Mode-coupling effects in anisotropic flow in heavy-ion collisions

TL;DR

This paper investigates how nonlinear mode coupling influences anisotropic flow in heavy-ion collisions, revealing sensitivity to initial fluctuation spectra and shear viscosity, which impacts the interpretation of flow measurements.

Contribution

It demonstrates that nonlinear mode coupling coefficients are sensitive to initial density fluctuations and shear viscosity, challenging previous assumptions of their insensitivity.

Findings

Significant sensitivity of mode coupling coefficients to initial fluctuation spectrum.

Strong dependence of coefficients on shear viscosity at freeze-out.

Weak dependence on shear viscosity during hydrodynamic evolution.

Abstract

Higher-order anisotropic flows in heavy-ion collisions are affected by nonlinear mode coupling effects. It has been suggested that the associated nonlinear hydrodynamic response coefficients probe the transport properties and are largely insensitive to the spectrum of initial density fluctuations of the medium created in these collisions. To test this suggestion, we explore nonlinear mode coupling effects in event-by-event viscous fluid dynamics, using two different models for the fluctuating initial density profiles, and compare the nonlinear coupling coefficients between the initial eccentricity vectors before hydrodynamic expansion and the final flow vectors after the expansion. For several mode coupling coefficients we find significant sensitivity to the initial fluctuation spectrum. They all exhibit strong sensitivity to the specific shear viscosity at freeze-out, but only weak dependence on the shear viscosity during hydrodynamic evolution.