Elliptic flow of $\varphi$ mesons as a sensitive probe for the onset of the deconfinement transition in high energy heavy-ion collisions

The elliptic flow ($v_2$) of $\varphi$ mesons is shown to be a sensitive probe of the partonic collectivity using a multiphase transport (AMPT) model. Within the ambit of the AMPT model with partonic interactions (string melting version), the $\varphi$-meson $v_2$ at midrapidity is found to have negligible contribution from hadronic interactions for Au+Au collisions at $\sqrt{s_{{}_{NN}}}=200$ GeV. Changing the hadron cascade time in the model calculations does not change the $\varphi$-meson $v_2$, while it reduces the $v_2$ of protons at low transverse momentum ($p_T$). These observations indicate that a substantial reduction in $\varphi$-meson $v_2$ as a function of colliding beam energy would suggest the dominance of hadronic interactions over partonic interactions.

Figure 1

$\varphi$-meson $v_2$ for Au+Au minimum-bias (0–80$\%$) collisions at midrapidity ($\pm$1.0) at $\sqrt{s_{{}_{NN}}}=200$ GeV from the STAR experiment at RHIC [10] compared to the corresponding AMPT and UrQMD model calculations. The AMPT model calculations are shown for three different parton-parton interaction cross sections of 3, 10, and 14 mb. The errors shown are statistical.

Figure 2

$\varphi$-meson $v_2$ for Au+Au minimum-bias (0–80$\%$) collisions at midrapidity ($\pm$1.0) at $\sqrt{s_{{}_{NN}}}=200$ GeV from the AMPT model. Panels (a) and (b) shows the results as a function of $p_T$ for parton-parton interaction cross sections of 0 and 10 mb and calculations before and after relativistic transport calculations for hadrons, respectively. Panels (c) and (d) show the differences in $v_2$ from panels (a) and (b), respectively. The errors shown are statistical.

Figure 3

(a) $v_2$ of protons as a function of $p_T$ for Au+Au 0–80$\%$ collisions at $\sqrt{s_{{}_{NN}}}=200$ GeV from the AMPT model at midrapidity. The results are shown for a parton-parton cross section of 10 mb and three different values of hadronic cascade time periods. (b) The same plot as in panel (a) for the $\varphi$ mesons. (c) Ratio of $v_2$ of protons for hadron cascade time of 0.6 fm$/c$ to corresponding $v_2$ for time periods of 15 and 30 fm$/c$. (d) The same plot as in panel (c) for the $\varphi$ mesons. The error bars shown are statistical.

Figure 4

Average transverse momentum ($\langle p_T\rangle$) for protons and $\varphi$ mesons produced in 0–80$\%$ Au+Au collisions at midrapidity for $\sqrt{s_{{}_{NN}}}=200$ GeV from AMPT model. The results are shown as a function of hadron cascade time and for two values of parton-parton cross sections.