Beam-Energy Dependence of the Directed Flow of Protons, Antiprotons, and Pions in Au plus Au Collisions

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Rapidity-odd directed flow (upsilon 1) measurements for charged pions, protons, and antiprotons near midrapidity (y = 0) are reported in root(S)(NN) = 7.7, 11.5, 19.6, 27, 39, 62.4, and 200 GeVAu+Au collisions as recorded by the STAR detector at the Relativistic Heavy Ion Collider. At intermediate impact parameters, the proton and net-proton slope parameter d upsilon(1) = d upsilon(1)vertical bar (y=0) shows a minimum between 11.5 and 19.6 GeV. In addition, the net-proton d upsilon(1) = d upsilon(1)vertical bar (y=0) changes sign twice between 7.7 and 39 GeV. The proton and net-proton results qualitatively resemble predictions of a hydrodynamic model with a first-order phase transition from hadronic matter to deconfined matter, and differ from hadronic transport calculations.