Imaginary part of the zero angle light-by-light scattering tensor
  We calculate the bilinear combination of Dirac tensors describing the creation of a pair of charged particle (pi-meson or leptons) by virtual photons integrated on the final particle phase volume. It can be interpreted as an s channel discontinuity of the zero angle light-by-light scattering tensor with both photons off mass shell. The expression of light-by-light scattering tensor is represented in the form where the gauge invariance and the Bose symmetry are explicitly shown. Some applications and the checks are discussed. 
A. I. Ahmadov, S. Bakmaev, E. A. Kuraev
hep-ph