Investigation of possible hadronic flow in $\sqrt{s_{\mathit{NN}}}=5.02\mathrm{TeV}$ $p-\mathrm{Pb}$ collisions

Using the ultrarelativistic quantum molecular dynamics (UrQMD) model, we investigate azimuthal correlations in $p-\mathrm{Pb}$ collisions at $\sqrt{s_{\mathit{NN}}}=5.02\mathrm{TeV}$. Comparison with the experimental data shows that UrQMD cannot reproduce the multiplicity dependence of two- and four-particle cumulants, especially the transition from positive to negative values of $c_2\left\{4\right\}$ in high-multiplicity events, which has been taken as experimental evidence of collectivity in $p-\mathrm{Pb}$ collisions. Meanwhile, UrQMD cannot describe the differential elliptic flow $v_2\left(p_{\mathrm{T}}\right)$ of all charged hadrons at various multiplicity classes. These discrepancies show that the simulated hadronic $p-\mathrm{Pb}$ systems cannot generate enough collective flow as observed in experiment, the associated hadron emissions are largely influenced by nonflow effects. However, the characteristic $v_2\left(p_{\mathrm{T}}\right)$ 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.

Figure 1

Pseudorapidity density of all charged hadrons in minimum bias $p-\mathrm{Pb}$ collisions at $\sqrt{s_{\mathit{NN}}}=5.02$ TeV, measured by ALICE [61] and calculated from UrQMD.

Figure 2

The $m_{\mathrm{T}}$ spectra of pions, kaons, and protons in $p-\mathrm{Pb}$ collisions at $\sqrt{s_{\mathit{NN}}}=5.02$ TeV measured by ALICE [60] and calculated from UrQMD. Here the multiplicity class determination in UrQMD is based on Table 2.

Figure 3

$c_2\left\{2\right\}$ of all charged hadrons in $p-\mathrm{Pb}$ collisions at $\sqrt{s_{\mathit{NN}}}=5.02$ TeV, calculated UrQMD (left) and measured by ALICE (right) [42]. The circle, square, and diamond markers represent various pseudorapidity gap cuts without $\eta$ gap, with gap $\left|\mathrm{\Delta }\eta \right|>0.4$, and $\left|\mathrm{\Delta }\eta \right|>1.0$, respectively. Here the multiplicity class determination in UrQMD is based on Table 1.

Figure 4

$c_2\left\{4\right\}$ of charged particles in $p-\mathrm{Pb}$ collisions at $\sqrt{s_{\mathit{NN}}}=5.02$ TeV, calculated with UrQMD and measured by ALICE [42]. Here the multiplicity-class determination in UrQMD is based on Table 1.

Figure 5

$c_3\left\{2\right\}$ of all charged hadrons in $p-\mathrm{Pb}$ collisions at $\sqrt{s_{\mathit{NN}}}=5.02$ TeV, calculated UrQMD (left) and measured by ALICE (right) [42]. Here the multiplicity class determination in UrQMD is based on Table 1.

Figure 6

$v_2\left({\mathit{p}}_{\mathrm{T}}\right)$ of all change hadrons in $p-\mathrm{Pb}$ collisions at $\sqrt{s_{\mathit{NN}}}=5.02$ TeV, calculated from UrQMD and measured by ALICE [43]. Here the multiplicity-class determination in UrQMD is based on Table 2.

Figure 7

$v_2\left({\mathit{p}}_{\mathrm{T}}\right)$ of pions, kaons, and protons in $p-\mathrm{Pb}$ collisions at $\sqrt{s_{\mathit{NN}}}=5.02$ TeV, calculated from UrQMD (left panels) and measured by ALICE (right panels) [43]. Here the multiplicity-class determination in UrQMD is based on Table 2.

Figure 8

The $m_{\mathrm{T}}$ spectra of of pions, kaons, and protons in $p-\mathrm{Pb}$ collisions at $\sqrt{s_{\mathit{NN}}}=5.02$ TeV, calculated from UrQMD with and without M-M and M-B collisions. Here the multiplicity-class determination in UrQMD is based on Table 2.

Figure 9

$v_2\left({\mathit{p}}_{\mathrm{T}}\right)$ of pions, kaons, and protons in $p-\mathrm{Pb}$ collisions at $\sqrt{s_{\mathit{NN}}}=5.02$ TeV, calculated from UrQMD with and without M-M and M-B collisions. Here the multiplicity-class determination in UrQMD is based on Table 2.