Bulk Properties of the Medium Produced in Relativistic Heavy-Ion Collisions from the Beam Energy Scan Program

Abstract: We present measurements of bulk properties of the matter produced in Au+Au collisions at $\sqrt{s_{NN}}=$ 7.7, 11.5, 19.6, 27, and 39 GeV using identified hadrons ($\pi^\pm$, $K^\pm$, $p$ and $\bar{p}$) from the STAR experiment in the Beam Energy Scan (BES) Program at the Relativistic Heavy Ion Collider (RHIC). Midrapidity ($|y|<$0.1) results for multiplicity densities $dN/dy$, average transverse momenta $\langle p_T \rangle$ and particle ratios are presented. The chemical and kinetic freeze-out dynamics at these energies are discussed and presented as a function of collision centrality and energy. These results constitute the systematic measurements of bulk properties of matter formed in heavy-ion collisions over a broad range of energy (or baryon chemical potential) at RHIC.

• The paper presents systematic measurements of particle yields, transverse momentum distributions, and freeze-out parameters across an energy range of 7.7 to 39 GeV.
• The analysis employs the STAR experiment’s TPC and TOF detectors with rigorous event selection and correction procedures to ensure high data precision.
• The results indicate energy and centrality-dependent trends in baryon density and radial flow, offering critical insights into QCD phase transitions.

Overview of the Bulk Properties of the Medium Produced in Relativistic Heavy-Ion Collisions

The paper "Bulk Properties of the Medium Produced in Relativistic Heavy-Ion Collisions from the Beam Energy Scan Program" reports on systematic measurements of the properties of matter created in Au+Au collisions over a broad range of energies using the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). By analyzing the results in terms of particle production, transverse momentum distributions, and freeze-out dynamics, the study aims to explore Quantum Chromodynamics (QCD) phase transitions, examining both chemical and kinetic processes in heavy-ion collisions.

Experimental Context and Methodology

The authors conducted the Beam Energy Scan (BES) program, focusing on collisions at center-of-mass energies $\sqrt{s_{NN}}$ = 7.7, 11.5, 19.6, 27, and 39 GeV. The research utilizes midrapidity identified hadron measurements — namely, $\pi^\pm$, $K^\pm$, $p$, and $\bar{p}$ — through the STAR experiment’s Time Projection Chamber (TPC) and Time-Of-Flight (TOF) detectors. Event and track selection criteria were meticulously followed to maintain data integrity and precision. The centrality of collisions was determined using reference multiplicity and subsequent corrections for detector acceptance, efficiency, and other systematic factors like track reconstruction, particle identification, and feed-down corrections were implemented.

Analysis and Key Results

Particle Yields and Ratios:

The study provides detailed centrality and energy dependence of particle yields ($dN/dy$) and their ratios. Yields of charged pions, kaons, and anti-protons indicated a decrease with decreasing energy, whereas proton yields are maximal at the lowest energy, evidencing high baryon density and stopping. Anti-particle to particle ratios were analyzed to infer the contributions of pair versus associated production mechanisms, showing larger effects of baryon stopping at lower energies.

Transverse Momentum:

The transverse momentum spectra demonstrate mass-dependent inverse slopes, with heavier particles exhibiting flatter distributions, suggesting collective radial flow. Mean transverse momentum ($\langle p_T \rangle$) measurements display a dependency on hadron mass and show increased values with centrality, reflecting varying freeze-out conditions and flow characteristics across energies.

Freeze-out Parameters:

The paper reports on chemical and kinetic freeze-out conditions inferred from thermal and hydrodynamic model fits. Chemical freeze-out parameters ($T_{\rm{ch}$, $\mu_B$) indicate a distinct energy dependence, characteristic of a QCD phase transition landscape, with significant centrality dependence in baryon chemical potential $\mu_B$ at lower energies. Kinetic freeze-out parameters, estimated from blast-wave fits, highlight decreasing freeze-out temperatures $T_{\rm{kin}$ and increasing radial flow velocities $\langle \beta \rangle$ with energy, indicating prolonged hadronic rescattering at higher energies.

Implications and Future Directions

This research provides crucial insights into the QCD phase diagram, particularly around the possible onset of deconfinement and chiral symmetry restoration thresholds. The observed trends and dependencies are pertinent to understanding the early universe's conditions and the nature of QCD matter under extreme conditions.

The findings have significant theoretical implications for modeling approaches to heavy-ion collisions, potentially guiding future experimental designs at RHIC and other facilities like FAIR and NICA. The documented centrality dependence of baryon chemical potential presents an opportunity for future experimental efforts to probe the QCD critical point and phase transitions. Future work may also explore longitudinal dynamics, multi-particle correlation studies, and comparisons with lattice QCD predictions to further solidify the understanding of the QCD phase structure.