Is the existence of a softest point in the directed flow excitation function an unambiguous signal for the creation of a quark–gluon plasma?

Abstract

We investigate the claim that a minimum of the excitation function of the in-plane directed flow of nucleons in ultrarelativistic heavy-ion collisions is a ‘smoking gun’ signature for the creation of a quark–gluon plasma (QGP). Employing a non-equilibrium transport approach we demonstrate that such a minimum is expected in heavy ion collisions and not related to the formation of a QGP. The minimum has its origin in the interplay between a decreasing scattering angle of the elastic collisions with increasing beam energy, which lowers the in-plane flow, and the onset of particle production which increases the in-plane flow. Thus, the interpretation of this minimum as a ‘smoking gun’ signature for the creation of a QGP seems premature.