Measurements of Higher-Order Flow Harmonics in Au+Au Collisions at sqrt(s_NN) = 200 GeV (1105.3928v2)

Abstract: Flow coefficients v_n for n = 2, 3, 4, characterizing the anisotropic collective flow in Au+Au collisions at sqrt(s_NN) = 200 GeV, are measured relative to event planes \Psi_n determined at large rapidity. We report v_n as a function of transverse momentum and collision centrality, and study the correlations among the event planes of different order n. The v_n are well described by hydrodynamic models which employ a Glauber Monte Carlo initial state geometry with fluctuations, providing additional constraining power on the interplay between initial conditions and the effects of viscosity as the system evolves. This new constraint improves precision of the extracted viscosity to entropy density ratio eta/s.

• The paper demonstrates that v3 and v4 show minimal centrality dependence compared to v2, emphasizing the impact of initial state fluctuations.
• The paper leverages a robust dataset and multiple detector systems to precisely measure flow coefficients as functions of transverse momentum and centrality.
• The paper constrains the QGP shear viscosity by comparing measured v3 values with hydrodynamic models, favoring scenarios with 4π(η/s)≈1.

Analyzing Higher-Order Flow Harmonics in Au+Au Collisions at √s_NN = 200 GeV

This article presents a detailed analysis of higher-order flow harmonics in gold-gold (Au+Au) collisions at the relativistic energy scale of √s_NN = 200 GeV. The research is conducted by the PHENIX Collaboration, focusing on flow coefficients $v_n$ for $n$=2, 3, and 4, which describe anisotropic collective flow in collisions relative to event planes $\Psi_n$ at large rapidities.

Overview and Results

The paper leverages a robust dataset—comprising approximately $3.0 \times 10^9$ Au+Au events collected by the PHENIX detector—to evaluate $v_n$ as a function of transverse momentum ($p_T$) and collision centrality. The study utilizes multiple detector systems to ascertain event planes, which are essential in deciphering flow patterns.

The significant insights presented in the paper include the identification and measurement of correlations among different order event planes. These correlations substantiate fluctuations in the initial state geometry, which are pivotal in enhancing the precision of hydrodynamic modeling of quark-gluon plasma (QGP) formation and evolution.

Key Findings

1. Higher-Order Harmonics: The research reports that $v_3$ and $v_4$ are less dependent on collision centrality as compared to $v_2$. Notably, $v_3$ exhibits magnitudes that are significant in more central collisions, matching results predicted by Glauber model calculations, which suggest substantial fluctuations in the participant geometry.
2. Viscosity Constraints: The paper emphasizes the constraining power of $v_3$ measurements, which help refine the extracted values of the shear viscosity to entropy density ratio $(\eta/s)$ of the QGP. Through comparisons with various hydrodynamic models—particularly those employing Glauber initial conditions—it becomes evident that models predicting $4\pi\eta/s \approx 1$ align more closely with the observed data.
3. Event Plane Correlations: The results confirm the absence of significant correlations between $\Psi_2$ and $\Psi_3$, reinforcing the notion that initial state fluctuations dominate higher order harmonics.

Implications and Future Directions

The implications of this study are profound for both theoretical and experimental nuclear physics. By offering tighter constraints on model parameters, such as $\eta/s$, this work provides critical insights into the underlying physics of QGP and enhances predictive capabilities for other relativistic heavy ion collision experiments. Future research directions could involve expanding this analysis to different systems and collision energies, or incorporating additional theoretical improvements to continuum models to further disentangle the influence of initial state fluctuations and medium properties.

In summary, this paper contributes significantly to the understanding of anisotropic flow phenomena in high-energy nuclear collisions. These results not only refine our comprehension of QGP dynamics but also guide the ongoing development and calibration of relativistic hydrodynamic models used to simulate heavy ion collisions.