Speaker
Description
Directed flow ($v_1$) is a good probe of the early-stage dynamics of collision systems, and the $v_1$ slope ($dv_1/dy$) at mid-rapidity is sensitive to the system's equation of state. Previously, a coalescence picture has been used to relate the elliptic flow ($v_2$) of identified particles to the $v_2$ of their constituent quarks [1]. In this talk, the coalescence idea is extended to study the $v_1$ of the constituent quarks - this includes quarks produced in the collision, as well as those transported from the initial-state nuclei. Rather than probe coalescence of $v_2$ versus $p_T$, we focus on $dv_1/dy$ versus $\sqrt{s_{NN}}$.
We report STAR measurements of $v_1(y)$ for ten particle species ($\pi^\pm$, $p$, $\bar{p}$, $\Lambda$, $\bar{\Lambda}$, $\phi$, $K^\pm$ and $K^0_{S}$) in Au+Au collisions at eight beam energies from $\sqrt{s_{NN}} = 7.7$ GeV to 200 GeV. The $v_1$ slopes of $\bar{p}$, $K^-$ and $\bar{\Lambda}$ are used to test the coalescence sum rule for produced quarks. We hypothesize that the $v_n$ of observed mesons and baryons is the summed $v_n$ of their constituent quarks, and it is assumed that deconfined quarks have already acquired azimuthal anisotropy. Data involving produced quarks support the coalescence picture at $\sqrt{s_{NN}} = 11.5$ GeV to 200 GeV, and a sharp deviation from this picture is observed at 7.7 GeV [2].
The $dv_1/dy$ of transported quarks are studied via net particles (net $p$, net $K$ and net $\Lambda$). The net-particle $dv_1/dy$ data for $p$, $K$ and $\Lambda$ agree at and above $\sqrt{s_{NN}} = 14.5$ GeV, but net kaons begin to diverge with decreasing beam energy. Tests of the coalescence sum rule are carried out for net particles, and implications for transported quark $v_1$ are discussed.
References
[1] J. Adams et al. (STAR Collaboration), Nucl. Phys. A757, 102 (2005).
[2] L. Adamczyk et al. (STAR Collaboration), arXiv:1708.07132.
| Content type | Experiment |
|---|---|
| Collaboration | STAR |
| Centralised submission by Collaboration | Presenter name already specified |
