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Characterizing radial flow fluctuations in relativistic heavy-ion collisions at top RHIC and LHC energies

Lipei Du

TL;DR

This work investigates the differential radial-flow observable $v_0(p_T)$ in relativistic heavy-ion collisions across top RHIC and LHC energies using a multistage hydrodynamic model. By systematically varying bulk viscosity, off-equilibrium corrections at particlization, hadronic afterburners, and initial-state nucleon width, and by applying a double-scaling $v_0(p_T)/v_0$ vs $p_T/raket{p_T}$, the authors demonstrate a universal low-$p_T$ behavior that reflects collective dynamics and identify model-sensitive features at higher $p_T$. The results show that bulk viscosity and $ d f$ choices leave distinct imprints on the $p_T$-dependent shape of $v_0(p_T)$, while the hadronic stage plays a subleading role in the scaled observable; initial-state granularity leaves a unique curvature signature. These findings underscore the utility of $v_0(p_T)$ as a complementary constraint on QGP transport properties and initial-state fluctuations, advocating its inclusion in future Bayesian model-to-data inferences to sharpen our understanding of the QCD medium.

Abstract

We present a systematic investigation of the transverse-momentum differential radial flow fluctuations observable $v_0(p_T)$ in relativistic heavy-ion collisions at top RHIC ($\sqrt{s_\mathrm{NN}}{\,=\,}200$ GeV) and LHC ($\sqrt{s_\mathrm{NN}}{\,=\,}2.76$ and 5.02 TeV) energies. Using a multistage hydrodynamic model, we assess the sensitivity of $v_0(p_T)$ to a wide range of physical effects, including bulk and shear viscosities, off-equilibrium corrections at particlization, the presence of a hadronic afterburner, and the nucleon size in the initial conditions. By employing complementary rescaling strategies, we demonstrate how different physical effects leave distinct imprints on the shape of $v_0(p_T)$. A combined double-rescaling of $v_0(p_T)/v_0$ versus $p_T/\langle p_T \rangle$ reveals a universality across a wide range of energies and model assumptions in the low-$p_T$ regime, a robust signature of collective behavior. This allows us to disentangle the universal dynamics of the bulk medium from model-specific features that emerge at higher $p_T$. Our results establish $v_0(p_T)$ as a powerful and complementary observable for constraining QGP transport properties and initial-state granularity, offering a unique probe of the created QCD medium.

Characterizing radial flow fluctuations in relativistic heavy-ion collisions at top RHIC and LHC energies

TL;DR

This work investigates the differential radial-flow observable in relativistic heavy-ion collisions across top RHIC and LHC energies using a multistage hydrodynamic model. By systematically varying bulk viscosity, off-equilibrium corrections at particlization, hadronic afterburners, and initial-state nucleon width, and by applying a double-scaling vs , the authors demonstrate a universal low- behavior that reflects collective dynamics and identify model-sensitive features at higher . The results show that bulk viscosity and choices leave distinct imprints on the -dependent shape of , while the hadronic stage plays a subleading role in the scaled observable; initial-state granularity leaves a unique curvature signature. These findings underscore the utility of as a complementary constraint on QGP transport properties and initial-state fluctuations, advocating its inclusion in future Bayesian model-to-data inferences to sharpen our understanding of the QCD medium.

Abstract

We present a systematic investigation of the transverse-momentum differential radial flow fluctuations observable in relativistic heavy-ion collisions at top RHIC ( GeV) and LHC ( and 5.02 TeV) energies. Using a multistage hydrodynamic model, we assess the sensitivity of to a wide range of physical effects, including bulk and shear viscosities, off-equilibrium corrections at particlization, the presence of a hadronic afterburner, and the nucleon size in the initial conditions. By employing complementary rescaling strategies, we demonstrate how different physical effects leave distinct imprints on the shape of . A combined double-rescaling of versus reveals a universality across a wide range of energies and model assumptions in the low- regime, a robust signature of collective behavior. This allows us to disentangle the universal dynamics of the bulk medium from model-specific features that emerge at higher . Our results establish as a powerful and complementary observable for constraining QGP transport properties and initial-state granularity, offering a unique probe of the created QCD medium.

Paper Structure

This paper contains 16 sections, 5 equations, 9 figures, 1 table.

Table of Contents

  1. Introduction
  2. Model and framework
  3. Multistage hydrodynamic model
  4. Model performance at MAP parameters
  5. Results and discussion
  6. Impact of method and kinematic cuts
  7. Sensitivity to bulk viscosity
  8. Sensitivity to off-equilibrium corrections
  9. Sensitivity to hadronic afterburner
  10. Sensitivity to nucleon width
  11. Universality under double rescaling
  12. Conclusion
  13. Computational framework for v0(pT) analysis
  14. Event-level processing
  15. Ensemble-level analysis
  16. ...and 1 more sections

Figures (9)

  • Figure 1: Validation of the multistage hydrodynamic model with MAP parameters, against experimental data from Au+Au collisions at $\sqrt{s_\mathrm{NN}} = 200$ GeV PHENIX:2003iijSTAR:2004jwm. (a) Transverse momentum spectra of identified hadrons ($\pi^+$, $K^+$, $p$) in the 30--40% centrality class. (b) Elliptic flow $v_2(p_T)$ of charged hadrons for two centrality classes (0--5% and 30--40%).
  • Figure 2: Comparison of model results for $v_0(p_T)$ with ALICE ALICE:2025iud and ATLAS ATLAS:2025ztg measurements for charged hadrons in 30--40% Pb+Pb collisions at 5.02 TeV. Solid lines represent model results with low-$p_T$ cuts matching experimental acceptance, while dashed lines show results with no low-$p_T$ cut applied. For the no-cut case, the model results corresponding to both measurements are largely overlapping.
  • Figure 3: The $p_T$-differential $v_0(p_T)$ for identified hadrons ($\pi^\pm$, $K^\pm$, $p/\bar{p}$) in 30--40% Pb+Pb collisions at 5.02 TeV from ALICE ALICE:2025iud. Solid lines show results from the fluctuation-based method, while dashed lines show the covariance-based method.
  • Figure 4: Upper panels: Transverse momentum dependence of the scaled radial flow $v_0(p_T)/v_0$ for charged hadrons at three beam energies: (a) $\sqrt{s_\mathrm{NN}} = 200$ GeV, (b) $\sqrt{s_\mathrm{NN}} = 2.76$ TeV, and (c) $\sqrt{s_\mathrm{NN}} = 5.02$ TeV, all for the 30--40% centrality class, as a function of $p_T$. Lower panels: Unscaled $v_0(p_T)$ as a function of the rescaled variable $p_T/\langle p_T \rangle$. Results are shown for three scenarios: full viscous evolution with shear and bulk viscosity (solid blue), shear-only viscosity (dashed green), and ideal hydrodynamics (dotted red).
  • Figure 5: Scaled radial flow observable $v_0(p_T)/v_0$ for charged hadrons in the 30--40% centrality class at three collision energies: (a) $\sqrt{s_\mathrm{NN}} = 200$ GeV, (b) 2.76 TeV, and (c) 5.02 TeV. Four model scenarios are compared: the default case with both shear and bulk viscosity and Grad viscous corrections ("shear+bulk", solid blue), shear viscosity only ("shear only", dashed green), no viscous corrections to the distribution function ("no $\delta f$", dot-dashed megenta), and Chapman--Enskog viscous corrections ("CE", dotted red).
  • ...and 4 more figures