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

We report the first measurement of rapidity-odd directed flow ($v_1$) for $D^0$ and $\overline{D^0}$ mesons at mid-rapidity ($\mathrm{\mid }y\mathrm{\mid }<0.8$) in Au+Au collisions at $\sqrt{s_{\mathrm{N}\mathrm{N}}}$ = 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 ($d{v}_1\mathrm{/}dy$), averaged over $D^0$ and $\overline{D^0}$ mesons, is -0.080 $\pm$ 0.017 (stat.) $\pm$ 0.016 (syst.) for transverse momentum $p_{\mathrm{T}}$ above 1.5~GeV/$c$. The absolute value of $D^0$-meson $d{v}_1\mathrm{/}dy$ 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.