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

TL;DR

This paper investigates the persistent challenge in hydrodynamic models to accurately reproduce ultra-central flow data in heavy-ion collisions, highlighting the need for new modeling approaches.

Contribution

It reassesses the ultra-central flow puzzle using multiple Bayesian models, showing existing models cannot fully resolve the discrepancy without compromising other fits.

Findings

Models describe central flow data better than before

Tension with experimental data increases in ultra-central collisions

Adjusting model parameters cannot resolve the tension without affecting other observables

Abstract

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. As such, new elements are likely needed in the standard modeling of heavy-ion collisions.