Beam energy dependence of the expansion dynamics in relativistic heavy ion collisions: Indications for the critical end point?

Abstract

The flow harmonic $v_n$ and the emission source radii $R_{\text{out}}$, $R_{\text{side}}$ and $R_{\text{long}}$ are studied for a broad range of centrality selections and beam collision energies in Au+Au ($\sqrt{s_{NN}}=7.7-200$ GeV) and Pb+Pb ($\sqrt{s_{NN}}=2.76$ TeV) collisions at RHIC and the LHC respectively. They validate the acoustic scaling patterns expected for hydrodynamic-like expansion over the entire range of beam energies studied. The combined data sets allow estimates for the \sqsn\ dependence of the mean expansion speed $⟨c_s⟩$, emission duration $⟨\Delta \tau ⟩$ and the viscous coefficients $⟨{\beta }^{\prime \prime }⟩$ that encode the magnitude of the specific shear viscosity $⟨\eta /s⟩$. The estimates indicate initial-state model independent values of $⟨\eta /s⟩$ which are larger for the plasma produced at 2.76 TeV (LHC) compared to that produced at 200 GeV (RHIC) (${⟨4\pi \eta /s⟩}_{\text{LHC}}=2.2\pm 0.2$ and ${⟨4\pi \eta /s⟩}_{\text{RHIC}}=1.3\pm 0.2$). They also show a non-monotonic \sqsn\ dependence for $⟨{\beta }^{\prime \prime }⟩$, $⟨c_s⟩$ and $⟨\Delta \tau ⟩$, with minima for $⟨{\beta }^{\prime \prime }⟩$ and $⟨c_s⟩$, and a complimentary maximum for $⟨\Delta \tau ⟩$. These dependencies signal a significant change in reaction dynamics in a narrow span of $\sqrt{s_{NN}}$, which may be linked to reaction trajectories close to the critical end point (CEP) in the phase diagram for nuclear matter.

4 pages, 4 figures, proceedings for International Conference on Quark Matter (QM2014)

References in corpus (6)

• The order of the quantum chromodynamics transition predicted by the standard model of particle physics
• On the Strongly-Interacting Low-Viscosity Matter Created in Relativistic Nuclear Collisions
• Has the QCD Critical Point been Signaled by Observations at RHIC ?
• Inclusive charged hadron elliptic flow in Au + Au collisions at $\sqrt{s_{NN}}$ = 7.7 - 39 GeV
• Hadron spectra and elliptic flow for 200 A GeV Au+Au collisions from viscous hydrodynamics coupled to a Boltzmann cascade
• An estimate for the location of QCD critical end point

Cited by in corpus (2)

• Observation of the critical end point in the phase diagram for hot and dense nuclear matter
• Energy dependent chemical potentials of light hadrons and quarks based on transverse momentum spectra and yield ratios of negative to positive particles