Photon-photon collision: ambiguity and duality in QCD factorization theorem

Abstract

We discuss duality in ``two-photon''-like processes in the scalar ${\phi }_E^3$ model and also in the process ${\gamma }^{\ast }\gamma \to \pi \pi$ in QCD. Duality implies the equivalence between two distinct nonperturbative mechanisms. These two mechanisms, one involving a twist-3 Generalized Distribution Amplitude, the other employing a leading-twist Transition Distribution Amplitude, are associated with different regimes of factorization. In the kinematical region, where the two mechanisms overlap, duality is observed for the scalar ${\phi }_E^3$ model, while in the QCD case the appearance of duality turns out to be sensitive to the particular nonperturbative model applied and can, therefore, be used as a tool for selecting the most appropriate one.

6 pages, 3 figures. Talk given at Conference PHOTON-09, DESY, Hamburg, May 11-15, 2009

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