Impact of Initial-State Nuclear and Sub-Nucleon Structures on Ultra-Central Puzzle in Heavy Ion Collisions

Hydrodynamic models fail to describe the near-equal $v_2/v_3$ ratio observed in ultra-central heavy-ion collisions, despite their success in other centrality classes. This discrepancy stems from shear viscosity suppressing higher-order geometric eccentricities, resulting in underestimated $v_3$ when using the conventional QGP viscosity coefficient. We explore two initial-state modifications to resolve this puzzle: (1) enforcing a minimum nucleon separation distance to homogenize distributions, and (2) amplifying sub-nucleon structures to reduce initial eccentricity. 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.