# First observation of the directed flow of $D^{0}$ and $\overline{D^0}$ in Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 200~GeV

## TL;DR

This paper reports the first measurement of the directed flow ($v_1$) for $D^{0}$ and $ar{D}^{0}$ mesons in Au+Au collisions at 200 GeV, revealing a significant $v_1$ slope that provides insights into the initial tilt of the quark-gluon plasma.

## Contribution

It presents the first observation of $D^{0}$ and $ar{D}^{0}$ meson directed flow in heavy-ion collisions, offering new constraints on the initial conditions and transport properties of the quark-gluon plasma.

## Key findings

- $D^{0}$ meson $dv_1/dy$ is about 25 times larger than charged kaons.
- The measured $dv_1/dy$ slope is -0.080 with statistical and systematic uncertainties.
- The results provide insights into the initial tilt and transport properties of the hot QCD medium.

## Abstract

We report the first measurement of rapidity-odd directed flow ($v_{1}$) for $D^{0}$ and $\overline{D^{0}}$ mesons at mid-rapidity ($|y| < 0.8$) in Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 200\,GeV using the STAR detector at the Relativistic Heavy Ion Collider. In 10--80\% Au+Au collisions, the slope of the $v_{1}$ rapidity dependence ($dv_{1}/dy$), averaged over $D^{0}$ and $\overline{D^{0}}$ mesons, is -0.080 $\pm$ 0.017 (stat.) $\pm$ 0.016 (syst.) for transverse momentum $p_{\rm T}$ above 1.5~GeV/$c$. The absolute value of $D^0$-meson $dv_1/dy$ is about 25 times larger than that for charged kaons, with 3.4$\sigma$ significance. These data give a unique insight into the initial tilt of the produced matter, and offer constraints on the geometric and transport parameters of the hot QCD medium created in relativistic heavy-ion collisions.