[Paper Review] Probing collision dynamics at RHIC

Olga Barannikova|ArXiv.org|Mar 11, 2004

Superconducting Materials and ApplicationsEngineering18 citations

TL;DR

This study analyzes hadron spectra from 200 GeV pp and Au+Au collisions at RHIC using chemical and kinetic freeze-out models. It finds that all centralities reach the same chemical freeze-out temperature (~160 MeV), followed by sequential kinetic freeze-out with multi-strange baryons decoupling first, indicating a long evolution time (~6 fm/c) between chemical and kinetic freeze-out, and radial flow at chemical freeze-out is inferred from $Ω$ and $Χ$ baryons.

ABSTRACT

Measurements of a variety of hadron species in pp and Au+Au collisions at 200 GeV are presented and studied within the framework of chemical and local kinetic equilibrium models. The extracted chemical and final kinetic freeze-out temperatures and collective flow velocities are discussed as function of centrality. The results suggest that Au+Au collisions of various centralities at RHIC always evolve toward the same temperature at chemical freeze-out, followed by cooling and expansion toward kinetic freeze-out.

Motivation & Objective

• To investigate final-state hadronic properties in Au+Au collisions at 200 GeV using identified particle spectra.
• To determine whether chemical and kinetic freeze-out conditions are universal across collision centralities.
• To test the hypothesis of sequential kinetic freeze-out based on particle interaction cross-sections.
• To estimate radial flow velocity at chemical freeze-out using multi-strange baryons.
• To compare experimental particle yield ratios and spectra with statistical and blast-wave models.

Proposed method

• Measured particle spectra (π±, K±, p, Λ, Ξ, Ω, etc.) in pp and Au+Au collisions at √sNN = 200 GeV using the STAR detector.
• Applied corrections for tracking inefficiency, detector acceptance, and decay feed-down in particle identification.
• Fitted charged pion, kaon, and proton spectra with a blast-wave model to extract kinetic freeze-out temperature (Tkin) and mean flow velocity (⟨β⟩).
• Fitted particle yield ratios using a statistical model with parameters Tch, μB, μs, and γs to determine chemical freeze-out conditions.
• Extended blast-wave fits to multi-strange baryons (Ξ, Ω) and resonances (K*, φ) to probe sequential freeze-out.
• Analyzed centrality dependence of freeze-out parameters using charged particle multiplicity in |η| < 0.5.

Experimental results

Research questions

• RQ1 Is the chemical freeze-out temperature independent of collision centrality in Au+Au collisions at √sNN = 200 GeV?
• RQ2 Do multi-strange baryons exhibit kinetic freeze-out at the same time as chemical freeze-out?
• RQ3 Is radial flow velocity at chemical freeze-out measurable from the spectra of heavy baryons?
• RQ4 Does the observed drop in kinetic freeze-out temperature with increasing centrality indicate a long evolution time between chemical and kinetic freeze-out?
• RQ5 Do particle yield ratios and spectra deviate from statistical model predictions due to post-chemical freeze-out effects like regeneration?

Key findings

• Chemical freeze-out temperature Tch ≈ 160 ± 6 MeV is independent of centrality and consistent with the Lattice QCD predicted critical temperature (Tc ≈ 170 MeV).
• Kinetic freeze-out temperature for multi-strange baryons (Ξ, Ω) coincides with chemical freeze-out temperature across all centralities, suggesting early decoupling.
• Radial flow velocity ⟨β⟩ ≈ 0.45 ± 0.1c extracted from Ξ⁻ and Ξ̄⁺ spectra is interpreted as the radial flow velocity at chemical freeze-out.
• For 5% most central Au+Au collisions, kinetic freeze-out temperature Tkin = 89 ± 10 MeV and ⟨β⟩ = 0.59 ± 0.05c, indicating strong collective flow.
• The observed decrease in Tkin with increasing centrality and the large radial flow imply a significant evolution time of at least 6 fm/c between chemical and kinetic freeze-out.
• Blast-wave fits suggest sequential kinetic freeze-out: Ω, Ξ, φ decouple first, followed by Λ, π, K, p, and K*.