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

Abstract

Using the Ultra-relativistic Quantum Molecular Dynamics (UrQMD) model, we investigate azimuthal correlations in p--Pb collisions at $\sqrt{s_{{}_{\mathrm{NN}}}}=5.02$ 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.

Comments: 12 pages 9 figures