# Quark Matter 2018

May 13 – 19, 2018
Venice, Italy
Europe/Zurich timezone
The organisers warmly thank all participants for such a lively QM2018! See you in China in 2019!

## Directed Flow Due to the Initial Source Tilt and Density Asymmetry in Cu+Au and Au+Au Collisions at STAR

May 15, 2018, 5:00 PM
2h 40m
First floor and third floor (Palazzo del Casinò)

### First floor and third floor

#### Palazzo del Casinò

Poster Initial state physics and approach to equilibrium

### Speaker

Takafumi Niida (Wayne State University)

### Description

Despite of a generally very successful description of the elliptic and higher harmonic flow in heavy-ion collisions by theoretical models, there is no single model that explains the dependence of the directed flow on pseudorapidity, collision energy, system size, and the particle type. This indicates that an important piece in our picture of ultrarelativistic heavy-ion collisions is still missing. Directed flow is thought to arise from two main mechanisms: the so-called "tilted" source and the dipole-like initial density asymmetry. The asymmetric Cu+Au collisions provide a unique possibility to identify the roles of theses two mechanisms.

We present the results of directed flow measurements for charged and identified particles in Cu+Au and Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 200 GeV as a function of centrality, pseudorapidity, and transverse momentum. We show how the comparison of pseudorapidiy dependence of $v_1$ and $\langle p_x\rangle$ can be used to quantify the contribution of different mechanisms of directed flow formation. We compare the results with Pb+Pb collisions at the LHC energies, and discuss the centrality and energy dependence of the relative contributions to the directed flow from the initial source tilt and from the density asymmetry.

Centralised submission by Collaboration Presenter name already specified STAR Experiment

### Primary author

Zhenyu Ye (University of Illinois at Chicago)