Directed flow in Au+Au collisions from the RHIC Beam Energy Scan at the STAR experiment

Abstract

We report results of $v_1(y)$ and $d{v}_1/dy$ near mid-rapidity for ${\pi }^{\pm }$, $K^{\pm }$, $K_s^0$, $p$, $\overline{p}$, $\Lambda$, $\overline{\Lambda }$ and $\varphi$ from Beam Energy Scan Au+Au collisions at $\sqrt{s_{NN}}=$ 7.7 - 200 GeV using the STAR detector at RHIC. The $d{v}_1/dy$ of ${\pi }^{\pm }$, $K^{\pm }$ and $K_s^0$ mesons remains negative over all beam energies. The $d{v}_1/dy$ of $p$ and $\Lambda$ baryons shows a sign change around 10 - 15 GeV, while net baryons (net p and net $\Lambda$) indicate a double sign change. The $d{v}_1/dy$ of $\overline{p}$, $\overline{\Lambda }$ and $\varphi$ show a similar trend for $\sqrt{s_{NN}}>$ 14.5 GeV. For the first time, $v_1$ measurements are used to test a quark coalescence hypothesis. Many measurements are found to be consistent with the particles being formed via coalescence of constituent quarks. The observed deviations from that consistency offer a new approach for probing the collision process at the quark level.

Comments: 7 pages, 4 figures to appear in CPOD 2017 proceedings, PoS(CPOD2017)004