  Despite the fact that a system created in relativistic heavy ion collisions
is an isolated quantum system, which cannot increase its entropy in the course
of unitary quantum evolution, hydrodynamical analysis of experimental data
seems to indicate that the matter formed in the collisions is thermalized very
quickly. Based on common consideration of hydrodynamics as an effective theory
in the domain of slow- and long-length modes, we discuss the physical
mechanisms responsible for the decoherence and emergence of the hydrodynamic
behavior in such collisions, and demonstrate how such physical mechanisms work
in the case of the scalar field model. We obtain the evolution equation for the
Wigner function of a long-wavelength subsystem that describes its decoherence,
isotropization, and approach to thermal equilibrium induced by interaction with
short-wavelength modes. Our analysis supports the idea that decoherence,
quantum-to-classical transition and thermalization in isolated quantum systems
are attributed to the experimental context, and are related to a particular
procedure of decomposition of the whole quantum system into relevant and
irrelevant from an observational viewpoint subsystems.
