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

The flow harmonics vn and the emission source radii R_out, R_side and R_long are studied for a broad range of centrality selections and beam collision energies in Au+Au (√sNN=7.7-200GeV) and Pb+Pb (√sNN=2.76TeV) 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 √sNN dependence of the mean expansion speed c_s, emission duration δτ and the viscous coefficients β^" that encode the magnitude of the specific shear viscosity η/s. The estimates indicate initial-state model independent values of η/s which are larger for the plasma produced at 2.76 TeV (LHC) compared to that produced at 200 GeV (RHIC) (4πη/s_LHC=2.2±0.2 and 4πη/s_RHIC=1.3±0.2). They also show a non-monotonic √sNN dependence for β^", c_s and δτ, with minima for β^" and c_s, and a complimentary maximum for δτ. These dependencies signal a significant change in reaction dynamics in a narrow span of √sNN, which may be linked to reaction trajectories close to the critical end point (CEP) in the phase diagram for nuclear matter.