Elliptic flow: Transition from out-of-plane to in-plane emission in Au + Au collisions

We have measured the proton elliptic flow excitation function for the Au + Au system spanning the beam energy range (2-8)A GeV. The excitation function shows a transition from negative to positive elliptic flow at a beam energy, Etr ∼ 4A GeV. Detailed comparisons with calculations from a relativistic Boltzmann equation are presented. The comparisons suggest a softening of the nuclear equation of state from a stiff form (K ∼ 380 MeV) at low beam energies (Ebeam ≤ 2A GeV) to a softer form (K ∼ 210 MeV) at higher energies (Ebeam ≥ 4A GeV) where the calculated baryon density ρ ∼ 4ρ0. © 1999 The American Physical Society.