Session RA: Hydrodynamic Simulations of the QGP: Successes and Challenges

Hydrodynamic flow in large and small QCD systems formed in relativistic collisions

The quark-gluon plasma (QGP) produced in ultra-relativistic collisions between large nuclei, such as gold or lead, is a state of QCD matter with extremely high temperature and energy density. The particles produced in these collisions exhibit collective behavior, which is well described by viscous hydrodynamics with very low specific viscosity. The success of hydrodynamics in describing simultaneously a large number of observables led to the notion that QGP is perhaps the most perfect liquid in nature. In the quest for understanding how the perfect fluid emerges, experiments at the Large Hadron Collider (CERN, Switzerland) and the Relativistic Heavy Ion Collider (BNL) studied collisions between protons or other small nuclei with large nuclei, which were not expected to produce QGP but only cold QCD matter. Surprisingly, collective behavior was also found in these small-system collisions. Hydrodynamical models again provide an excellent simultaneous description of a large body of data. How small can a system be and still behave as a liquid, and what are the limits of applicability of hydrodynamics? This talk will focus on the successes and challenges of hydrodynamics applied to large and small QCD systems formed in relativistic collisions.   

Hydrodynamic simulations in challenging environments

Relativistic viscous hydrodynamics is an effective description of the bulk dynamics of high energy relativistic heavy-ion collisions. Understanding the collective origin in the high multiplicity pp and pA collisions experiments at Relativistic Heavy-Ion Collider (RHIC) and the Large Hadron Collider (LHC) has been pushing the successful fluid paradigm to its limits. In this talk, I will discuss the current theoretical uncertainties in the hydrodynamic modeling of these small collision systems and highlight recent developments to go beyond hydrodynamics. Furthermore, I will also present the theoretical challenge of describing the bulk dynamics of relativistic heavy-ion collisions at lower RHIC Beam Energy Scan (BES). At those collision energies, the overlapping time for the two nuclei to pass each other becomes comparable to the system lifetime.   

Recent theoretical developments in hydrodynamics

In this talk we focus on two recents developments in relativistic hydrodynamics which is an important tool in understanding the physics of heavy ion collisions. i) We review the theory of fluctuations in relativistic hydrodynamics and its implementation in numerical simulations. In particular we present a general systematic formalism describing dynamics of fluctuations in an arbitrary relativistic hydrodynamic flow. We derive a deterministic evolution equation for the fluctuation modes which nontrivially matches the kinetic equation for phonons propagating on an arbitrary background, including relativistic inertial and Coriolis forces due to acceleration and vorticity of the flow. We also describe the procedure of renormalization of short-distance singularities which eliminates cutoff dependence, allowing efficient numerical implementation of these equations. ii) We briefly discuss the asymptotic nature of the derivative expansion and show that the way the expansion diverges is related to the existence of certain non-hydrodynamic modes and show how quantitative information about these modes can be extracted from the late terms in the derivative expansion.