Novel insights into the $γγ^*\to π^0$ 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 $γγ^*\toπ^0$ 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 $\ddot{I}{€}^0$ and $\hat{I}\cdot$ TFFs can be estimated.

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