Collective Dynamics of Heavy Ion Collisions in ATLAS

n overview of the latest ATLAS measurements of collective behaviour in a variety of collision systems, including 13 TeV pp , 5.44 TeV Xe + Xe and 5.02 TeV Pb + Pb collisions is presented. The results shown include measurements of v_n -mean p T correlations and longitudinal flow decorrelations in Xe + Xe and Pb + Pb collisions, which carry important information about the initial-state geometry of the Quark-Gluon Plasma; heavy flavour flow harmonics testing the quark energy loss mechanism in QGP; and measurements of the sensitivity of collective behaviour in pp collisions to the presence of jets, which provide insight on the role of semi-hard processes in the origin of the flow phenomena in small systems. These results provide stringent tests of the theoretical understanding of the initial state in heavy ion collisions.