Kaon and Lambda Production at Intermediate $p_T$: Insights into the Hadronization of the Bulk Partonic Matter Created in Au+Au Collisions at RHIC

Abstract

Measurements of identified particles over a broad $p_T$ range may provide strong evidence for the existence of a thermalized partonic state in heavy-ion collisions (\textit{i.e.} a quark-gluon plasma). Of particular interest are the centrality dependence and the azimuthal anisotropy in the yield of baryons and mesons at intermediate $p_T$. The first measurements of $v_2$ and the number-of-binary-collisions scaled centrality ratio $R_{CP}$ for $K_S^0$ and $\Lambda+\bar{\Lambda}$ production in Au+Au collisions at RHIC are presented. These measurements from the STAR detector establish the particle-type dependence of $v_2$ and $R_{CP}$ within $0.4 < p_T < 6.0$. At intermediate $p_T$ (1.5--4.5 GeV/c) $v_2$ of $K_S^0$ and $\Lambda+\bar{\Lambda}$ is shown to follow a number-of-constituent-quark scaling with v_2^{kaon}(p_T/2)/2 = v_2^{lambda}(p_T/3)/3. $R_{CP}$ shows that lambda production at intermediate $p_T$ increases more rapidly with system size than kaon production: consistent with multi-parton dynamics in particle production. At $p_T = 5.5$ GeV/c lambda, kaon, and charged hadron production are suppressed by a similar factor ($R_{CP} \approx 0.33$): establishing the extent of the centrality dependent baryon enhancement. The particle- and $p_T$-dependence of $v_2$, and $R_{CP}$ are consistent with expectations based on hadronization of bulk partonic matter by coalescence. As such, the constituent-quark-number scaled $v_2$ may reflect the anisotropy established in a partonic stage providing strong evidence for the existence of a quark-gluon plasma in Au+Au collisions at RHIC.