The unreasonable effectiveness of hydrodynamics in heavy ion collisions

Abstract

Event-by-event hydrodynamic simulations of AA and pA collisions involve initial energy densities with large spatial gradients. This is associated with the presence of large Knudsen numbers ($K_n\approx 1$) at early times, which may lead one to question the validity of the hydrodynamic approach in these rapidly evolving, largely inhomogeneous systems. A new procedure to smooth out the initial energy densities is employed to show that the initial spatial eccentricities, ${\epsilon }_n$, are remarkably robust with respect to variations in the underlying scale of initial energy density spatial gradients, $\lambda$. For $\sqrt{s_{NN}}=2.76$ TeV LHC initial conditions generated by the MCKLN code, ${\epsilon }_n$ (across centralities) remains nearly constant if the fluctuation scale varies by an order of magnitude, i.e., when $\lambda$ varies from 0.1 to 1 fm. Given that the local Knudsen number $K_n\approx \frac{1}{\lambda }$, the robustness of the initial eccentricities with respect to changes in the fluctuation scale suggests that the vn's cannot be used to distinguish between events with large $K_n$ from events where $K_n$ is in the hydrodynamic regime. We use the 2+1 Lagrangian hydrodynamic code v-USPhydro to show that this is indeed the case: anisotropic flow coefficients computed within event-by-event viscous hydrodynamics are only sensitive to long wavelength scales of order $\frac{1}{{\Lambda }_{QCD}}\approx 1$ fm and are incredibly robust with respect to variations in the initial local Knudsen number. This robustness can be used to justify the somewhat unreasonable effectiveness of the nearly perfect fluid paradigm in heavy ion collisions.

Proceedings of the 25th International Conference on Ultrarelativistic Nucleus-Nucleus Collisions (Quark Matter 2015), Kobe, Japan,4 pages, 4 figures

References in corpus (10)

• Glauber Modeling in High Energy Nuclear Collisions
• Effects of fluctuations on the initial eccentricity from the Color Glass Condensate in heavy ion collisions
• Origin of the Relaxation Time in Dissipative Fluid Dynamics
• Bulk viscosity-driven suppression of shear viscosity effects on the flow harmonics at RHIC
• Linear and cubic response to the initial eccentricity in heavy-ion collisions
• Decorrelation of anisotropic flows along the longitudinal direction
• Viscous Effects on the Mapping of the Initial to Final State in Heavy Ion Collisions
• Hydrodynamic predictions for 5.02 TeV Pb-Pb collisions
• Fast Equilibration of Hadrons in an Expanding Fireball
• Extracting shear viscosity of the Quark Gluon Plasma in the presence of bulk viscosity

Cited by in corpus (8)

• Dense Nuclear Matter Equation of State from Heavy-Ion Collisions
• Fluctuating relativistic dissipative hydrodynamics as a gauge theory
• Bubble expansion at strong coupling
• Shear flows in far-from-equilibrium strongly coupled fluids
• Hydrodynamic attractors for the speed of sound in holographic Bjorken flow
• Hydrodynamic generators in relativistic kinetic theory
• Critical behaviour of hydrodynamic series
• Infinite Order Hydrodynamics: An Analytical Example