Freeze-out conditions from net-proton and net-charge fluctuations at RHIC (1403.4903v2)

Abstract: We calculate ratios of higher-order susceptibilities quantifying fluctuations in the number of net protons and in the net-electric charge using the Hadron Resonance Gas (HRG) model. We take into account the effect of resonance decays, the kinematic acceptance cuts in rapidity, pseudo-rapidity and transverse momentum used in the experimental analysis, as well as a randomization of the isospin of nucleons in the hadronic phase. By comparing these results to the latest experimental data from the STAR collaboration, we determine the freeze-out conditions from net-electric charge and net-proton distributions and discuss their consistency.

Freeze-out Conditions from Fluctuations at RHIC

The paper "Freeze-out conditions from net-proton and net-charge fluctuations at RHIC," introduces a detailed investigation utilizing the Hadron Resonance Gas (HRG) model to analyze event-by-event fluctuations of net-protons and net-electric charges in heavy-ion collisions at the RHIC. The research aims to determine the chemical freeze-out conditions, such as freeze-out temperature ($T_{ch}$) and baryo-chemical potential ($\mu_{B,ch}$), by comparing experimental data from the STAR collaboration with HRG model calculations. The paper focuses on the ratios of susceptibilities, particularly those of higher orders, which quantify fluctuations and are influenced by the underlying freeze-out conditions.

Key Findings and Methods

The authors calculate ratios of susceptibilities using HRG, a model well-appreciated for its ability to describe bulk properties of hadronic matter at thermal and chemical equilibrium. By incorporating factors such as resonance decays, isospin randomization, and experimental acceptance cuts in rapidity and transverse momentum, the paper provides refined freeze-out parameters by comparing theoretical predictions with experimental results.

The HRG model's strength lies in its capacity to incorporate a range of chemical potential values, thus extending the theoretical analysis beyond the limitations often encountered in lattice QCD simulations. Such simulations are constrained by small chemical potential values and experimental acceptance limitations. The kinematic acceptance cuts in rapidity, pseudo-rapidity, and transverse momentum are adjusted to match those employed in experimental analysis, which are essential for aligning theoretical predictions with empirical data.

Results and Implications

The paper presents comparisons of experimental data with HRG calculations for most central RHIC collisions at varying energies. The outcomes demonstrate that a simultaneous analysis of the ratios of lowest-order susceptibilities for net protons and net-electric charge can derive consistent freeze-out points on the ($T,\mu_B$)-plane of QCD matter. The analysis shows variations in susceptibility ratios, notably $\sigma^2/M$ and $S\sigma$, across different collision energies. Moreover, challenges are identified in aligning higher-order cumulants for net protons, hinting at potential modifications needed in the HRG approach to fully encapsulate the observed phenomena.

The implications of these results are profound for QCD phase diagrams, especially concerning the existence of a critical point where transition characteristics may alter. The work enriches the understanding of freeze-out parameters, providing insightful opportunities for tuning theoretical frameworks to align with experimental observations.

Future Directions

The research underscores the necessity for continued exploration into higher-order cumulants and their role in elucidating freeze-out dynamics. Further investigations may integrate alternative computational models or enhanced algorithms to reconcile discrepancies observed at lower collision energies. Additionally, bridging the gap between simplified hadronic descriptions like those offered by HRG and more complex quantum phenomena underscored by lattice QCD represents a promising frontier.

Overall, the paper contributes to the meticulous understanding of thermal conditions under which the QGP transitions into hadronic matter. As experiments evolve and data complexity increases, refining models like HRG and leveraging lattice QCD simulations will be pivotal in advancing theoretical propositions pertinent to heavy-ion physics.