Understanding non-linear hydrodynamic response in heavy-ion collisions via event-plane correlations

Recently the ATLAS collaboration measured correlations between pairs and triplets of event-plane angles in Pb+Pb collisions at √S_NN = 2.76 TeV and observed interesting centrality dependent patterns. In this work, the origin of these event-plane correlations and the quantative physical implications of their measured values are discussed. Such correlations can originate due to correlations between the eccentricities, ε_n, of different orders in the initial geometry. They can also be generated dynamically during the hydrodynamic expansion of the medium. Comparisons to a Glauber initial geometry model, and to AMPT transport model calculations show that the measured correlations are generated during hydrodynamic expansion of the medium. The measured correlations are shown to unambiguously establish that the higher order flow harmonics, v₄-v₆, are in fact largely driven by non-linear hydrodynamic response to lower order eccentricities ε₂-ε₃ of the initial geometry.