Dynamics of high-energy nuclear collisions: present and future*

In this talk, I first summarize the current understanding of dynamics of high-energy nuclear collisions from a hydrodynamic point of view. State-of-the-art dynamical models have been utilized towards comprehensive understanding of the deconfined nuclear matter, the quark gluon plasma (QGP). Bayesian parameter estimation is a well-known powerful tool to constrain the transport properties of the QGP such as shear and bulk viscosity. However, this can be valid only within an applicability of the model itself. In particular, given a fact that non-equilibrium initial stages just after the collisions are not well understood yet, one needs to keep it in mind in interpretation of the results from Baysian parameter estimation.

I discuss to include a non-equilibrium effect based on the core-corona picture into the dynamical approach for complehensive description of the dynamics from small to large colliding systems. At extremely large multiplicity in A+A collisions, the system is dominated by the equilibrated matter. While at low multiplicity in p+p collisions, pQCD based approach would be more desirable to describe the reaction. This suggests importance of the interpolation of these two approaches in a unified manner. I show some results from the core-corona picture in this presentation.

Relativistic hydrodynamics has been successful in description of space-time evolution in high-energy nuclear collisions. However, it is not at all trivial at which stage one can apply hydrodynamics to the description of dynamics. In particular, at the very early stage, non-equilibrium effects play an essential role before the system is close to the local equilibrium. On the other hand, at the very late stage, the system is too diluted to maintain the local equilibrium. From the viewpoint of the causality, I discuss the applicability of relativistic viscous hydrodynamics and constrain the initial conditions of fluids in a simle case. This turns out that, when the system is far from the local equilibirium, the causality is violated in relativistic hydrodynamics.

We finally present the future of dynamical approaches in high-energy nuclear collisions.