We investigate the effects of nuclear mean-field as well as the formation and decay of nuclear clusters on the directed flow $v_1$ in high energy nucleus-nucleus collisions from $sqrt{s_{NN}}=7.7$ GeV to 27 GeV incident energies within a transport model. Specifically, we use the JAM transport model in which potentials are implemented based on the framework of the relativistic quantum molecular dynamics. Our approach reproduces the rapidity dependence of directed flow data up to $sqrt{s_{NN}}approx 8$ GeV showing the significant importance of mean-field. However, the slopes of $dv_1/dy$ at mid-rapidity are calculated to be positive at $sqrt{s_{NN}}=11.7$ and 19.6 GeV, and becomes negative above 27 GeV. Thus the result from the JAM hadronic transport model with nuclear mean-field approach is incompatible with the data. Therefore within our approach, we conclude that the excitation function of the directed flow cannot be explained by the hadronic degree of freedom alone.
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