  We investigate the effects of nonequilibrium dynamics in small colliding
systems by comparing a nonequilibrium transport approach, the
Parton-Hadron-String-Dynamics (PHSD), with a (2+1)D viscous hydrodynamic model,
VISHNew. Focusing on p+Pb collisions at LHC energy, we extract the initial
conditions for the hydrodynamic model from PHSD, in order to reduce the impact
of the early out-of-equilibrium dynamics and focus on the traces of
nonequilibiurm in the ensuing medium evolution. We find that in the transport
approach quantities like energy density and bulk viscous pressure are highly
inhomogeneous on the transverse plane during the whole evolution, whereas the
hydrodynamic simulations dissolve more efficiently the initial spatial
irregularities, still keeping a high degree of inhomogeneity due to the smaller
size and lifetime of the medium produced in p+Pb collisions with respect to
heavy-ion reactions. As a first step that will help to identify the impact of
these nonequilibrium effects on final observables in proton-nucleus collisions,
we perform an analysis of the transverse spherocity, an event-shape observable
able to distinguish between jetty and isotropic configurations of transverse
momenta. We found that the spherocity distribution in PHSD is slightly shifted
towards the isotropic limit with respect to the hydrodynamic result. Even
though this dissimilarity is partially due to the difference in the final
charged particle production, it mainly comes from the different description
within the two frameworks of the medium produced in small colliding systems.
This finding supports the idea that multi-differential measurements, such as
those based on event categorization according to multiplicity and spherocity,
are useful to study final-state observables in ultrarelativistic proton-nucleus
collisions.
