The enhancement of $v_4$ in nuclear collisions at the highest densities signals a first order phase transition

Abstract

The beam energy dependence of $v_4$ (the quadrupole moment of the transverse radial flow) is sensitive to the nuclear equation of state (EoS) in mid-central Au + Au collisions at the energy range of $3 < \sqrt{s_{NN}} < 30$ GeV, which is investigated within the hadronic transport model JAM. Different equations of state, namely, a free hadron gas, a first-order phase transition and a crossover are compared. An enhancement of $v_4$ at $\sqrt{s_{{NN}}}\approx 6$ GeV is predicted for an EoS with a first-order phase transition. This enhanced $v_4$ flow is driven by both the enhancement of $v_2$ as well as the positive contribution to $v_4$ from the squeeze-out of spectator particles which turn into participants due to the admixture of the strong collective flow in the shocked, compressed nuclear matter.