Evidence of partonic collectivity in ultra-relativistic heavy-ion collisions with NCQ scaling of radial flow

TL;DR

This work investigates whether radial flow and its NCQ scaling, previously established for elliptic flow, reflect partonic collectivity in ultra-relativistic heavy-ion collisions. By contrast­ing AMPT-SM simulations for Au+Au at $\\sqrt{s_{\rm NN}}=200$ GeV with ALICE Pb+Pb data at $\\sqrt{s_{\rm NN}}=5.02$ TeV (and using PYTHIA8/Angantyr as a non-collective baseline), the study shows that the radial-flow observable $v_{0}(p_{ m T})$ exhibits mass ordering at low $p_{ m T}$ and a meson–baryon separation at intermediate $p_{ m T}$ in a manner consistent with collective expansion. A quantitative NCQ-scaling analysis reveals robust scaling when $v_{0}(p_{ m T})$ is expressed as $v_{0}(p_{ m T})/n_q$ versus $(m_{ m T}-m_{0})/n_q$, particularly for central collisions and at RHIC energies, while scaling weakens toward peripheral collisions and at LHC energies. These findings provide strong evidence that radial collectivity is established predominantly at the partonic stage of fireball evolution, extending the NCQ-collectivity paradigm to isotropic radial dynamics and reinforcing the picture of a strongly interacting, parton-level medium prior to hadronization.

Abstract

We report the first observation of \textit{Number of Constituent Quark} (NCQ) scaling of the radial flow observable $v_{0}(p_{\rm T})$ in relativistic heavy-ion collisions. Au+Au collisions at $\sqrt{s_{\rm NN}}=200$ GeV are investigated using the string-melting version of the \texttt{AMPT} model, while Pb+Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV are analyzed using published ALICE data, with calculations from \texttt{PYTHIA8/Angantyr} serving as a non-collective baseline. The $v_{0}(p_{\rm T})$ spectra from \texttt{AMPT-SM} exhibit clear collective signatures, including mass ordering at lower-$p_{\rm T}$ and a meson-baryon separation at intermediate-$p_{\rm T}$, which are absent in the baseline calculations. In central collisions, $v_{0}(p_{\rm T})/n_q$ follows robust NCQ scaling when expressed as a function of transverse kinetic energy per quark, $(m_{\rm T}-m_{0})/n_q$, while significant deviations emerge toward peripheral events, indicating a stronger manifestation of collectivity in central collisions. The scaling is more precise at RHIC than at LHC energies, consistent with earlier observations for elliptic flow $v_2$. These findings provide strong evidence that radial collectivity is established predominantly at the partonic stage of the fireball evolution.

Figures (7)

• Figure 1: (Color online) Transverse momentum ($p_{\rm T}$) dependence of the radial flow observable $v_{0}(p_{\rm T})$ for identified hadrons in Au+Au collisions generated with the String Melting version of AMPT model, shown for 0-20% (most central) and 20-40% (semi-central) centrality selections. Shaded bands indicate bootstrap-estimated statistical uncertainties.
• Figure 2: Cartoon illustrating the $\eta$ regions used in the analysis. Regions A (red) and B (green) are separated by a gap $\eta_{\rm gap}$. The observable $F_{\rm A}^{p_{\rm T}}$ is correlated with the transverse momentum $[p_{\rm T}]_{\rm B}$ measured in the opposite region.
• Figure 3: (Color online) The $v_{0}(p_{\rm T})$ observable as a function of transverse kinetic energy $\rm (m_{T} - m_{0})$ for identified hadrons in AMPT-SM simulations of Au+Au collisions, displayed for 0-20% (most central) and 20-40% (semi-central) centrality bins. Bootstrap-derived statistical uncertainties are shown as shaded regions.
• Figure 4: (Color online) Comparison of $v_{0}(p_{\rm T})$ between PYTHIA8/Angantyr predictions and ALICE measurements alice2025 for identified hadrons in Pb+Pb collisions at = $\sqrt{s_{\rm NN}}$ = 5.02 TeV, presented for 10-20% (left panel) and 30-40% (right panel) centrality classes. Model calculations are represented by solid curves with shaded bands denoting bootstrap uncertainties, while experimental data are shown as filled markers.
• Figure 5: (Color online) $v_{0}(p_{\rm T})$ of identified hadrons presented in terms of transverse kinetic energy, ($m_{\rm T} - m_{\rm 0}$) for Pb+Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV. The plot is compiled from the experimental data of ALICE alice2025.