We report results of $v_1(y)$ and $dv_1/dy$ near mid-rapidity for $\pi^{ \pm}$, $K^{ \pm}$, $K_s^0$, $p$, $\overline{ p}$, $\Lambda$, $\overline{ \Lambda}$ and $\phi$ from Beam Energy Scan Au+Au collisions at $\sqrt{ s_{ NN}} = $ 7.7 - 200 GeV using the STAR detector at RHIC. The $dv_{ 1}/dy$ of $\pi^{ \pm}$, $K^{ \pm}$ and $K_s^0$ mesons remains negative over all beam energies. The $dv_1/dy$ 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 $dv_1/dy$ of $\overline{ p}$, $\overline{ \Lambda}$ and $\phi$ 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.
Categories: nucl-ex hep-ex nucl-th