[Paper Review] Azimuthal Anisotropy of Identified Hadrons in 200 GeV Au+Au Collisions

M. Oldenburg|arXiv (Cornell University)|Oct 7, 2005

High-Energy Particle Collisions Research4 citations

TL;DR

This paper presents high-statistics measurements of azimuthal anisotropy ($v_2$) for identified hadrons in 200 GeV Au+Au collisions at RHIC, revealing strong $v_2$ at high $p_T$ for pions and protons, evidence of number-of-constituent-quark scaling in the intermediate $p_T$ region, and significant $v_2$ for multi-strange hadrons ($\phi$, $\Xi$, $\Omega$), all indicating early-stage partonic collectivity and supporting quark coalescence as the dominant hadronization mechanism.

ABSTRACT

The azimuthal anisotropy parameter v2 has been measured with high statistics for identified hadrons in sqrt(s_NN) = 200 GeV Au+Au collisions with the STAR experiment. At high transverse momentum (pT) a strong v2 for pi+ + pi- and p + pbar is observed. In the intermediate pT region, number-of-constituent-quark scaling was tested to high precision. A detailed comparison of v2 for the multi-strange hadrons phi, Xi + anti-Xi, and Omega + anti-Omega with other particle species substantiates the development of collectivity among partons in the early phase of the collisions at RHIC.

Motivation & Objective

• To measure the azimuthal anisotropy ($v_2$) of identified hadrons in 200 GeV minimum bias Au+Au collisions with high statistical precision.
• To test number-of-constituent-quark (NCQ) scaling in the intermediate $p_T$ region to probe the hadronization mechanism.
• To investigate the $v_2$ of multi-strange hadrons ($\phi$, $\Xi$, $\Omega$) as a probe of early-stage partonic collectivity.
• To determine whether $v_2$ scaling is a meson-baryon effect or mass-dependent, using comparisons to $K^0_S$ and $\Lambda$.
• To assess the role of parton energy loss and jet quenching at high $p_T$ by measuring residual $v_2$.

Proposed method

• Measured $v_2$ for identified hadrons ($\pi^\pm$, $K^0_S$, $\Lambda$, $p$, $\phi$, $\Xi$, $\Omega$) using the STAR experiment at RHIC.
• Used event plane reconstruction in opposite pseudorapidity ($\eta$) subevents to suppress non-flow effects and reduce systematic uncertainties.
• Applied $v_2$ scaling via $v_2(p_T/n)/n$ to test number-of-constituent-quark scaling, where $n$ is the number of valence quarks.
• Performed a common fit to $K^0_S$ and $\Lambda + \bar{\Lambda}$ $v_2(p_T)$ data to compare with multi-strange hadrons.
• Calculated $\chi^2$/ndf to quantitatively assess agreement between measured $v_2$ and the NCQ-scaling fit function.
• Used systematic error estimates from method comparisons (full-acceptance vs. subevent plane) and proton-kaon contamination.

Experimental results

Research questions

• RQ1 Does $v_2$ for identified hadrons at high $p_T$ ($>6$ GeV/$c$) remain significant, challenging parton energy loss models?
• RQ2 To what extent does $v_2$ follow number-of-constituent-quark scaling in the intermediate $p_T$ region ($p_T/n < 2$ GeV/$c$)?
• RQ3 Do multi-strange hadrons ($\phi$, $\Xi$, $\Omega$) exhibit $v_2$ comparable to other mesons and baryons, indicating early partonic collectivity?
• RQ4 Is the observed $v_2$ scaling a meson-baryon effect or a mass-dependent effect, given the similar mass of $\phi$ and proton?
• RQ5 What is the significance of $v_2$ deviations from ideal NCQ scaling, and do they reflect non-ideal recombination or medium effects?

Key findings

• A strong $v_2$ signal persists for $\pi^+ + \pi^-$ and $p + \bar{p}$ up to $p_T = 9$ GeV/$c$, contradicting predictions from parton energy loss models.
• The $v_2(p_T/n)/n$ values for mesons and baryons collapse onto a common curve at $p_T/n < 2$ GeV/$c$, supporting quark coalescence as the dominant hadronization mechanism.
• Small deviations from ideal NCQ scaling are observed, detectable with high statistics, consistent with expectations from recombination models.
• The $\chi^2$/ndf comparison shows $\Xi^{-} + \bar{\Xi}^{+}$ and $\Omega^{-} + \bar{\Omega}^{+}$ have $v_2$ consistent with baryonic scaling (27.4/6 and 2.1/3, respectively), while $\phi$ favors mesonic scaling (2.0/5).
• The $\phi$ meson, despite having a mass similar to the proton, flows like a meson and not like a baryon, proving that $v_2$ scaling is a meson-baryon effect, not a mass effect.
• The strong $v_2$ of multi-strange hadrons confirms that collectivity develops at the partonic stage, providing strong evidence for the formation of a quark-gluon plasma at RHIC.