# Quark Matter 2012

12-18 August 2012
US/Eastern timezone

## Event anisotropy v_2 in Au+Au collisions at 7.7 - 62.4 GeV with STAR

17 Aug 2012, 15:20
20m
Regency 2/3 ()

### Regency 2/3

Oral Presentation Exploring the QCD phase diagram

Shusu Shi (CCNU)

### Description

The exploration of the QCD phase diagram in the region of a possible phase transition between the Quark Gluon Plasma (QGP) and the hadron gas phase is one of the main goals of the RHIC Beam Energy Scan(BES). One of the most important observables from high-energy nuclear collisions to study the early evolution of the expanding system is the elliptic flow $v_{2}$. At the top RHIC energy, $\sqrt{s_{NN}}$ = 200 GeV, the number-of-constituent quark (NCQ) scaling of $v_{2}(p_{T})$ is interpreted as a signature of deconfinement and the formation of the QGP phase. A disappearance of the NCQ scaling is expected for a pure hadronic system at low beam energies. Hence it is a necessary signature to identify the phase transition. We present the $v_2$ measurement at midrapidity from Au+Au collisions at $\sqrt{s_{NN}}=$ 7.7, 11.5, 19.6, 27, 39 and 62.4 GeV for inclusive charged hadrons, light nuclei ($d$, $\bar{d}$) and identified hadrons ($\pi^{\pm}$, $K^{\pm}$, $K_{S}^{0}$, $p$, $\bar{p}$, $\phi$, $\Lambda$, $\bar{\Lambda}$, $\Xi^{-}$, $\bar{\Xi}^{+}$, $\Omega^{-}$, $\bar{\Omega}^{+}$) up to 4 GeV/$c$ in $p_{T}$. The beam energy and centrality dependence of charged hadron $v_2$ are presented with comparison to higher energies at RHIC and LHC. The identified hadron $v_{2}$ are used to discuss the NCQ scaling for different beam energies. Significant difference in $v_{2}(p_{T})$ is observed between particles and corresponding anti-particles for $\sqrt{s_{NN}} <$ 39 GeV. These differences are more pronounced for baryons compared to mesons and they increase with decreasing energy and increasing centrality. The $v_{2}$ difference between particles and anti-particles shows an almost linear dependency with the baryon chemical potential $\mu_{B}$. The transport model calculations of UrQMD and AMPT models are compared with the experimental data.

 Slides