Abstract
We study the origin of the directed flow of charged particles produced in relativistic heavy-ion collisions. Three different initial conditions, $\mathrm{Boz}\stackrel{\ifmmode \dot{}\else \.{}\fi{}}{\text{e}}\mathrm{k}$-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 comparison to experimental data at both the Relativistic Heavy-Ion Collider and the Large Hadron Collider, 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 midrapidity, the counterclockwise 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.
Highlights
Heavy-ion collisions at the BNL Relativistic Heavy-Ion Collider (RHIC) and the CERN Large Hadron Collider (LHC) suggest that a hot and dense nuclear matter, known as quarkgluon plasma (QGP), is formed in the reaction region
By comparison to experimental data at both the Relativistic Heavy-Ion Collider and the Large Hadron Collider, 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
Our calculation indicates the counterclockwise tilt of the initial energy density profile yields an increasingdecreasing average pressure gradient − ∂xP from zero with respect to time at backward-forward space-time rapidity within |ηs| < 2. This further leads to a negative slope of the average QGP flow velocity vx with respect to ηs, and in the end the same behavior of v1 vs η for the final-state charged particles√, which is consistent with the experimental observations√in sNN = 200 GeV Au + Au collisions at RHIC [55] and sNN = 2.76 TeV Pb + Pb collisions at LHC [56]
Summary
Heavy-ion collisions at the BNL Relativistic Heavy-Ion Collider (RHIC) and the CERN Large Hadron Collider (LHC) suggest that a hot and dense nuclear matter, known as quarkgluon plasma (QGP), is formed in the reaction region. Following the idea of Bozek-Wyskiel, we developed an alternative initialization ansatz (Central China Normal University — CCNU — parametrization) of the longitudinal distribution of the nuclear matter in an earlier study [51] It grasps the key feature of the tilted medium geometry and is able to describe the charged particle v1 at RHIC and LHC. Our calculation indicates the counterclockwise tilt of the initial energy density profile yields an increasingdecreasing average pressure gradient − ∂xP from zero with respect to time at backward-forward space-time rapidity within |ηs| < 2 This further leads to a negative slope of the average QGP flow velocity vx with respect to ηs, and in the end the same behavior of v1 vs η for the final-state charged particles√, which is consistent with the experimental observations√in sNN = 200 GeV Au + Au collisions at RHIC [55] and sNN = 2.76 TeV Pb + Pb collisions at LHC [56].
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