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

TL;DR

This paper develops an event-by-event framework combining pQCD, saturation, and hydrodynamics to analyze QCD matter in heavy-ion collisions, successfully describing multiple flow observables and constraining shear viscosity across energies.

Contribution

It introduces a comprehensive EbyE pQCD+saturation+hydrodynamics model that accurately reproduces experimental data and constrains the temperature dependence of QCD shear viscosity.

Findings

Consistent description of flow observables across energies.

Constraints on eta/s(T) independent of collision energy.

Validation of initial state and hydrodynamic applicability.

Abstract

We introduce an event-by-event pQCD + saturation + hydro ("EKRT") framework for high-energy heavy-ion collisions, where we compute the produced fluctuating QCD-matter energy densities from next-to-leading order (NLO) perturbative QCD (pQCD) using saturation to control soft particle production, and describe the space-time evolution of the QCD matter with viscous hydrodynamics, event by event (EbyE). We compare the computed centrality dependence of hadronic multiplicities, p_T spectra and flow coefficients v_n against LHC and RHIC data. We compare also the computed EbyE probability distributions of relative fluctuations of v_n, as well as correlations of 2 and 3 event-plane angles, with LHC data. Our systematic multi-energy and -observable analysis not only tests the initial state calculation and applicability of hydrodynamics, but also makes it possible to constrain the temperature dependence of the shear viscosity-to-entropy ratio, eta/s(T), of QCD matter in its different phases. Remarkably, we can describe all these different flow observables and correlations consistently with eta/s(T) that is independent of the collision energy.