Charged particle's $p_T$ spectra and elliptic flow in $\sqrt{s_{NN}}$=200 GeV Au+Au collisions: QGP vs. hadronic resonance gas

Abstract

We show that if the hadronic resonance gas (HRG), with viscosity to entropy ratio $\eta /s\approx$0.24, is physical at temperature $T\approx$220 MeV, charged particles $p_T$ spectra and elliptic flow in Au+Au collisions at RHIC, over a wide range of collision centrality do not distinguish between initial QGP fluid and initial hadronic resonance gas. Unambiguous identification of bulk of the matter produced in Au+Au collisions require clear demonstration that HRG is unphysical at temperature $T<$200 MeV. It calls for precise lattice simulations with realistic boundary conditions.

Revised version. Some discussions are added, also a new figure. 5 pages, 4 figures