Longitudinal distribution of initial energy density and directed flow of charged particles in relativistic heavy-ion collisions

TL;DR

This study investigates how different initial energy density conditions influence the directed flow of charged particles in relativistic heavy-ion collisions, using hydrodynamic modeling and experimental data comparison.

Contribution

It introduces a systematic comparison of three initial condition models and links the tilt of the initial medium profile to the observed directed flow patterns.

Findings

The initial tilt of the medium profile constrains directed flow.

Counter-clockwise tilt causes a negative slope in flow velocity.

The model matches experimental data at RHIC and LHC.

Abstract

We study the origin of the directed flow of charged particles produced in relativistic heavy-ion collisions. Three different initial conditions, Boz$\dot{\textrm{e}}$k-Wyskiel, CCNU and Shen-Alzhrani, of energy density distributions are coupled to the (3+1)-dimensional viscous hydrodynamic model CLVisc, and their effects on the development of the anisotropic medium geometry, pressure gradient and radial flow are systematically compared. By comparing to experimental data at both RHIC and LHC, we find that the directed flow provides a unique constraint on the tilt of the initial medium profile in the plane spanned by the impact parameter and space-time rapidity. Within mid-rapidity, the counter-clockwise tilt is shown to be a crucial source of the positive/negative force by the pressure gradient along the impact parameter ($x$) direction at backward/forward rapidity, which drives a negative slope of the $x$ component of the medium flow velocity with respect to rapidity, and in the end the same feature of the charged particle directed flow.