Multiple exchanges in lepton pair production in high--energy heavy ion   collisions
  The recent analysis of nuclear distortions in DIS off nuclei revealed a breaking of the conventional hard factorization for multijet observable. The related pQCD analysis of distortion effects for jet production in nucleus-nucleus collisions is as yet lacking. As a testing ground for such an analysis we consider the Abelian problem of higher order Coulomb distortions of the spectrum of lepton pairs produced in peripheral nuclear collisions. We report an explicit calculation of the contribution to the lepton pair production in the collision of two photons from one nucleus with two photons from the other nucleus, $2\gamma + 2\gamma \to \m{l}^+\m{l}^-$. The dependence of this amplitude on the transverse momenta has a highly nontrivial form the origin of which can be traced to the mismatch of the conservation of the Sudakov components for the momentum of leptons in the Coulomb field of the oppositely moving nuclei.   The result suggests that the familiar eikonalization of Coulomb distortions breaks down for the oppositely moving Coulomb centers, which is bad news from the point of view of extensions to the pQCD treatment of jet production in nuclear collisions. On the other hand, we notice that the amplitude for the $2\gamma + 2\gamma \to \m{l}^+\m{l}^-$ process has a logarithmic enhancement for the lepton pairs with large transverse momentum, which is absent for $n\gamma + m\gamma \to \m{l}^+\m{l}^-$ processes with $m,n > 2$.   We discuss the general structure of multiple exchanges and show how to deal with higher order terms which cannot be eikonalized. 
E. Bartos, S. R. Gevorkyan, E. A. Kuraev, N. N. Nikolaev