We present our calculation of electromagnetic effects, induced by the spectator charge on Feynman-xF distributions of charged pions in peripheral Pb + Pb 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 .

中文翻译：

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重离子碰撞中的旁观者感应电磁效应和π介子发射的时空动量条件

我们展示了我们对电磁效应的计算，由观察者对 CERN SPS 能量下外围 Pb + Pb 碰撞中带电π子的 Feynman-xF 分布的电荷引起的计算，包括π 发射的实际初始时空动量条件。计算是在火条纹模型的框架内进行的，该模型用于产生 π− 和 π 介子。考虑到 π 和 π− 高速产生的不对称性，包括同位旋效应。与文献中的简化模型进行了比较。我们对 NA49 数据的 xF 和 pT 依赖于横截面 π/π 的比率进行了很好的描述。实验数据有利于短时间 (0.5 < τ < 2 fm/c) 在局部火条纹静止帧中快速产生π。在我们的计算中考虑了观众膨胀的可能性，并讨论了它对观察到的 π/π 比的电磁效应的影响。我们得出的结论是，火焰条纹模型正确地描述了 CERN SPS 能量下 π− 快速谱的中心性依赖性，也为 pion 产生提供了现实的初始条件。因此，它提供了对作为 xF 函数的 π/π 比的电磁效应的定量描述。