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

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, pT spectra and flow coefficients vn against LHC and RHIC data. We compare also the computed EbyE probability distributions of relative fluctuations of vn, 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, η/s(T), of QCD matter in its different phases. Remarkably, we can describe all these different flow observables and correlations consistently with η/s(T) that is independent of the collision energy.