Charge splitting of directed flow and space-time picture of pion emission from the electromagnetic interactions with spectators

Abstract

We estimate the effect of the spectator-induced electromagnetic interaction on the directed flow of charged pions. For intermediate centrality Au+Au collisions at $\sqrt{s_{NN}}=7.7$~GeV, we demonstrate that the electromagnetic interaction between spectator charges and final state pions results in charge splitting of positive and negative pion directed flow. Such a charge splitting is visible in the experimental data reported by the STAR Collaboration. The magnitude of this charge splitting appears to strongly depend on the actual distance between the pion emission site (pion at freeze-out) and the spectator system. As such, the above electromagnetic effect brings new, independent information on the space-time evolution of pion production in heavy ion collisions. From the comparison of our present analysis to our earlier studies made for pions produced at higher rapidity, we formulate conclusions on the rapidity dependence of the distance between the pion emission site and the spectator system. This distance appears to decrease with increasing pion rapidity, reflecting the longitudinal expansion of the strongly-interacting system responsible for pion emission. Thus for the first time, information on the space-time characteristics of the system is being provided by means of the spectator-induced electromagnetic interaction. The above electromagnetic effect being in fact a straight-forward consequence of the presence of spectator charges in the collision, we consider that it should be considered as a baseline for studies of other phenomena, like those related to the electric conductivity of the quark-gluon plasma.