12-18 August 2012
US/Eastern timezone

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Centrality dependence of freeze-out parameters from the Beam Energy Scan at STAR

17 Aug 2012, 15:00
Regency 2/3

Regency 2/3




The RHIC Beam Energy Scan (BES) program aims to study the QCD phase diagram. The main focus is to search for signals of the hypothesized critical point and the onset of the quark-hadron phase transition. The BES program covers a large part of the QCD phase diagram ($T$ vs.$mu_{B}$). With its uniform acceptance and excellent particle identification, STAR has collected large event samples in the $mu_{B}$ range 100-400 MeV. This makes it possible to study, in addition to the energy dependence, the centrality dependence of freeze-out parameters such as temperature, baryon chemical potential, and radial collectivity. We present the first results on the centrality dependence of freeze-out parameters in Au+Au collisions at $\sqrt{s_{NN}} =$ 7.7, 11.5 and 39 GeV from the STAR experiment. The chemical freeze-out conditions are obtained by comparing the measured particle ratios at midrapidity (involving $\pi^{\pm}$, $K^{\pm}$, $K_{s}$, $p(\bar{p})$, $\phi$, $\Lambda$, $\Xi$, and $\Omega$) to those from the statistical-thermal model calculations (THERMUS). We observe a clear centrality dependence in both $T$ and $mu_{B}$ values at the lower beam energies (11.5 and 7.7 GeV). Whereas no such dependence is observed for higher beam energies (200 - 62.4 GeV). The implications of including multi-strange hadrons and choosing different ensembles (Grand canonical versus Strangeness canonical) in THERMUS model on the $T$ and $mu_{B}$ values will be discussed. The kinetic freeze-out parameters are obtained by fitting the invariant yields of the produced particles ($\pi^{\pm}$, $K^{\pm}$, and $p(\bar{p})$), as a function of transverse momentum, to the Blast-Wave and Tsallis models. The kinetic freeze-out temperature and the radial collectivity which shows an anti-correlation at all the energies studied will be presented.

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