Properties of hot and dense matter from relativistic heavy ion collisions (1510.00442v3)

Abstract: We review the progress achieved in extracting the properties of hot and dense matter from relativistic heavy ion collisions at the relativistic heavy ion collider (RHIC) at Brookhaven National Laboratory and the large hadron collider (LHC) at CERN. We focus on bulk properties of the medium, in particular the evidence for thermalization, aspects of the equation of state, transport properties, as well as fluctuations and correlations. We also discuss the in-medium properties of hadrons with light and heavy quarks, and measurements of dileptons and quarkonia. This review is dedicated to the memory of Gerald E. Brown.

• The paper demonstrates that rapid thermalization in heavy-ion collisions enables hydrodynamic modeling to extract the QGP’s equation of state.
• The paper finds that flow measurements constrain transport properties, with the shear viscosity to entropy density ratio nearing its quantum lower bound.
• The paper details that fluctuations of conserved charges and in-medium hadron modifications via dilepton spectra probe the QCD phase structure and chiral restoration.

Overview of "Properties of Hot and Dense Matter from Relativistic Heavy Ion Collisions"

The paper by Braun-Munzinger et al. provides a thorough review of the advancements in extracting properties of hot and dense matter from relativistic heavy-ion collisions, primarily conducted at RHIC and LHC. These facilities are crucial in exploring the phase transition from hadronic matter to quark-gluon plasma (QGP) and studying nuclear matter under extreme conditions.

The paper discusses several key areas:

1. Thermalization and the Equation of State: The authors address the evidence for thermalization in heavy-ion collisions, a cornerstone for applying hydrodynamics to describe the subsequent evolution. The QGP's equation of state, revealed through lattice QCD computations and experimental data, shows a rapid rise in the energy density around the critical temperature, indicating a crossover transition for QCD with physical quark masses.
2. Transport Properties: The review sheds light on estimating transport properties, particularly the shear viscosity to entropy density ratio, $\eta/s$, which interestingly approaches the conjectured lower bound in a strongly coupled plasma. These properties are constrained by experimental data on collective flow and elliptic flow measurements, providing insights into the fluid-like properties of the QGP.
3. Fluctuations and Correlations: The authors explore fluctuations of conserved charges, emphasizing their sensitivity to the phase structure of QCD. These fluctuations are potential signatures for critical phenomena, and the ongoing beam energy scan at RHIC aims to map out the QCD phase diagram at finite baryon density.
4. In-Medium Properties of Hadrons and Dilepton Measurements: Modifications to hadronic properties in medium, probed through dilepton and photon measurements, are essential for understanding chiral symmetry restoration. The broadening and potential shifting of vector meson masses, especially the $\rho$ meson, serve as a probe for in-medium effects and have been investigated with high precision in measurements from the CERES and NA60 collaborations.
5. Heavy Quarks and Quarkonia: The thermalization and suppression of heavy quarks are discussed, alongside models explaining charmonium production in heavy-ion collisions. The recombination models accounting for charm quark thermalization and subsequent formation of quarkonia at hadronization provide a compelling framework, especially in light of recent LHC results showing J/$\psi$ regeneration.

Implications and Future Prospects

The synthesis of theoretical models and experimental data has provided an intricate understanding of QGP properties and dynamics. The paper underscores the significance of continued experimental efforts at varying collision energies and configurations to refine our comprehension of the QCD phase diagram. Additionally, future theoretical advancements in modeling initial state fluctuations, viscosities, and critical dynamics will enhance the precision of heavy-ion collision simulations.

Looking forward, further studies of small systems ($pA$, $pp$) offer a new dimension to traditional heavy-ion physics, deepening insights into the limits of hydrodynamic descriptions. As experimental techniques advance, they will likely yield precise measurements of heavy quark diffusion constants, elucidate the conditions necessary for QGP formation, and explore novel phenomena such as color superconductivity at high densities.

In conclusion, the paper by Braun-Munzinger et al. offers a comprehensive view of the current understanding of hot and dense QCD matter, firmly placing heavy-ion collision experiments at the forefront of exploring fundamental aspects of nuclear matter.