Isotropization and hydrodynamization in weakly coupled heavy-ion collisions (1506.06647v2)

Abstract: We numerically solve 2+1D effective kinetic theory of weak coupling QCD under longitudinal expansion relevant for early stages of heavy-ion collisions. We find agreement with viscous hydrodynamics and classical Yang-Mills simulations in the regimes where they are applicable. By choosing initial conditions that are motivated by color-glass-condensate framework we find that for Q=2GeV and $\alpha_s$=0.3 the system is approximately described by viscous hydrodynamics well before $\tau \lesssim 1.0$ fm/c.

• The paper demonstrates that energy density evolution follows viscous hydrodynamics predictions within 10% accuracy by Qsτ ∼ 5.0 under realistic coupling conditions.
• The study employs a 2+1D effective kinetic theory under longitudinal expansion to bridge classical Yang-Mills dynamics with viscous hydrodynamics.
• The research reveals hydrodynamic behavior emerges earlier than expected (τ ≲ 1.0 fm/c for Qs=2 GeV), refining models of early-time heavy-ion collisions.

Isotropization and Hydrodynamization in Weakly Coupled Heavy-Ion Collisions

This paper investigates the complex processes of isotropization and hydrodynamization in the context of weakly coupled heavy-ion collisions which are indicative of the thermal evolution of the quark-gluon plasma. Initially, post-collision debris in the mid-rapidity region undergo multiple stages characterized by different physics and degrees of freedom. Employing a numerical solution to a 2+1D effective kinetic theory (EKT) under longitudinal expansion, the paper explores transitions between classical Yang-Mills fields to viscous hydrodynamics within the weak coupling limit of QCD.

The research aligns with established hydrodynamic models in applicable regimes, bridging the gap between saturated gluon fields and fluid dynamics. It provides insights into utilizing EKT as an improvable framework, especially in systems where gluon occupancies are not nonperturbative $f \ll 1/\lambda$ and momenta significantly surpass the in-medium screening scale. This clear transition from classical Yang-Mills theory for early conditions to EKT and later hydrodynamics is crucial for effective modeling of heavy-ion collisions.

Key Numerical Results

By selecting initial conditions aligned with the color-glass-condensate framework, the paper demonstrates that the system approximates viscous hydrodynamics well before $\tau \lesssim 1.0$ fm/c for $Q_s=2$ GeV and $\alpha_s=0.3$. The research finds substantial agreement with viscous hydrodynamics and classical Yang-Mills simulations under conditions where these models are valid. The numerical results show that, for realistic coupling $\lambda = 10$, corresponding to $\alpha_s \approx 0.3$, the energy density evolution adheres to viscous hydrodynamics predictions within $10\%$ accuracy after $Q_s\tau \sim 5.0$.

Implications and Future Directions

The paper challenges certain theoretical assumptions about early-time dynamics. The findings suggest that at realistic values of coupling, hydrodynamic descriptions can be valid far earlier than previously anticipated, indicating robustness of hydrodynamics in QCD even in anisotropic scenarios. Chromo-Weibel instabilities—while formally impactful in anisotropic screening—are numerically small for relevant $\lambda$ values, suggesting an isotropic treatment in screening can suffice until more comprehensive models are developed.

For future research, the paper establishes foundational work in connecting different simulation regimes, opening avenues for refining initial conditions in EKT simulations further validated by 3+1D simulations. Discrepancies in scaling estimates at varying coupling levels hint at potential areas for deeper investigation, particularly concerning plasma instabilities and their role in isotropization.

Concluding Remarks

This paper contributes a systematic framework for studying prethermal evolution in heavy-ion collisions, enhancing our understanding of isotropization and hydrodynamization. Utilizing numerical simulations within EKT and integrating these into hydrodynamic models offers a significant step in modeling the complex dynamics of the early stages of heavy-ion collisions. The results bear relevance for both theoretical explorations and practical applications in understanding quark-gluon plasma behaviors under extreme conditions.