Exclusive production of heavy mesons in photon-photon collision: the double scattering mechanism

Two mechanisms are presented which can compete with constituent interchange in the exclusive production at wide angle of heavy mesons in photon-photon collisions. It is shown that they have the same energy behaviour as constituent interchange, and that their amplitudes are free from large logarithms. This is exploited in a calculation of the double scattering diagrams numerically. It is found that while constituent interchange dominates at 900, double scattering becomes dominant for angles less than about 400. (author)