LAUSR

Abstract We study the influence and interplay of initial state and final state effects in the dynamics of small systems, focusing on azimuthal correlations at different multiplicities. To this end we introduce a new model, matching the classical Yang-Mills dynamics of pre-equilibrium gluon fields (IP-GLASMA) to a perturbative QCD based parton cascade for the final state evolution (BAMPS) on an event-by-event basis. Depending on multiplicity of the event, we see transverse momentum dependent signatures of the initial, but also the final state in azimuthal correlation observables, such as v 2 {2 PC }( p T ). In low-multiplicity events, initial state correlations dominate for transverse momenta p T > 2 GeV, whereas in high-multiplicity events and at low momenta final state interactions dominate and initial state correlations strongly affect v 2 {2 PC }( p T ) for p T > 2 GeV as well as the p T integrated v 2 {2 PC }. Nearly half of the final pT integrated v 2 {2 PC } is contributed by the initial state in low-multiplicity events, whereas in high-multiplicity the share is much less. Based on Ref. [M. Greif, C. Greiner, B. Schenke, S. Schlichting, Z. Xu, Phys. Rev. D96 (9) (2017) 091504], we are now able to carry out a systematic multiplicity scan, probing the dynamics on the border of initial state dominated to final state dominated – but not yet hydrodynamic regime.