Event-by-event fluctuations in perturbative QCD + saturation + hydro model: pinning down QCD matter shear viscosity in ultrarelativistic heavy-ion collisions

TL;DR

This paper develops an event-by-event framework combining perturbative QCD, saturation, and hydrodynamics to analyze fluctuations in ultrarelativistic heavy-ion collisions, aiming to constrain the QCD matter shear viscosity.

Contribution

It introduces a novel integrated model that links initial QCD calculations with hydrodynamic evolution and compares multiple observables across energies to constrain shear viscosity.

Findings

Consistent description of multiplicities, spectra, and flow coefficients across LHC and RHIC.

Constraints on the temperature dependence of shear viscosity-to-entropy ratio.

Validation of initial state models and hydrodynamics applicability through multi-observable analysis.

Abstract

We introduce an event-by-event perturbative-QCD + saturation + hydro ("EKRT") framework for ultrarelativistic heavy-ion collisions, where we compute the produced fluctuating QCD-matter energy densities from next-to-leading order perturbative QCD using a saturation conjecture to control soft particle production, and describe the space-time evolution of the QCD matter with dissipative fluid dynamics, event by event. We perform a simultaneous comparison of the centrality dependence of hadronic multiplicities, transverse momentum spectra, and flow coefficients of the azimuth-angle asymmetries, against the LHC and RHIC measurements. We compare also the computed event-by-event probability distributions of relative fluctuations of elliptic flow, and event-plane angle correlations, with the experimental data from Pb+Pb collisions at the LHC. We show how such a systematic multi-energy and multi-observable analysis tests the initial state calculation and the applicability region of hydrodynamics, and in particular how it constrains the temperature dependence of the shear viscosity-to-entropy ratio of QCD matter in its different phases in a remarkably consistent manner.