Predictions for isobaric collisions at $\sqrt{s_{NN}}=200$ GeV from a multiphase transport model

The isobaric collisions of ${}_{44}{}^{96}\mathrm{Ru}+{}_{44}{}^{96}\mathrm{Ru}$ and ${}_{40}{}^{96}\mathrm{Zr}+{}_{40}{}^{96}\mathrm{Zr}$ have recently been proposed to discern the charge separation signal of the chiral magnetic effect (CME). In this article, we employ the string melting version of a multiphase transport model to predict various charged-particle observables, including $dN/d\eta ,p_T$ spectra, elliptic flow $\left(v_2\right)$, and particularly possible CME signals in Ru + Ru and Zr + Zr collisions at $\sqrt{s_{{}_{NN}}}=200\mathrm{GeV}$. Two sets of the nuclear structure parametrization have been explored, and the difference between the two isobaric collisions appears to be small, in terms of $dN/d\eta ,p_T$ spectra, and $v_2$ for charged particles. We mimic the CME by introducing an initial charge separation that is proportional to the magnetic field produced in the collision, and study how the final-state interactions affect the CME observables. The relative difference in the CME signal between the two isobaric collisions is found to be robust, insensitive to the final-state interactions.

Figure 1

Upper: the impact parameter dependence of the magnetic field in different heavy-ion collisions at $\sqrt{s_{{}_{NN}}}=200\mathrm{GeV}$. Lower: the relative difference in the magnetic field between Ru + Ru and Zr + Zr collisions.

Figure 2

The centrality dependence of ${\gamma }_{\alpha \beta }=\langle cos({\varphi }_{\alpha }+{\varphi }_{\beta }-2{\mathrm{\Psi }}_2)\rangle$ for Au + Au and Cu + Cu collisions at $\sqrt{s_{{}_{NN}}}=200\mathrm{GeV}$, where ${\mathrm{\Psi }}_2$ is the second-order event plane. The open symbols represent the results from the AMPT model with a given initial charge separation, and the solid symbols represent the STAR data [15].

Figure 3

The impact parameter dependence of the initial charge separation fraction for four different collision systems at $\sqrt{s_{{}_{NN}}}=200\mathrm{GeV}$, as discussed in Sec. 2c.

Figure 4

Upper: the $\langle N_{\mathrm{part}}\rangle$ dependence of the charged particle multiplicity $d{N}_{\mathrm{ch}}/d\eta$ at mid-pseudorapidity $\left(\left|\eta \right|<0.5\right)$ from the AMPT model with two settings of the isobar collisions at $\sqrt{s_{{}_{NN}}}=200\mathrm{GeV}$. The PHENIX data for Au+Au collisions at $\sqrt{s_{{}_{NN}}}=200\mathrm{GeV}$ [43] are also shown in comparison. Lower: the relative difference in $d{N}_{\mathrm{ch}}/d\eta$ between Ru + Ru and Zr + Zr.

Figure 5

Upper: the $p_T$ spectra of the charged hadrons at mid-pseudorapidity $\left(\left|\eta \right|<1\right)$ from the string melting model of AMPT with two settings of central $\left(0–5\%\right)$ isobar collisions at $\sqrt{s_{{}_{NN}}}=200\mathrm{GeV}$. Lower: the corresponding relative differences.

Figure 6

Upper: the $N_{\mathrm{part}}$ dependence of elliptic flow $\left(v_2\right)$ of charged hadrons at mid-pseudorapidity $\left(\left|\eta \right|<1\right)$ from the string melting model of AMPT, with two settings of isobar collisions at $\sqrt{s_{{}_{NN}}}=200\mathrm{GeV}$. Lower: the corresponding relative difference in $v_2$.

Figure 7

Upper: the centrality dependence of the correlator ${\gamma }_{\alpha \beta }$ for Ru + Ru and Zr + Zr collisions at $\sqrt{s_{{}_{NN}}}=200\mathrm{GeV}$ from the AMPT model, with and without introduction of the initial charge separation to mimic the CME. The STAR data for Au + Au and Cu + Cu collisions at $\sqrt{s_{{}_{NN}}}=200\mathrm{GeV}$ [15] are also shown with lines in comparison. Lower: the relative difference in $\mathrm{\Delta }\gamma$ between the two collisions. Here, $\mathrm{\Delta }$ means the difference between the opposite-charge and the same-charge correlations.

Figure 8

The centrality dependence of $H_{\mathrm{opp}}$ and $H_{\mathrm{same}}$ for Ru + Ru and Zr + Zr collisions at $\sqrt{s_{{}_{NN}}}=200\mathrm{GeV}$.

Figure 9

Upper: the centrality dependence of $H_{\mathrm{opp}}-H_{\mathrm{same}}$ for Ru + Ru and Zr + Zr collisions at $\sqrt{s_{{}_{NN}}}=200\mathrm{GeV}$. Lower: the corresponding relative difference.

Figure 10

Upper: the impact parameter dependence of the final charge separation fraction compared with the initial charge separation fraction for Ru + Ru and Zr + Zr collisions at $\sqrt{s_{{}_{NN}}}=200\mathrm{GeV}$. Lower: the relative difference in the charge separation fraction between Ru + Ru and Zr + Zr collisions for the initial and the final states.