Mean-field update in the JAM microscopic transport model: Mean-field effects on collective flow in high-energy heavy-ion collisions atsNN=2–20 GeV energies

Abstract

The beam energy dependence of the directed flow is a sensitive probe for the properties of strongly interacting matter. Hybrid models that simulate dense (core) and dilute (corona) parts of the system by combining the fluid dynamics and hadronic cascade model describe well the bulk observables, strangeness productions, and radial and elliptic flows in high-energy heavy-ion collisions in the high baryon density region. However, the beam energy dependence of the directed flow cannot be described in existing hybrid models. We focus on improving the corona part, i.e., the nonequilibrium evolution part of the system, by introducing the mean-field potentials into a hadronic cascade model. For this purpose, we consider different implementations of momentum-dependent hadronic mean fields in the relativistic quantum molecular dynamics (RQMD) framework. First, Lorentz scalar implementation of a Skyrme type potential is examined. Then, full implementation of the Skyrme type potential as a Lorentz vector in the RQMD approach is proposed. We find that scalar implementation of the Skyrme force is too weak to generate repulsion explaining observed data of sideward flows at $\sqrt{{s}_{NN}}<10$ GeV, while vector implementation gives collective flows compatible with the data for a wide range of beam energies $2.7<\sqrt{{s}_{NN}}<20$ GeV. We show that our approach reproduces the negative proton directed flow at $\sqrt{{s}_{NN}}>10$ GeV discovered by experiments. We discuss the dynamical generation mechanisms of the directed flow within a conventional hadronic mean field. A positive slope of proton directed flow is generated predominantly during compression stages of heavy-ion collisions by the strong repulsive interaction due to high baryon densities. In contrast, at the expansion stages of the collision, the negative directed flow is generated more strongly than the positive one by the tilted expansion and shadowing by the spectator matter. At lower collision energies $\sqrt{{s}_{NN}}<10$ GeV, the positive flow wins against the negative flow because of a long compression time. On the other hand, at higher energies $\sqrt{{s}_{NN}}>10$ GeV, negative flow wins because of shorter compression time and longer expansion time. A transition beam energy from positive to negative flow is highly sensitive to the strength of the interaction.