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Center for Nonlinear Studies |
The buildup of collective flow in relativistic heavy-ion collisions is routinely described using relativistic hydrodynamics. As an effective theory for high-multiplicity, near-equilibrium fluids, the applicability of hydrodynamics is questionable. By considering a comparison to relativistic kinetic theory, this talk aims to establish the applicability bounds of hydrodynamics in terms of a dimensionless parameter called opacity, which is a measure of the total interaction rate in the system. The preequilibrium dynamics play an important role in this comparison, as it is not described correctly in hydrodynamics. This leads to significant differences in late-time observables. At large opacity reached in Pb+Pb and central O+O collisions, agreement can be achieved by either altering the initial conditions in hydrodynamics or by using hybrid simulations, in which preequilibrium is modeled in kinetic theory. At small opacities characterizing e.g. p+p, p+A and non-central O+O, when hydrodynamics loses its applicability, we find significant discrepancies to kinetic theory in all previously-mentioned approaches.
Host: Daniel Livescu and Alessandro Gabbana (CCS-2)