Centrality dependence of charged hadron and strange hadron elliptic flow from sqrt(s_NN) = 200 GeV Au+Au collisions

TL;DR

<p>We investigate the centrality dependence of elliptic flow $v_2$ for charged, strange, and multi-strange hadrons in $ ext{Au+Au}$ collisions at $ oot o 200$ GeV. Using multiple flow-analysis methods (Event Plane, $ta$-subevent, four-particle cumulants, and Lee-Yang Zero), we quantify non-flow effects and extract $v_2$ across a broad $p_T$ range. We observe $m_T-m$ scaling for identified hadrons at low $p_T$ and number-of-quark scaling at intermediate $p_T$, with Omega consistent with both behaviors, indicating early partonic collectivity and quark-coalescence formation. Integrated $v_2$ scaled by the participant eccentricity grows with centrality, signaling stronger collective flow in more central collisions, while no universal centrality scaling emerges across all hadron species. Comparisons to ideal hydrodynamics reveal partial agreement at low $p_T$ but overprediction at higher $p_T$, underscoring a nuanced picture of thermalization and the role of partonic dynamics.</p>

Abstract

We present STAR results on the elliptic flow v_2 of charged hadrons, strange and multi-strange particles from sqrt(s_NN) = 200 GeV Au+Au collisions at RHIC. The detailed study of the centrality dependence of v_2 over a broad transverse momentum range is presented. Comparison of different analysis methods are made in order to estimate systematic uncertainties. In order to discuss the non-flow effect, we have performed the first analysis of v_2 with the Lee-Yang Zero method for K_s^0 and Lambda. In the relatively low p_T region, p_T <= 2 GeV/c, a scaling with m_T - m is observed for identified hadrons in each centrality bin studied. However, we do not observe v_2(p_T) scaled by the participant eccentricity to be independent of centrality. At higher p_T, 2 GeV/c <= p_T <= 6 GeV/c, v_2 scales with quark number for all hadrons studied. For the multi-strange hadron Omega, which does not suffer appreciable hadronic interactions, the values of v_2 are consistent with both m_T -m scaling at low p_T and number-of-quark scaling at intermediate p_T. As a function of collision centrality, an increase of p_T-integrated v_2 scaled by the participant eccentricity has been observed, indicating a stronger collective flow in more central Au+Au collisions.

Figures (15)

• Figure 1: (Color online) Plots (a1)--(d1) represent the invariant mass distributions for $\mathrm{K}^{0}_{S}$ (1.4 $\le p_T \le$ 1.6 $\mathrm{GeV}/c$), $\Lambda$ (1.4 $\le p_T \le$ 1.6 $\mathrm{GeV}/c$), $\Xi$ (2.3 $\le p_T \le$ 2.6 $\mathrm{GeV}/c$), and $\Omega$ (2.5 $\le p_T \le$ 3.0 $\mathrm{GeV}/c$), respectively, from $\sqrt{\mathrm{s}_{_{\mathrm{NN}}}}$ = 200 GeV minimum bias (0--80%) $Au + Au$ collisions. The dashed lines are the background distributions. The corresponding data for the $v_2$ distributions are shown in plots (a2)--(d2) as open circles. The thick-dashed, thin-dashed and the dot-dashed lines represent the relative contributions of $v_2(Sig)$, $v_2(Bg)$, and $v_2(Sig+Bg)$, respectively. For clarity, the invariant mass plots for $\mathrm{K}^{0}_{S}$ , $\Lambda$ , $\Xi$, and $\Omega$, are scaled by 1/50000, 1/170000, 1/2.5, and 1/3, respectively. The error bars are shown only for the statistical uncertainties.
• Figure 2: (Color online) Examples of the modulus of the second harmonic Lee-Yang Zero Generating Functions plotted as a function of the imaginary axis coordinate, $r$. The Sum Generating Function is shown in (a) and the Product Generating Function in (b). The vertical arrows indicate the positions of the first minimum, called $r_0$. Note that in (b) the horizontal scale does not go out as far because the calculations were terminated. All data are from $\sqrt{\mathrm{s}_{_{\mathrm{NN}}}}$ = 200 GeV $Au + Au$ collisions.
• Figure 3: (Color online) $v_2$ for charged hadrons from the Lee-Yang Zero Product Generating Function (solid circles) and from the Event-Plane method (open circles), as a function of pseudorapidity. Both sets of data have been averaged over $p_T$ from 0.15 to 2.0 $\mathrm{GeV}/c$ and centrality from 10 to 40% of $\sqrt{\mathrm{s}_{_{\mathrm{NN}}}}$ = 200 GeV $Au + Au$ collisions. For comparison, the PHOBOS data (10--40%) phobos (crosses) are also shown. The error bars are shown only for the statistical uncertainties.
• Figure 4: (Color online) (a) $v_2$ as a function of $p_T$ for charged hadrons with $\mid \eta \mid \hbox{$\ <\ $} 1.0$ in 10--40% $Au + Au$ collisions, at $\sqrt{\mathrm{s}_{_{\mathrm{NN}}}}$ = 200 GeV, from the Event-Plane method (open circles), 4-particle cumulant method (solid squares), and Lee-Yang Zero method (solid circles) with Sum Generating Function. (b) The ratios to the polynomial fit to $v_2${EP} are shown for $v_2${4}/$v_2${EP} and $v_2${LYZ}/$v_2${EP} as a function of transverse momentum. The error bars are shown only for the statistical uncertainties.
• Figure 5: (Color online) (a) $p_T$-integrated charged hadron $v_2$ in the TPC as a function of geometrical cross section. Shown are the Event-Plane method ($v_2${EP}) (open circles), Lee-Yang Zero method with Sum Generating Function (solid circles), Lee-Yang Zero method with Product Generating Function (open stars), and 4-particle cumulant method ($v_2\{4\}$) (solid squares). For the TPC, $\mid\eta\mid < 1.0$ was used, except for Lee-Yang Zero method with Product Generating Function where the $\eta$ limit went to 1.3. (b) $v_2$ divided by $v_2${EP}. All data are from $\sqrt{\mathrm{s}_{_{\mathrm{NN}}}}$ = 200 GeV $Au + Au$ collisions. The error bars are shown only for the statistical uncertainties.