Assessing the ultracentral flow puzzle in hydrodynamic modeling of heavy-ion collisions

Understand

An outstanding problem in heavy-ion collisions is the inability for models to accurately describe ultra-central experimental flow data, despite that being precisely the regime where a hydrodynamic description should be most applicable.

• We reassess the status of this puzzle by computing the flow in ultra-central collisions obtained from multiple recent Bayesian models that were tuned to various observables in different collision systems at typical centralities.
• While central data can now be described with better accuracy than in previous calculations, tension with experimental observation remains and worsens as one goes to ultra-central collisions.
• Tuning the model parameters cannot remove this tension without destroying the fit at other centralities.