Multiplicity scaling in ideal and viscous hydrodynamics

TL;DR

This paper investigates how elliptic flow and entropy production scale with particle multiplicity in ideal and viscous hydrodynamics, revealing dependencies on the equation of state and shear viscosity, and proposing their use to constrain quark-gluon plasma properties.

Contribution

It introduces a detailed analysis of multiplicity scaling in hydrodynamics, highlighting the effects of viscosity and freeze-out conditions on flow and entropy, and suggests using these effects to measure quark-gluon plasma viscosity.

Findings

Approximate scaling of v2/epsilon and Delta S/S0 with multiplicity density.

Scaling violations increase with shear viscosity and decrease with system size.

Viscous effects cause larger deviations from ideal scaling, useful for constraining eta/s.

Abstract

Using numerical results from ideal and viscous relativistic hydrodynamic simulations with three different equations of state, for Au+Au and Cu+Cu collisions at different centralities and initial energy densities, we explore the dependence of the eccentricity-scaled elliptic flow, v_2/epsilon, and the produced entropy fraction, Delta S/S_0, on the final charged hadron multiplicity density dN_ch/dy per unit transverse overlap area S, (1/S)(dN_ch/dy). The viscous hydrodynamic simulations are performed with two different versions of the Israel-Stewart kinetic evolution equations, and in each case we investigate the dependence of the physical observables on the kinetic relaxation time. We find approximate scaling of v_2/epsilon and Delta S/S_0 with (1/S)(dN_ch/dy), with scaling functions that depend on the EOS and, in particular, on the value of the specific shear viscosity eta/s. Small scaling violations are seen even in ideal hydrodynamics, caused by a breaking of the scale invariance of ideal fluid dynamics by the freeze-out condition. Viscous hydrodynamics shows somewhat larger scale-breaking effects that increase with increasing eta/s and decreasing system size and initial energy density. We propose to use precision studies of these scaling violations to help constrain the shear viscosity eta/s of the quark-gluon plasma created in relativistic heavy ion collisions.