Nonlinear hydrodynamic response confronts LHC data

TL;DR

This paper introduces nonlinear response coefficients to better understand higher order harmonic flow in heavy-ion collisions, showing they are independent of initial conditions and aligning well with LHC data.

Contribution

It proposes a novel set of nonlinear response coefficients for higher harmonic flow that are independent of initial state conditions and validated through hydrodynamic and AMPT simulations.

Findings

Nonlinear response coefficients agree with LHC data.

Predictions made for $v_7$ and $v_8$ flow harmonics.

Good simulation-experiment agreement.

Abstract

Higher order harmonic flow $v_n$ (with $n\ge4$) in heavy-ion collisions can be measured either with respect to their own plane, or with respect to a plane constructed using lower-order harmonics. By assuming that higher flow harmonics are the superposition of medium nonlinear and linear responses to initial anisotropies, we propose a set of nonlinear response coefficients $\chi_n$'s, which are independent of initial state by construction. In experiments, $\chi_n$'s can be extracted as the ratio between higher order harmonic flow measured in the plane constructed by $v_2$ and $v_3$, and moments of lower order harmonic flow. Simulations with single-shot hydrodynamics and AMPT model lead to results of these nonlinear response coefficients in good agreement with the experimental data at the LHC energy. Predictions for $v_7$ and $v_8$ measured with respect to plane of lower order harmonics are given accordingly.