# Probing non-linearity of higher order anisotropic flow in Pb--Pb collisions

## TL;DR

This paper investigates the non-linear contributions to higher order anisotropic flow in Pb--Pb collisions at LHC, revealing significant mode-coupling effects and introducing a new observable to better understand the system's dynamics.

## Contribution

It presents the first detailed analysis of non-linear flow modes and introduces the non-linear response coefficient as a new measurement in heavy-ion collisions.

## Key findings

- Significant non-linear mode contributions to higher order flow.
- Observation of mode-coupling effects in flow measurements.
- Comparison with theory constrains system dynamics at freeze-out.

## Abstract

The second and the third order anisotropic flow, $V_{2}$ and $V_3$, are determined by the corresponding initial spatial anisotropy coefficients, $\varepsilon_{2}$ and $\varepsilon_{3}$, in the initial density distribution. On the contrary, the higher order anisotropic flow $V_n$ ($n > 3$), in addition to their dependence on the same order initial anisotropy coefficient $\varepsilon_{n}$, have a significant contribution from lower order initial anisotropy coefficients, which leads to mode-coupling effects. In this contribution, we present the investigations on linear and non-linear modes in higher order anisotropic flow ($V_{4}$, $V_{5}$ and $V_{6}$) in Pb--Pb collisions at $\sqrt{s_{\rm NN}} =$ 2.76 TeV using the ALICE detector at the Large Hadron Collider (LHC). A significant contribution from a non-linear mode is observed. A new observable, the non-linear response coefficient, is measured as well. The comparison to theoretical calculations provides crucial information on dynamic of the created system especially at the freeze-out conditions, which are poorly known from previous flow measurements.