Electromagnetic effects and the longitudinal evolution of the system at CERN SPS energies
Proceedings of Corfu Summer Institute 2018 "School and Workshops on Elementary Particle Physics and Gravity" — PoS(CORFU2018)
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
... 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 π + /π − 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 , 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 √ s NN = 17.3 GeV  . We also explain the broadening of this distri-* Speaker.