Collective flow and viscosity in relativistic heavy-ion collisions (1301.2826v1)

Abstract: Collective flow, its anisotropies and its event-to-event fluctuations in relativistic heavy-ion collisions, and the extraction of the specific shear viscosity of quark-gluon plasma (QGP) from collective flow data collected in heavy-ion collision experiments at RHIC and LHC are reviewed. Specific emphasis is placed on the similarities between the Big Bang of our universe and the Little Bangs created in heavy-ion collisions.

• The paper elucidates how advanced hydrodynamic modeling extracts QGP shear viscosity from heavy-ion collision data at RHIC and LHC.
• It demonstrates that both ideal and viscous fluid dynamics capture the influence of initial geometric fluctuations on elliptic and triangular flow.
• The findings confirm QGP's near-perfect fluidity with shear viscosity near the theoretical lower bound, guiding future QCD research.

Overview of "Collective Flow and Viscosity in Relativistic Heavy-Ion Collisions"

The paper "Collective Flow and Viscosity in Relativistic Heavy-Ion Collisions" by Ulrich Heinz and Raimond Snellings provides an extensive review of the phenomena of collective flow and its anisotropies in relativistic heavy-ion collisions. It also discusses the extraction of the specific shear viscosity of quark-gluon plasma (QGP) utilizing data from the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC). Central to the paper is the comparison between the early universe Big Bang and the mini or "Little Bangs" that occur during heavy-ion collisions, emphasizing the similar underlying physical principles.

Historical Context

The concept of using relativistic fluid dynamics to describe high-energy hadron and nuclear collisions is not new; its foundations were laid in the mid-20th century. Early observations demonstrated collective transverse flow, driving theoretical efforts to solve the relativistic fluid dynamics equations under conditions relevant to heavy-ion collisions. These models initially predicted momentum distributions that qualitatively matched experimental observations but faced quantitative discrepancies at varying collision energies.

The advent of RHIC marked a pivotal transition, providing data that aligned well with ideal fluid dynamics predictions, particularly for QGP. This led to the realization that QGP behaves as a strongly-coupled liquid with low viscosity, in contrast to earlier gas-like expectations based on asymptotic freedom.

Hydrodynamic Modeling and Viscosity

In-depth hydrodynamic models, both ideal and viscous, have been developed to better capture the complexities of heavy-ion collisions. The paper explores second-order viscous relativistic fluid dynamics, emphasizing the need to consider shear and bulk viscosities along with relaxation times. The evolution from the early ideal hydrodynamic approach to more sophisticated hybrid models that integrate microscopic hadron cascades illustrates the progression towards more accurate representation of the late-stage hadronic phase of these collisions.

Extraction of Shear Viscosity

A key scientific objective addressed by the paper is the determination of QGP shear viscosity, $\eta/s$. Theoretical analyses offer lower bounds around $1/4\pi$, derived using AdS/CFT correspondence principles. The extraction of these transport coefficients from experimental data relies heavily on the interplay between the specific viscosities and the fluctuating initial states of QGP. Event-by-event fluctuations play a significant role, as seen in the harmonic flow components generated by such fluctuations.

Observable Consequences and Model Comparisons

The study identifies various manifestations of collective flow, reflecting both radial and anisotropic expansion. It highlights the intricate relationship between initial-state geometry, its fluctuations, and the resultant flow harmonics like elliptic flow ($v_2$) and triangular flow ($v_3$). The paper stresses the importance of accurately characterizing the initial eccentricity coefficients ($\varepsilon_n$) that drive flow harmonics, advocating for the utilization of different initial state models such as MC-Glauber, MC-KLN, and IP-Glasma.

Moreover, the paper underscores the significance of non-linear hydrodynamic response, which affects harmonic flows at different order $n$ and can be sensitive to shear viscosity. Through meticulous comparisons of hydrodynamic model predictions with experimental data, the paper illustrates the power of these models to replicate detailed features of the measured anisotropic flow patterns, providing constraints on $(\eta/s)_{QGP}$.

Implications and Future Directions

The insights derived from this comprehensive analysis have profound implications for understanding the fundamental properties of QCD matter under extreme conditions. The meticulous extraction of transport coefficients not only heralds advancements in the precision characterization of QGP but also promises to inform future explorations into bulk viscosity and relaxation phenomena.

Looking ahead, the continued comparison of theoretical models with an expanding array of experimental data from varying collision energies and system sizes appears promising. These studies will refine our understanding of the QGP's transport properties, advancing our grasp of early universe conditions shortly after the Big Bang.

This paper provides a robust foundation for future investigations aiming to complete the "Little Bang Standard Model," aspiring to mirror the precise and predictive capabilities of the cosmological Big Bang model, albeit in the vastly different context of high-energy nuclear physics.