Examination of directed flow as a signature of the softest point of the equation of state in QCD matter

We analyze the directed flow of protons and pions in high-energy heavy-ion collisions in the incident energy range from $\sqrt{{s}_{NN}}=7.7$ to 27 GeV within a microscopic transport model. Standard hadronic transport approaches do not describe the collapse of directed flow below $\sqrt{{s}_{NN}}\ensuremath{\simeq}20$ GeV. By contrast, a model that simulates effects of a softening of the equation of state descibes well the behavior of directed flow data recently obtained by the STAR Collaboration [Phys. Rev. Lett. 112, 162301 (2014)]. We give a detailed analysis of how directed flow is generated. Particularly, we found that softening of the effective equation of state at the overlapping region of two nuclei, i.e., the reaction stages where the system reaches high baryon density state, is needed to explain the observed collapse of proton directed flow within a hadronic transport approach.