Subleading power corrections to the pion-photon transition form factor in QCD

Abstract

We reconsider QCD factorization for the leading power contribution to the $\gamma^{\ast} \gamma \to \pi^0$ form factor $F_{\gamma^{\ast} \gamma \to \pi^0} (Q^2)$ at one loop using the evanescent operator approach, and demonstrate the equivalence of the resulting factorization formulae derived with distinct prescriptions of $\gamma_5$ in dimensional regularization. Applying the light-cone QCD sum rules (LCSRs) with photon distribution amplitudes (DAs) we further compute the subleading power contribution to the pion-photon form factor induced by the "hadronic" component of the real photon at the next-to-leading-order in ${\cal O}(\alpha_s)$, with both naive dimensional regularization and 't Hooft-Veltman schemes of $\gamma_5$. Confronting our theoretical predictions of $F_{\gamma^{\ast} \gamma \to \pi^0} (Q^2)$ with the experimental measurements from the BaBar and the Belle Collaborations implies that a reasonable agreement can be achieved without introducing an "exotic" end-point behaviour for the twist-2 pion DA.