Production of rho meson pair in gamma* gamma* collisions
  We have shown that the lowest order QCD amplitude, i.e. the quark exchange contribution, to the forward production of a pair of longitudinally polarized rmesons in the scattering of two virtual photons gamma (Q1) gamma (Q2) rho rho factorizes in two different ways: the part with transverse photons is described by the QCD factorization formula involving the generalized distribution amplitude of two final rho mesons, whereas the part with longitudinally polarized photons takes the QCD factorized form with the gamma rho transition distribution amplitude. Perturbative expressions forthese, in general, non-perturbative functions are obtained in terms of the rho meson distribution amplitude. 
B. Pire, M. Segond, L. Szymanowski and S. Wallon