Systematic Measurements of Identified Particle Spectra in pp, d+Au and Au+Au Collisions from STAR (0808.2041v2)

Abstract: Identified charged particle spectra of $\pi^{\pm}$, $K^{\pm}$, $p$ and $\pbar$ at mid-rapidity ($|y|<0.1$) measured by the $\dedx$ method in the STAR-TPC are reported for $pp$ and d+Au collisions at $\snn = 200$ GeV and for Au+Au collisions at 62.4 GeV, 130 GeV, and 200 GeV. ... [Shortened for arXiv list. Full abstract in manuscript.]

• The paper demonstrates that identified particle spectra, measured via the STAR TPC, reveal distinct transverse momentum trends influenced by collision centrality and energy.
• It shows systematic variations in particle ratios and a near-constant freeze-out temperature, supporting the universal chemical freeze-out hypothesis near the QCD phase transition.
• Utilizing Glauber model comparisons and resonance contributions, the study refines our understanding of collective flow and particle production in high-energy nuclear collisions.

Systematic Analysis of Identified Particle Spectra in Various Collisions

This paper presents a comprehensive paper of identified particle spectra, focusing on charged pions ($\pi^{\pm}$), kaons ($K^{\pm}$), protons, and antiprotons ($\bar{p}$), across different collision systems: proton-proton ($pp$), deuteron-gold (d+Au), and gold-gold (Au+Au) at varying energies ($\sqrt{s_{NN}}$ of 62.4, 130, and 200 GeV). Measurements from the STAR detector's Time Projection Chamber (TPC) are used, employing the $\dedx$ method for particle identification. This paper systematically investigates particle production rates as influenced by collision system and centrality, extracting average transverse momenta and discussing the underlying physics mechanisms, including chemical and kinetic freeze-out parameters.

Results Summary and Key Findings

1. Transverse Momentum ($\pt$) Spectra: The paper reveals that transverse momentum spectra are flatter for heavy particles (protons and kaons) than for lighter ones (pions), particularly in central Au+Au collisions. This indicates a significant collective radial flow or other mechanisms leading to spectral broadening.
2. Centrality and Energy Dependence: The analysis highlights that particle production and spectra shapes vary significantly with collision centrality and energy. Central Au+Au collisions at 62.4 GeV show more pronounced net-baryon density than those at higher energies, reflecting stronger stopping.
3. Particle Ratios and Freeze-Out Parameters: It is found that the $\pi^{-}/\pi^{+}$ and $\bar{p}/p$ ratios remain largely independent of centrality in 200 GeV Au+Au collisions, but the $K^{-}/K^{+}$ ratio displays a decreasing trend with energy and centrality, particularly at lower energies (62.4 GeV). The freeze-out temperature ($\Tch$) remains approximately 156 MeV across all systems, mirroring the QCD-predicted phase-transition temperature.
4. Impact of Resonances: Resonance decays, notably those of the $\rho$ meson, affect the spectral shape, particularly for pions, although within the limited $\pt$ range of the measurements, they have little influence on extracted freeze-out parameters.
5. Glauber Model Implications: Discrepancies between Monte Carlo (MC) and optical Glauber calculations illustrate the importance of model choice in interpreting collision centralities through $\Npart$ and $\Ncoll$. The paper advocates for a careful consideration of uncertainties in deriving these quantities.

Practical and Theoretical Implications

The smooth evolution of freeze-out parameters across different energies and centrality classes suggests that heavy-ion collisions at RHIC energies reach a universal chemical freeze-out condition. The results support the hypothesis that chemical freeze-out occurs near hadronization during the QCD phase transition. Furthermore, the observed enhancement in $K/\pi$ ratios in central collisions relative to $pp$ collisions is consistent with partial strangeness saturation, hinting at the formation of a dense medium possibly characterized by quark-gluon degrees of freedom.

Future Directions

The systematic analysis calls for future exploration into disentangling the contributions of different physical processes to the spectra, including jet interactions, flow dynamics, and resonance contributions. Moreover, extending the paper to higher transverse momentum with improved detectors could yield insights into hard scattering processes and the dynamics of high-energy QCD. Additionally, the differing behavior of the $K^-/K^+$ ratio with collision energy and system centrality invites further theoretical investigations into baryon transport models, particularly at lower beam energies.

By establishing a consistent baseline across energies and systems, this paper provides a crucial reference point for understanding the interplay of thermalization, collective flow, and chemical freeze-out in high-energy nuclear collisions.