Directed flow and freeze-out in relativistic heavy-ion collisions at NICA and FAIR energies

Abstract

Directed flow of charged particles is studied in nucleus-nucleus collisions simulated within the energy range accessible for NICA and FAIR facilities. Two transport cascade models, UrQMD and QGSM, are employed. These models use different mechanisms of the string excitation and string fragmentation. Despite of the differences, directed flows of charged pions and charged kaons in both models remain antiflow-oriented with reduction of the collision energy from $\sqrt{s} = 11.5$~GeV to 3.5~GeV. In contrast, the directed flow of protons changes its sign from antiflow to normal flow within the investigated energy interval. Both models favor continuous non-uniform emittence of hadrons from the expanding fireball rather than sharp, or sudden, freeze-out picture adopted by majority of hydrodynamic models. We found that the earlier frozen hadrons carry the strongest directed flow at midrapidity, although the flow development even at $|y| \leq 0.5$ takes about 8-12~fm/$c$ for different hadron species.