Investigating Directed Flow In High-energy Heavy-ion Collisions

Quantum Chromodynamics(QCD)theory predicted that the confined hadronic matter undergoes a phase transition to a deconfined state of matter named the Quark-Gluon Plasma(QGP)in the environment of ultra-high temperature and high density.Heavy-ion collisions can create high-temperature and high-density physical environment,so we can study the phase diagram of the nuclear matter,properties of extreme hot and dense QGP matter,and QCD phase transition through the heavy-ion collision.In non-central collisions,the isotropic collective motion of particles forms collective flows.It is a powerful probe exposing the characteristics of the QGP matter.The early hydrodynamic studies have shown that the directed flow,especially that for baryons,is sensitive to the equation of state(EoS)of the produced matter,and may provide information about the critical point in the phase diagram.If the hadron-quark phase transition exists,the slope of the directed flow for charged particles near mid-rapidity region changes from positive to negative,presumedly caused by the collapse of the EoS.Recently,measurements of both proton and net-proton directed flows from the RHIC-STAR Collaboration have shown nonmonotonic behavior of the directed flow as a function of beam energy.In this paper we investigate the directed flow in relativistic heavy-ion collisions within a multi-phase transport(AMPT)model.We mainly focus on the formation and evolution mechanism of the directed flow in the partonic phase,during the hadronization process,and in the hadronic phase,based on the phase space information generated by the AMPT model.As the partonic phase evolves with time,the slope of the parton directed flow at midrapidity region changes from negative to positive as a result of the later dynamics at 200 GeV,while it remains negative at 39 GeV due to the shorter life time of the partonic phase.The directed flow splitting for various quark species due to their different initial eccentricities is observed at 39 GeV,while the splitting is very small at 200 GeV.From a dynamical coalescence algorithm with Wigner functions,we found that the directed flow of hadrons is a result of competition between the coalescence in momentum and coordinate space as well as further modifications by the hadronic rescatterings...