Speaker
Description
Transverse-momentum ($p_T$) dependence of elliptic flow ($v_2$) for identified particles at the top RHIC energy has well established a number-of-constituent-quark (NCQ) scaling [1], supportive of the coalescence sum rule that determines $v_2$ of mesons and baryons as the sum of contributions from their constituent quarks. The NCQ scaling implicitly assumes that the produced drops of partonic matter are thermally equilibrated, and constituent quarks at hadronization do not remember their past history. This assumption is found to be violated with the RHIC Beam Energy Scan (BES) data [2]. In this talk, we will focus on the $p_T$-integrated $v_2$ data instead of $v_2(p_T)$, and quantitatively explain the results on $\pi^\pm$, $K^\pm$, $p$, $\bar{p}$, $\Lambda$ and $\bar{\Lambda}$ in Au+Au collisions at $\sqrt{s_{\rm NN}} = 7.7$ GeV to 62.4 GeV by differentiating quarks produced in the collision and those transported from the initial-state nuclei. After verification of the coalescence sum rule, we extract $v_2$ for both produced ($u/d/{\bar u}/{\bar d}$, $s$, and ${\bar s}$, separately) and for transported ($u/d$) quarks. The latter is found to be larger than the former, and accounts for the observed $v_2$ difference between particles and antiparticles. We also relate the $v_2$ measurements of multi-strange hadrons to the different formation times of $\phi$, $\Omega^\pm$ and $\Xi^+$. At the RHIC BES regime, although the coalescence sum rule seems to be valid and the partonic degrees of freedom are important, the produced nuclear matter may not reach a full thermal equilibrium. We will discuss the beam energy dependence of the baryon transport and its impact on thermal equilibration.
[1] J. Adams et al. (STAR Collaboration), Nucl. Phys. A 757 (2005) 102.
[2] L. Adamczyk et al. (STAR Collaboration), Phys. Rev. C 93, 014907 (2016).
