First Observation of the Directed Flow of $D^{0}$ and $\overline{D^0}$ in Au+Au Collisions at $\sqrt{s_{\rm NN}}$ = 200 GeV

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 ± 0.017 (stat.) ± 0.016 (syst.) for transverse momentum $p_T$ 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.