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Description
Anisotropic flow measurements in heavy-ion collisions are sensitive to the spatial distribution of the initial state and to the transport properties of the quark--gluon plasma, such as the shear viscosity to entropy density ratio ($\eta/s$). Hydrodynamic models provide a successful description of flow observables over a wide centrality range, yet deviations from data are observed in ultra-central collisions. An intrinsic octupole deformation of the $^{208}$Pb nucleus has been proposed as a possible explanation for these deviations in Pb--Pb collisions, as it could influence the ratio $v_{3}\{2\}/v_{2}\{2\}$ and induce characteristic fluctuations in triangular flow, quantified through $v_{3}\{4\}/v_{3}\{2\}$.
Measurements of the anisotropic flow coefficients in ultra-central Pb--Pb collisions at $\sqrt{s_{\mathrm{NN}}} = 5.36$~TeV with LHC Run 3 data collected by ALICE are presented. The ratios $v_{3}\{2\}/v_{2}\{2\}$ and $v_{3}\{4\}/v_{3}\{2\}$ are discussed in the context of possible octupole deformation, together with the performance of recent hydrodynamic models in reproducing these observations.
