Early anisotropic hydrodynamics and the RHIC early-thermalization and HBT puzzles

TL;DR

This paper proposes a hybrid anisotropic hydrodynamic model for early-stage heavy-ion collision dynamics, which successfully reproduces key experimental data and suggests that rapid thermalization assumptions can be relaxed, helping to resolve longstanding puzzles.

Contribution

The study introduces a hybrid model combining transverse hydrodynamics with perfect-fluid dynamics, providing a new approach to address early thermalization and HBT puzzles in heavy-ion collisions.

Findings

Reproduces RHIC data with about 20% accuracy

Suggests early thermalization assumption can be relaxed

Reduces discrepancies in HBT radii predictions

Abstract

We address the problem if the early thermalization and HBT puzzles in relativistic heavy-ion collisions may be solved by the assumption that the early dynamics of the produced matter is locally anisotropic. The hybrid model describing the purely transverse hydrodynamic evolution followed by the perfect-fluid hydrodynamic stage is constructed. The transition from the transverse to perfect-fluid hydrodynamics is described by the Landau matching conditions applied at a fixed proper time. The global fit to the RHIC data reproduces the soft hadronic observables (the pion, kaon, and the proton spectra, the pion and kaon elliptic flow, and the pion HBT radii) with the accuracy of about 20%. These results indicate that the assumption of the very fast thermalization may be relaxed. In addition, the presented model suggests that a large part of the inconsistencies between the theoretical and experimental HBT results may be removed.