Event-by-event anisotropic flow in heavy-ion collisions from combined Yang-Mills and viscous fluid dynamics (1209.6330v1)

Abstract: Anisotropic flow coefficients v_1-v_5 in heavy ion collisions are computed by combining a classical Yang-Mills description of the early time glasma flow with the subsequent relativistic viscous hydrodynamic evolution of matter through the quark-gluon plasma and hadron gas phases. The glasma dynamics, as realized in the IP-Glasma model, takes into account event-by-event geometric fluctuations in nucleon positions and intrinsic sub-nucleon scale color charge fluctuations; the pre-equilibrium flow of matter is then matched to the MUSIC algorithm describing viscous hydrodynamic flow and particle production at freeze-out. The IP-Glasma+MUSIC model describes well both transverse momentum dependent and integrated v_n data measured at the Large Hadron Collider (LHC) and the Relativistic Heavy Ion Collider (RHIC). The model also reproduces the event-by-event distributions of v_2, v_3 and v_4 measured by the ATLAS collaboration. The implications of our results for better understanding of the dynamics of the glasma as well as for the extraction of transport properties of the quark-gluon plasma are outlined.

• The paper demonstrates a hybrid model that integrates classical Yang-Mills initial conditions with viscous hydrodynamics for event-by-event flow analysis.
• The methodology leverages the IP-Glasma+music framework to simulate sub-nucleon fluctuations and early non-equilibrium dynamics in heavy-ion collisions.
• Results show strong agreement with RHIC and LHC data, highlighting the model’s capability to extract temperature-dependent viscosity features.

Event-by-Event Anisotropic Flow in Heavy-Ion Collisions from Combined Yang-Mills and Viscous Fluid Dynamics

The paper presents an intricate paper of anisotropic flow coefficients $v_1$ through $v_5$ in the context of heavy-ion collisions at RHIC and LHC. Using a novel hybrid model, the authors combine a classical Yang-Mills framework with a relativistic viscous hydrodynamic model, known as the IP-Glasma+music model. This model uniquely captures the evolution from the initial fluctuating glasma stage to the late-stage quark-gluon plasma and hadron gas phases.

Methodology

The IP-Glasma model is introduced for simulating the early-time dynamics of glasma fields with fluctuations down to sub-nucleon scales, providing an initial condition which leads to a more accurate description of early-stage non-equilibrium phenomena. The transition from the glasma phase to the quark-gluon plasma is executed by transitioning the system's description to a viscous hydrodynamics through the music algorithm. This approach accounts for geometric nucleon fluctuation and intrinsic color charge variations event-by-event.

The methodology involves starting the viscous hydrodynamic evolution at a switching time $\tau_{\text{switch}} \sim 1/Q_s$, with considerations on the efficiency of early-stage isotropization mechanisms which may adjust the initial viscous tensor. Various analyses were conducted to explore consistency over different switching times and parametrizations for shear viscosity, ensuring robustness in the model's fidelity to experimental data.

Results

The model successfully aligns with both transverse momentum-dependent and integrated $v_n$ data across NHIC and LHC energies. Specifically, results demonstrate strong agreement with ATLAS collaboration data regarding event-by-event distributions of $v_2$, $v_3$, and $v_4$, as well as the $p_T$-spectra for various particle types. The fidelity of the model under different switching times and viscous coefficients was scrutinized, highlighting a credible window where both classical Yang-Mills dynamics and hydrodynamics yield comparable conclusions.

A comparative analysis with experimental data under constant and temperature-dependent shear viscosity outlines the potential for extracting temperature-dependent viscosity characteristics, albeit with limitations in higher $p_T$ regions at RHIC. Further improvements in the $\eta/s(T)$ model were proposed for better experimental correspondence.

Implications and Future Directions

The confluence of classical Yang-Mills theory with viscous hydrodynamics in this paper provides a considerable enhancement to previous flow dynamic models. By integrating sub-nucleon dynamics into heavy-ion collision simulations, the model presents a pathway for probing early-stage glasma phenomena and refining the extraction of transport properties of the quark-gluon plasma. Importantly, the ability to describe a wide variety of experimental data with a single model underscores its potential utility in future research.

Future developments could focus on further refining pre-equilibrium dynamics within this framework, potentially incorporating instabilities and enhanced scattering mechanisms for more comprehensive early-time isotropization. Additionally, further parameter sensitivity analyses will enhance the robustness of the model and broaden its applicability across different energies and conditions. As ongoing work aims to reduce systematic uncertainties, the IP-Glasma+music model remains a promising candidate for advancing our understanding of the complex dynamics in heavy-ion collisions.