Collectivity in AA, pA and pp collisions at the LHC
Prof. Dr. Urs Wiedemann (CERN Theoretical Physics Department, Genf, Schweiz)

Large azimuthal momentum asymmetries vn (a.k.a. flow harmonics) have been measured since long in nucleus-nucleus (AA) collisions. They indicate collectivity in the sense that they characterize a phenomenon shared by all soft particles produced in the collision. Such vn -signals are not obtained in models of the underlying event of proton-proton (pp) collisions, such as those implemented in multi-purpose event generators. However, large asymmetries vn have been measured in recent years in proton-nucleus and in high multiplicity proton-proton collisions. In this talk, I shall first shortly review the experimental state of the art of characterizing flow harmonics vn as a function of system size. I shall then give a critical assessment of the dynamical mechanisms that are studied to account for the observed phenomenology.