Anisotropic Flow in Ultra-Central Pb$-$Pb Collisions at $\sqrt{\mathrm{s_{NN}}}=5.36$ 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.