Hadronic "flow" in p--Pb collisions at the Large Hadron Collider?

Using the Ultra-relativistic Quantum Molecular Dynamics (UrQMD) model, we investigate azimuthal correlations in p–Pb collisions at $\sqrt{s_{_{\rm NN}}}=5.02$ TeV. Comparison with the experimental data shows that UrQMD can not reproduce the multiplicity dependence of 2- and 4-particle cumulants, especially the transition from positive to negative values of $c_2\{4\}$ in high multiplicity events, which has been taken as experimental evidence of collectivity in p–Pb collisions. Meanwhile, UrQMD can not qualitatively describe the differential elliptic flow, $v_2(p_{\mathrm{T}})$, of all charged hadrons at various multiplicity classes. These discrepancies show that the simulated hadronic p–Pb systems can not generate enough collective flow as observed in experiment, the associated hadron emissions are largely influenced by non-flow effects. However, the characteristic $v_2(p_{\mathrm{T}})$ mass-ordering of pions, kaons and protons is observed in UrQMD, which is the consequence of hadronic interactions and not necessarily associated with strong fluid-like expansions.