Non-equilibrium Dynamics and Collectivity in Ultra-relativistic Proton–Nucleus Collisions

We investigate the medium evolution in p+Pb collisions at LHC energy by comparing a non-equilibrium transport approach, PHSD, with a (2+1)D viscous hydrodynamic model, VISHNew, with initial conditions extracted from PHSD. We find that the energy density in PHSD is highly inhomogeneous in the transverse plane during the whole evolution, whereas VISHNew dissolves efficiently the initial spatial irregularities. We perform also a comparison of event topology in the two approaches by means of the transverse spherocity observable. We found that the spherocity distribution in PHSD is slightly shifted towards the isotropic limit with respect to the hydrodynamic case, mainly due to the different descriptions within the two models of the medium produced in small systems. We have applied the spherocity selection to the elliptic flow of charged particles finding that it is predominantly determined by the most jetty events. This finding supports the idea that multi-differential measurements through multiplicity and spherocity are very useful to study final-state observables in ultra-relativistic proton-nucleus collisions.