Novel insights into the $\gamma {\gamma }^{\ast }\to {\pi }^0$ transition form factor

Abstract

BaBar's observation of significant deviations of the pion transition form factor (TFF) from the asymptotic expectation with $Q^2>9$ GeV${}^2$ has brought a serious crisis to a fundamental picture established for such a simplest $q\bar{q}$ system by perturbative QCD, i.e. the dominance of collinear factorization at high momentum transfers for the pion TFF. We show that non-factorizable contributions due to open flavors in $\gamma {\gamma }^{\ast }\to {\pi }^0$ could be an important source that contaminates the pQCD asymptotic limit and causes such deviations with $Q^2>9$ GeV${}^2$. Within an effective Lagrangian approach, the non-factorizable amplitudes can be related to intermediate hadron loops, i.e. $K^{(\ast )}$ and $D^{(\ast )}$ etc, and their corrections to the ${\pi }^0$ and $\eta$ TFFs can be estimated.

Comments: Revtex, 6 pages and 4 eps figures; Extended version accepted by Eur. Phys. J. C