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Excitation Function of the Longitudinal Expansion in Central Nuclear Collisions

Longitudinal hadron spectra from Proton-Proton (pp) and nucleus-nucleus (AA) collisions from $E_{lab}= 2$ AGeV to $\sqrt s=200$ AGeV are investigated. The widths of the rapidity spectra for various particle species increases monotonously with energy. The present calculation indicates no sign of a step like behaviour as excepted from the Kaon transverse mass systematics. For Pions, the transport simulation is consistent with a Landau type scaling of the rapidity widths, both in central AA reactions and in pp collisions. However, other hadron species do not follow the Landau scaling. The present model predicts a decreasing rapidity width with particle mass for newly produced particles, not supporting a Landau type flow interpretation.