Pinning down QCD-matter shear viscosity in A+A collisions via EbyE fluctuations using pQCD + saturation + hydrodynamics

TL;DR

This paper combines NLO pQCD, saturation, and hydrodynamics to model initial conditions and evolution in heavy-ion collisions, aiming to constrain QCD matter's shear viscosity and predict observables at LHC energies.

Contribution

It introduces a comprehensive framework integrating pQCD, saturation, and hydrodynamics to analyze initial states and constrain shear viscosity in QCD matter.

Findings

Constraints on shear viscosity-to-entropy ratio $\, ext{eta}/s$ from RHIC and LHC data.

Predictions for charged hadron multiplicities at 5.023 TeV Pb+Pb collisions.

Validation of initial state models against experimental measurements.

Abstract

We compute the initial energy densities produced in ultrarelativistic heavy-ion collisions from NLO perturbative QCD using a saturation conjecture to control soft particle production, and describe the subsequent space-time evolution of the system with hydrodynamics, event by event. The resulting centrality dependence of the low-$p_T$ observables from this pQCD + saturation + hydro ("EKRT") framework are then compared simultaneously to the LHC and RHIC measurements. With such an analysis we can test the initial state calculation, and constrain the temperature dependence of the shear viscosity-to-entropy ratio $\eta/s$ of QCD matter. Using these constraints from the current RHIC and LHC measurements we then predict the charged hadron multiplicities and flow coefficients for the 5.023 TeV Pb+Pb collisions.