Electromagnetic effects and the longitudinal evolution of the system at CERN SPS energies

We review our studies of spectator-induced electromagnetic (EM) effects on charged pion emission in nucleus-nucleus collisions at CERN SPS and RHIC BES energies. These we discuss in the context of (1) new data on Ar+Sc collisions from the NA61/SHINE experiment (2) new findings on the role of energy-momentum conservation for the longitudinal evolution of the system at SPS energies, and (3) new work on the space-time evolution of spectator fragmentation. Although the average Ar spectator charge in intermediate Ar+Sc collisions is only about 8 elementary units, the corresponding EM field is large enough to impose a visible distortion on final state $\pi^{+}/\pi^{-}$ ratios, and break isospin symmetry. A Monte Carlo simulation of this process provides new information on the space-time evolution of the system in Ar+Sc collisions, as well as that of spectator fragmentation. We compare this information to that obtained for Au+Au and Pb+Pb collisions from STAR, NA49 [3], and WA98 experiments. A uniform picture emerges where the distance $d_E$ between the pion formation zone at freeze-out and the spectator system decreases with increasing pion rapidity. At central rapidity our estimates agree with pion decoupling times obtained from standard femtoscopy. As a result, a specific picture of the longitudinal evolution of the system emerges. We construct a simple model of the heavy ion collision, local in the impact parameter plane, and appropriate for the SPS energy range. With some similarity to the original "fire-streak" approach, we start from local energy and momentum conservation, and nicely describe the centrality dependence of the pion rapidity distribution and total pion yields in heavy ion collisions at $\sqrt{s_{NN}} = 17.3$ GeV. We also explain the broadening of this distribution when going from central to peripheral collisions. We discuss the resulting implications on the role of energy and momentum conservation in the early stage of the A+A reaction. Finally, we comment on the possibility of using EM effects in relativistic heavy ion collisions to test the nuclear models of spectator break-up. This includes possible new measurements in the framework of the NA61/SHINE Phase II programme recommended by the SPSC.