ELECTROMAGNETIC EFFECTS IN Pb plus Pb COLLISIONS AT THE SPS; FROM NUCLEAR PHYSICS OF THE SPECTATOR SYSTEM TO THE SPACE-TIME EVOLUTION OF THE QGP

We review our studies of spectator-induced electromagnetic (EM) effects on charged pion emission in heavy-ion collisions at CERN SPS and RHIC BES energies. These effects are found to consist in the electromagnetic charge splitting of pion directed flow as well as very large distortions in spectra and ratios of produced charged particles. As it emerges from our analysis, they offer sensitivity to the actual distance d(E) between the pion formation zone at freeze-out and the spectator matter. As a result, this gives a new possibility of studying the space-time evolution of the dense and hot matter created in the course of the collision. A specific picture emerges, where the centrality dependence of pion rapidity spectra at CERN SPS energies may be interpreted as a pure consequence of local energy-momentum conservation. Furthermore, as follows from our analysis, EM effects also show sensitivity to the space-time scale of spectator breakup. We comment on a recent study of the dynamical evolution of the spectator system, which gives very different predictions for spectator excitation energy depending on the dynamical scenario assumed. In this context, we argue that EM effects in ultrarelativistic heavy-ion collisions can provide a unique, independent experimental input to test nuclear models in extreme conditions.