Charge-Dependent Directed Flow in Symmetric Nuclear Collisions

Abstract

The directed flow ($v_1$) of identified hadrons ($π^{\pm}, K^{\pm}, p, \bar{p}, φ, Λ$, and $\barΛ$) is studied in symmetric nuclear collisions (O+O, Cu+Cu, Ru+Ru, Au+Au, and U+U) at $\sqrt{s_{NN}} = 200$ GeV using the string-melting version of a multiphase transport model with improved quark coalescence. The mid-rapidity $v_1$-slope ($dv_1/dy$) and its charge-dependent splitting ($Δdv_1/dy$) between particles and anti-particles are investigated as a function of nuclear mass number ($A$) and collision centrality in both low-$p_\mathrm{T}$ (0.2$-$2.0 GeV/$c$) and high-$p_\mathrm{T}$ (2.0$-$5.0 GeV/$c$) regions. At low-$p_\mathrm{T}$, the $v_1$-slope shows weak system-size dependence, while at high-$p_\mathrm{T}$ strong system-size dependence is found and it becomes negative with nuclear mass number, reflecting the hard-soft asymmetry in particle production. The charge-dependent splitting $Δdv_1/dy$ reveals a striking baryon-meson dichotomy: baryon pairs ($p-\bar{p}$ and $Λ-\barΛ$) exhibit significant splitting that grows with system size, whereas meson pairs ($π^+-π^-$ and $K^+-K^-$) show minimal splitting. The effect of final state hadronic interactions on the $v_1$-slope is found to be negligible confirming that it is primarily generated during the partonic phase and coalescence process. A comparison of the AMPT results with measurements from the STAR experiment at RHIC in Au+Au collisions establish the transported quark contribution as a baseline for the observed charge-dependent $v_1$ splitting, on top of which electromagnetic field effects must be considered.

Figures (7)

• Figure 1: $v_1$($y$) of identified hadrons ($\pi^{\pm}$, K$^{\pm}$, $p$, $\bar{p}$, $\Lambda$, $\bar{\Lambda}$, and $\phi$) at low-$p_T$ in different colliding systems (O+O, Cu+Cu, Ru+Ru, Au+Au, and U+U) for 10--40$\%$ centrality at $\sqrt{s_{NN}}$ = 200 GeV using the AMPT-SM model.
• Figure 2: $v_1$($y$) of identified hadrons ($\pi^{\pm}$, K$^{\pm}$, $p$, $\bar{p}$, $\Lambda$, $\bar{\Lambda}$, and $\phi$) at high-$p_T$ in different colliding systems (O+O, Cu+Cu, Ru+Ru, Au+Au, and U+U) for 10--40$\%$ centrality at $\sqrt{s_{NN}}$ = 200 GeV using the AMPT-SM model.
• Figure 3: $v_1$-slope parameter $F$ versus mass number ($A$) for low- and high-$p_T$ identified hadrons in 10--40$\%$ centrality for O+O, Cu+Cu, Ru+Ru, Au+Au, and U+U collisions at $\sqrt{s_{NN}}$ = 200 GeV using the AMPT-SM model.
• Figure 4: Difference in $v_1$-slope parameter ($\Delta F$) between particles and anti-particles (upper panels) and $\Delta F$ scaled by $A^{1/3}$ (lower panels) in 10--40% central O+O, Cu+Cu, Ru+Ru, Au+Au, and U+U collisions at $\sqrt{s_{NN}}$ = 200 GeV using the AMPT-SM model.
• Figure 5: Difference in $v_1$-slope parameter ($\Delta F$) as a function of centrality for low-$p_T$ (a) $\pi^+-\pi^-$, (b) $K^+-K^-$, and (c) $p-\bar{p}$ in Au+Au collisions at $\sqrt{s_{NN}} =$ 200 GeV from the AMPT-SM model. The results are compared with the measurements fro the STAR experiment at RHIC STAR:2023jddstar_systsize_pid_v1 in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV.