Speaker
Description
Hydrodynamic models fail to describe the near-equal $v_3 / v_2$ ratio observed in ultra-central heavy-ion collisions, despite their success in other centrality classes. This failure can not be resolved by adjusting the shear viscous coefficient, as shear viscosity suppresses higher-order anisotropic flows more strongly, leading to an underestimation of $v_3$ when $v_2$ matches experimental data. To address this issue, we explore two initial-state modifications to resolve this puzzle: (1) impose a minimum distance between nucleons to simulate the homogenization effect arising from short-range nucleon-nucleon repulsion; and (2) introduce sub-nucleonic structures, specifically 'hot spots' within protons, to provide a more refined description of initial-state fluctuations. Using TRENTo initial conditions and 3+1D viscous hydrodynamic model CLVisc, both approaches significantly lower geometric eccentricity, reduce required viscosity, and narrow the $v_2$–$v_3$ gap in ultra-central collisions. Our results implicate initial-state nuclear and sub-nucleon structures as critical factors in addressing this puzzle. Resolving it would advance nuclear structure studies and improve precision in extracting QGP transport coefficients (e.g., shear viscosity), bridging microscopic nuclear features to macroscopic quark-gluon plasma properties.
