The Spectator Electromagnetic Effect on Charged Pion Spectra in   Peripheral Ultrarelativistic Heavy-Ion Collisions
  We estimate the electromagnetic effect of the spectator charge on the momentum spectra of $\pi^+$ and $\pi^-$ produced in peripheral Pb+Pb collisions at SPS energies. We find that the effect is large and results in strongly varying structures in the $x_F$ dependence of the $\pi^+/\pi^-$ ratio, especially at low transverse momenta where a deep valley in the above ratio is predicted at $x_F \sim$ 0.15 -- 0.20.   It appears that the effect depends on initial conditions. Thus, it provides new information on the space and time evolution of the non-perturbative pion creation process. 
Andrzej Rybicki and Antoni Szczurek