spectators and participants in   relativistic heavy-ion collisions
  The process of formation of the participant system in heavy-ion collisions is investigated in the framework of a simplified analytic Glauber-like model, which is based on the relativistic Boltzmann transport equation. The key point lies in the time-dependent partition of the nucleon system into two groups: nucleons, which did not take part in any interaction before a given time and nucleons, which already have interacted. In the framework of the proposed model we introduce a natural energy-dependent temporal scale $t_c$, which allows us to remove all dependencies of the model on the collision energy except for the energy dependence of the nucleon-nucleon cross-section. By investigating the time dependence of the total number of participants we conclude that the formation process of the participant system becomes complete at $t\simeq1.5 t_c$. Time dependencies of participant total angular momentum and vorticity are also considered and used to describe the emergence of rotation in the reaction plane. 
V. Vovchenko, D. Anchishkin, L.P. Csernai
nucl-th hep-ph