Spectator induced electromagnetic effects in heavy-ion collisions and space-time-momentum conditions for pion emission

Abstract

We present our calculation of electromagnetic effects, induced by the spectator charge on Feynman-xF distributions of charged pions in peripheral P b + P b collisions at CERN SPS energies, including realistic initial space-time-momentum conditions for pion emission. The calculation is performed in the framework of the fire-streak model, adopted to the production of both π − and π + mesons. Isospin effects are included to take into account the asymmetry in production of π + and π − at high rapidity. A comparison to a simplified model from the literature is made. We obtain a good description of the NA49 data on the xF -and pT -dependence of the ratio of cross sections π + /π − . The experimental data favors short times (0.5 < τ < 2 fm/c) for fast pion creation in the local fire-streak rest frame. The possibility of the expansion of the spectators is considered in our calculation, and its influence on the electromagnetic effect observed for the π + /π − ratio is discussed. We conclude that the fire-streak model, which properly describes the centrality dependence of π − rapidity spectra at CERN SPS energies, also provides realistic initial conditions for pion production. Consequently, it provides a quantitative description of the electromagnetic effect on the π + /π − ratio as a function of xF .