System size dependence of charged hadrons directed flow at $\sqrt{s_{NN}}$ = 200 GeV using a multi-phase transport model

TL;DR

The paper addresses how the size of the colliding system affects the directed flow $v_1$ in symmetric heavy-ion collisions at 200 GeV. It employs the string-melting AMPT-SM model with deformed nuclei to compute $v_1(η)$ and its mid-rapidity slope $dv_1/dη$ for both low- and high-$p_T$ hadrons across O+O to U+U, analyzing centrality dependence and scaling properties. The study finds that $dv_1/dη$ at low $p_T$ is largely system-size independent between Cu+Cu and Au+Au (consistent with STAR), while high-$p_T$ shows strong centrality- and size-dependent behavior with the opposite sign, signaling a hard-soft asymmetry and violation of simple geometric scaling via $A^{1/3}$. These results provide constraints on initial-state geometry, non-equilibrium dynamics, and particle production mechanisms, and offer concrete predictions for upcoming high-statistics experimental tests.

Abstract

The directed flow ($v_1$) of charged hadrons ($h^{\pm}$) in symmetric collision systems (O+O, Cu+Cu, Zr+Zr, Ru+Ru, Au+Au, and U+U) at $\sqrt{s_{\mathrm{NN}}} =$ 200 GeV using string-melting version of A Multiphase Transport (AMPT-SM) model is reported. The $v_1$ as a function of pseudo-rapidity ($η$) is obtained for transverse momentum ($p_{\mathrm{T}}$) ranges of 0.2-2.0 GeV/$c$ and 2.0-5.0 GeV/$c$. The dependence of $v_1$-slope ($dv_1/dη$) at mid-rapidity on $p_{\mathrm{T}}$ range, collision centrality, and system size are discussed particularly in the context of the hard-soft asymmetry in the flow profiles of produced particles. In the AMPT-SM model, a system size independence of the magnitude of $dv_1/dη$ between Cu+Cu and Au+Au collisions at low-$p_{\mathrm{T}}$ is observed, and this finding is similar to the observation from the STAR experiment at $\sqrt{s_{\mathrm{NN}}} =$ 200 GeV. In contrast, a strong centrality and system size dependence, with the opposite sign of $dv_1/dη$, is found for the high-$p_{\mathrm{T}}$ charged hadrons. The AMPT-SM model demonstrates a clear violation of the expected scaling of charged hadrons $(dv_1/dη)/A^{1/3}$ across different colliding systems.

Figures (6)

• Figure 1: (Color online) Schematic illustration of the orientation of the initial tilted QGP bulk relative to the symmetric hard scattering distribution profile of particle production projected onto the reaction plane (x, z) dflow2dflow3.
• Figure 2: (Color online) Charged particle multiplicity distributions within $|\eta| < 0.5$ for (a) O+O, (b) Cu+Cu, (c) Ru+Ru, (d) Zr+Zr, (e) Au+Au, and (f) U+U collisions at $\sqrt{s_{\mathrm{NN}}}$ = 200 GeV using the AMPT-SM model. The centrality classes 0-10%, 10-40%, and 40-80% are shown in different bands.
• Figure 3: $v_1(\eta)$ of charged hadrons in O+O, Cu+Cu, Ru+Ru, Zr+Zr, Au+Au, and U+U collisions for 10-40% centrality at $\sqrt{s_{\mathrm{NN}}}$ = 200 GeV using the AMPT-SM model. The red and blue markers correspond to low-$p_\mathrm{T}$ (0.2 $< p_\mathrm{T} <$ 2.0 GeV/$c$) and high-$p_\mathrm{T}$ (2.0 $< p_\mathrm{T} <$ 5.0 GeV/$c$), respectively. The solid lines indicate cubic polynomial function fit.
• Figure 4: The charged hadron $v_1$-slope parameter (F) and F/A$^{1/3}$ from the AMPT-SM model compared with the magnitude of the STAR measurements exdflow3 for low-$p_\mathrm{T}$ at $\sqrt{s_{\mathrm{NN}}}$ = 200 GeV. The corresponding results for high-$p_\mathrm{T}$ charged hadrons from the AMPT-SM model is also shown.
• Figure 5: The charged hadron $v_1$-slope as function of centrality (%) in O+O, Cu+Cu, Ru+Ru, Zr+Zr, Au+Au and U+U collisions at $\sqrt{s_{\mathrm{NN}}}$ = 200 GeV using the AMPT-SM model. The red and blue markers correspond to low-$p_\mathrm{T}$ (0.2 $< p_\mathrm{T} <$ 2.0 GeV/$c$) and high-$p_\mathrm{T}$ (2.0 $< p_\mathrm{T} <$ 5.0 GeV/$c$), respectively.