Anisotropic flow in ultra-central Pb{-}{-}Pb collisions at $\sqrt{s_{NN}}=5.36$ TeV with ALICE

Anisotropic flow measurements in heavy-ion collisions are sensitive to the spatial distribution of the initial state, and to QGP transport properties such as the shear viscosity to entropy density ratio $(\eta/s)$. Hydrodynamic models successfully describe such flow measurements over a wide centrality range. However, the hydrodynamic description of anisotropic flow deviates from the data in ultra-central collisions. An octupole deformation of the $\textbf{$^{208}$Pb}$ nuclei has been proposed as a remedy to improve the modeling of the measured $v_{3} \{2\} / v_{2} \{2\}$ ratio. Such a deformation should manifest in triangular flow fluctuations via measurements of the $v_3\{4\}/v_3 \{2\}$ ratio.

In this talk, we present multi-particle flow measurements of the coefficients $v_{n}\{m\}$ in Pb--Pb collisions with LHC Run 3 data over the full centrality range. We will also present measurements from ultra-central collisions and discuss whether there is experimental evidence for an octupole deformation.