Quantum Thermalization in Relativistic Nuclear Collisions

Based on common consideration of hydrodynamics as effective theory in domain of slow and long-length modes, we discuss the physical mechanisms responsible for the decoherence and emergence of the hydrodynamic behavior in relativistic heavy ion 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 long wavelength subsystem that describes its decoherence, isotropization and approach to thermal equilibrium induced by interaction with short wavelength modes. Our analysis supports an idea that decoherence, quantum-to-classical transition and thermalization in isolated quantum system are related to a particular procedure of decomposition of the whole quantum system into ”relevant” and ”unrelevant” from an observational viewpoint subsystems.