[Paper Review] Strangeness production in STAR

J. Takahashi, R. Derradi de Souza|ArXiv.org|Sep 4, 2008

Advanced X-ray Imaging Techniques3 citations

TL;DR

This study investigates strangeness enhancement in Au+Au and Cu+Cu collisions at √sNN = 200 GeV using thermal statistical models. It finds that central Cu+Cu collisions exhibit higher strangeness enhancement than peripheral Au+Au collisions with similar participant numbers, indicating system-size dependence beyond geometric scaling. The canonical ensemble model with a 5 fm strangeness correlation volume best describes the data, suggesting equilibrated strange quark production in large systems.

ABSTRACT

We present a summary of strangeness enhancement results comparing data from Cu+Cu and Au+Au collisions at sqrt(SNN) = 200GeV measured by the STAR experiment. Relative yields in central Cu+Cu data seem to be higher than the equivalent sized peripheral Au+Au collision. In addition, strange particle production from these two systems is compared in terms of a statistical model, applying a Grand-Canonical ensemble and also applying a canonical correlation volume for the strange particles. Thermal fit results from the Grand-Canonical formalism shows little dependence on the system size but, when considering a strange canonical ensemble, strangeness enhancement shows a strong dependency on the correlation volume.

Motivation & Objective

• To investigate the system-size dependence of strangeness enhancement in heavy-ion collisions at RHIC energy.
• To determine whether the observed enhancement in Cu+Cu collisions exceeds that in peripheral Au+Au collisions with comparable <N_part>.
• To assess the validity of Grand-Canonical vs. canonical statistical thermal models in describing strange particle yields.
• To extract the strangeness correlation volume from thermal fits and evaluate its impact on particle ratios.

Proposed method

• Measured particle yields (Λ, Ξ, Ω, ϕ, etc.) in Au+Au and Cu+Cu collisions at √sNN = 200 GeV across centrality classes.
• Normalized yields by <N_part> and scaled relative to p+p data to extract strangeness enhancement factors.
• Applied the THERMUS code for thermal model fits using both Grand-Canonical and strangeness canonical ensembles.
• Fitted particle ratios including corrections for feed-down from Λ → p and estimated Σ contributions using Σ/Λ = 0.35.
• Varied the strangeness correlation volume in the canonical model to find the best fit to experimental data.
• Used the γS parameter to quantify deviation from Grand-Canonical behavior and assess model validity.

Experimental results

Research questions

• RQ1 Does strangeness enhancement in Cu+Cu collisions exceed that in peripheral Au+Au collisions at the same <N_part>?
• RQ2 How does the thermal fit parameter T_ch depend on system size in Au+Au and Cu+Cu collisions?
• RQ3 To what extent does the canonical ensemble formalism improve the description of strange particle yields compared to the Grand-Canonical model?
• RQ4 What is the optimal strangeness correlation volume that reproduces the observed particle ratios in central Au+Au collisions?
• RQ5 Why does the anti-strange particle enhancement increase at RHIC energies despite decreasing trends in the Grand-Canonical model?

Key findings

• Central Cu+Cu collisions show higher strangeness enhancement than peripheral Au+Au collisions with equivalent <N_part>, indicating system-size dependence beyond geometric scaling.
• Thermal fits using the Grand-Canonical ensemble show no significant dependence of T_ch or μS on system size, with T_ch ≈ 155 MeV for both Au+Au and Cu+Cu.
• The γS parameter, indicating deviation from Grand-Canonical behavior, reaches unity only for <N_part> > 100, suggesting only central Cu+Cu events are well described by this model.
• The canonical ensemble model yields a strangeness correlation volume radius of approximately 5 fm for central Au+Au collisions, significantly larger than the ~1 fm found in SPS data.
• The φ/π ratio is insensitive to the correlation volume, while other ratios like Λ/π and Ξ/π show strong dependence, validating the model's predictive power.
• The fit to p+p data yields γS ≈ 0.6, indicating that strange particle ratios in p+p cannot be described by the Grand-Canonical model, highlighting the need for canonical corrections in small systems.