Shape and flow fluctuations in ultra-central Pb+Pb collisions at the LHC

TL;DR

This study investigates initial density fluctuations in ultra-central Pb+Pb collisions at the LHC, showing hydrodynamics can qualitatively explain flow factorization breaking but struggles with initial fluctuation spectrum accuracy.

Contribution

It introduces an initial entropy-based centrality definition and compares hydrodynamic predictions with CMS data, highlighting limitations of current initial fluctuation models.

Findings

Initial fluctuation spectra from MC-Glauber and MC-KLN models are incompatible with CMS flow data.

Hydrodynamics qualitatively reproduces flow factorization breaking effects.

Flow factorization breaking is significant in ultra-central collisions.

Abstract

In ultra-central heavy-ion collisions, anisotropic hydrodynamic flow is generated by density fluctuations in the initial state rather than by geometric overlap effects. For a given centrality class, the initial fluctuation spectrum is sensitive to the method chosen for binning the events into centrality classes. We show that sorting events by total initial entropy or by total final multiplicity yields event classes with equivalent statistical fluctuation properties, in spite of viscous entropy production during the fireball evolution. With this initial entropy-based centrality definition we generate several classes of ultra-central Pb+Pb collisions at LHC energies and evolve the events using viscous hydrodynamics with non-zero shear but vanishing bulk viscosity. Comparing the predicted anisotropic flow coefficients for charged hadrons with CMS data we find that both the Monte Carlo Glauber (MC-Glb) and Monte Carlo Kharzeev-Levin-Nardi (MC-KLN) models produce initial fluctuation spectra that are incompatible with the measured final anisotropic flow power spectrum, for any choice of the specific shear viscosity. In spite of this failure, we show that the hydrodynamic model can qualitatively explain, in terms of event-by-event fluctuations of the anisotropic flow coefficients and flow angles, the breaking of flow factorization for elliptic, triangular and quadrangular flow measured by the CMS experiment. For elliptic flow, this factorization breaking is large in ultra-central collisions. We conclude that the bulk of the experimentally observed flow factorization breaking effects are qualitatively explained by hydrodynamic evolution of initial-state fluctuations, but that their quantitative description requires a better understanding of the initial fluctuation spectrum.