Dynamical Freeze-out in 3-Fluid Hydrodynamics

Freeze-out procedure accepted in the model of 3-fluid dynamics (3FD) is analyzed. This procedure is formulated in terms of drain terms in hydrodynamic equations. Dynamics of the freeze-out is illustrated by 1-dimensional simulations. It is demonstrated that the resulting freeze-out reveals a nontrivial dynamics depending on initial conditions in the expanding ``fireball''. The freeze-out front is not defined just ``geometrically'' on the condition of the freeze-out criterion met but rather is a subject the fluid evolution. It competes with the fluid flow and not always reaches the place where the freeze-out criterion is met. Dynamics of the freeze-out in 3D simulations is analyzed. It is demonstrated that the late stage of central nuclear collisions at top SPS energies is of the form of three (two baryon-rich and one baryon-free) fireballs separated from each other.