Impact of the higher twist effects on the $\gamma {\gamma }^{\ast }\to {\pi }^0$ transition form factor

Abstract

We reanalyze the $\gamma {\gamma }^{\ast }\to {\pi }^0$ transition form factor $F_{\pi \gamma }(Q^2)$ within the QCD light-cone sum rules method with the twist-4 accuracy. In computations the pion leading twist distribution amplitude (DA) with two nonasymptotic terms, and the renormalon method-inspired twist-4 DAs are used. The latters allow us to estimate impact of effects due to higher conformal spin components in the pion twist-4 DAs on the form factor $F_{\pi \gamma }(Q^2)$. Obtained theoretical predictions are employed to deduce constraints on the pion DAs from CELLO and CLEO data.

Comments: 15 pages, 7 figures. Numerical errors in ${\Phi }_4^{(4)}(u)$ (Eq.(3.9)), and in ${\varphi }_4^1(u)$, \rho_4^4(t,s), H_4^4(t)$ (Appendix) corrected. The figures and Section IV revised. Conclusions changed