Origin of the mass splitting of elliptic anisotropy in a multiphase transport model

The mass splitting of elliptic anisotropy $\left(v_2\right)$ at low transverse momentum is considered as a hallmark of hydrodynamic collective flow. We investigate a multiphase transport (AMPT) model where the $v_2$ is mainly generated by an anisotropic escape mechanism, not of the hydrodynamic flow nature, and where mass splitting is also observed. We demonstrate that the $v_2$ mass splitting in AMPT is small right after hadronization (especially when resonance decays are included); the mass splitting mainly comes from hadronic rescatterings, even though their contribution to the overall charged hadron $v_2$ is small. These findings are qualitatively the same as those from hybrid models that combine hydrodynamics with a hadron cascade. We further show that there is no qualitative difference between heavy ion collisions and small system collisions. Our results indicate that the $v_2$ mass splitting is not a unique signature of hydrodynamic collective flow and thus cannot distinguish whether the elliptic flow is generated mainly from hydrodynamics or the anisotropic parton escape.

Figure 1

Parton $v_2$. Midrapidity ($\left|\eta \right|<1$) parton $v_2$ as a function of $p_{\perp }$ for $u$+$d$ (thick curves) and $s$ (anti)quarks (thin curves) in the final state before hadronization in $b=6.6$–8.1 fm Au+Au collisions at $\sqrt{s_{{}_{\mathrm{NN}}}}=200$ GeV by normal AMPT (solid) and $\varphi$-randomized AMPT (dashed) lines.

Figure 2

Mass splitting from coalescence. Midrapidity ($\left|\eta \right|<1$) constituent quark (c.q., dashed curves) and primordial hadron (solid curves) $v_2$ both as a function of hadron $p_{\perp }$ right after hadronization before hadronic rescatterings in $b=6.6$–8.1 fm Au+Au collisions at $\sqrt{s_{{}_{\mathrm{NN}}}}=200$ GeV by normal AMPT (a) and $\varphi$-randomized AMPT (b).

Figure 3

Effects of hadronic rescatterings. Midrapidity ($\left|\eta \right|<1$) pion and (anti)proton $v_2$ before (dashed) and after (solid) hadronic rescatterings in $b=6.6$–8.1 fm Au+Au (a) and $b=0$ fm $d$+Au (b) collisions at $\sqrt{s_{{}_{\mathrm{NN}}}}=200$ GeV by AMPT. Resonance decays are included.

Figure 4

Origins of $v_2$ mass splitting. Midrapidity ($\left|\eta \right|<1$) $v_2$ of ${\pi }^{\pm }, K^{\pm }, p(\overline{p}$), and charged hadrons ($h^{\pm }$) at $0.7<p_{\perp }<0.8$ GeV/$c$ at different stages of system evolution in $b=6.6$–8.1 fm Au+Au (a) and $b=0$ fm $d$+Au (b) collisions at $\sqrt{s_{{}_{\mathrm{NN}}}}=200$ GeV by AMPT: right after coalescence hadronization and before hadronic rescatterings (initial $v_2$ of primordial hadrons), hadron initial $v_2$ including decays, and after hadronic rescatterings at freeze-out (hadron final $v_2$).