The transition form factors in QCD

In an analysis by Kroll (2011) [11] the form factors for the transitions from a photon to a pseudoscalar meson P = π 0 , η , η ′ , η c within QCD are reported. The sharp rise of the π γ form factor with the photon virtuality demands strong power corrections to the results from collinear factorization in contrast to the other transition form factors. The power corrections are modeled as quark transverse momenta accompanied by gluon radiation.