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

Understand

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, $v_{2}(p_{\rm T})$, 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.