Proton & Antiproton Directed Flow In Au+Au Collisions At (~SNN)～(1/2)=200 GeV

Modern physics is challenged by the puzzle of quark confinement in a strongly interacting system. High-energy heavy-ion collisions can experimentally provide the high energy density required to generate Quark-Gluon Plasma (QGP), a deconfined state of quark matter. For this purpose, the Relativistic Heavy Ion Collider (RHIC) at BrookHaven National Laboratory (BNL) in USA has been constructed and is currently taking data.Directed flow is the first coefficient (Ï…₁) of Fourier expansion of the azimuthal distribution of emitted particles with respect to the reaction plane. It describes the collective sideward motion of produced particles and nuclear fragments, and it carries information from the very early stage of the collision. The magnitude and the shape of directed flow, in particular those for identified particles, are of the special interest because they are sensitive to the equation of the state (EOS). Recent theoretical work shows that in close-to-central collisions, due to the strong expansion of a QGP and the positive space-momentum correlation, directed flow as a function of rapidity crosses zero three times around mid-rapidity, and display a wiggle structure through the hydrodynamic evolution. For the same reason, protons in central collisions are expected to follow in the direction of pions instead of spectators, which results in a collapse of proton directed flow. In particular, proton, anti-proton and charged kaon near mid-rapidity may provide valuable insight into a possible 1st order phase transition to the QGP.We have found that proton v1 is less than 1%, anti-Proton v1 is less than 2% within our acceptance. With current statistics obtained from Au+Au collisions at 200 GeV, both of them show a weak centrality dependence.