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, 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, 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.