Anisotropic Flow in Ultra-Central Pb$-$Pb Collisions at $\sqrt{{\mathrm{s}}_{\mathrm{NN}}}=5.36$ TeV with ALICE

Abstract

Anisotropic flow measurements in heavy-ion collisions are sensitive to the spatial distribution of the initial state, and quark-gluon plasma transport properties such as the shear viscosity to entropy density ratio $(\eta /s)$. State-of-the-art relativistic hydrodynamic models successfully describe such flow measurements over a wide centrality range. However, the hydrodynamic description of anisotropic flow deviates from the experimental data in ultra-central collisions (UCC), where the average geometric anisotropy of the system becomes small and fluctuations dominate the initial-state geometry. This discrepancy constitutes the UCC puzzle, as the expected hierarchy of flow harmonics is not fully reproduced by current modeling approaches. Probing towards ultra-central collisions, effects on flow fluctuations due to the initial spatial anisotropies are suppressed. The measured flow can be explained by quantum fluctuations on the energy distribution of ${}^{208}$Pb nuclei. An octupole deformation of the ${}^{208}$Pb nuclei has been proposed as a remedy to improve the modeling of the measured $v_3\{2\}/v_2\{2\}$ ratio. In this contribution, we present measurements of $v_3\{2\}/v_2\{2\}$ ratio in ultra-central Pb\textendash Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.36$ TeV with ALICE Run 3 detector and compare them with recent hydrodynamic model calculations.