Azimuthal anisotropy: transition from hydrodynamic flow to jet suppression

TL;DR

This paper investigates how azimuthal anisotropy in heavy-ion collisions transitions from being dominated by hydrodynamic flow at low transverse momentum to jet suppression at higher transverse momentum, revealing viscous effects and a breakdown of hydrodynamics.

Contribution

It provides empirical analysis of anisotropy coefficients scaled by initial eccentricity, identifying the transition point and constraining viscosity and freeze-out temperature models.

Findings

Scaling violations for p_T < 3 GeV/c consistent with viscous corrections.

Viscous corrections show a maximum around p_T > 3 GeV/c, indicating a transition from flow to suppression.

Constraints on specific viscosity and freeze-out temperature from empirical viscous corrections.

Abstract

Measured 2nd and 4th azimuthal anisotropy coefficients v_{2,4}(N_{part}), p_T) are scaled with the initial eccentricity \varepsilon_{2,4}(N_{part}) of the collision zone and studied as a function of the number of participants N_{part} and the transverse momenta p_T. Scaling violations are observed for $p_T \alt 3$ GeV/c, consistent with a $p_T^2$ dependence of viscous corrections and a linear increase of the relaxation time with $p_T$. These empirical viscous corrections to flow and the thermal distribution function at freeze-out constrain estimates of the specific viscosity and the freeze-out temperature for two different models for the initial collision geometry. The apparent viscous corrections exhibit a sharp maximum for $p_T \agt 3$ GeV/c, suggesting a breakdown of the hydrodynamic ansatz and the onset of a change from flow-driven to suppression-driven anisotropy.