Pion transition form factor in k_T factorization

TL;DR

This paper explains the BaBar data on the pion transition form factor using k_T factorization, reconciling different Q^2 regimes with a flat pion distribution amplitude and resummation techniques.

Contribution

It demonstrates that the observed data can be explained by k_T factorization with a flat pion distribution amplitude and includes NLO corrections less than 20%.

Findings

High Q^2 data explained by k_T convolution with flat distribution amplitude.

Low Q^2 data fitted by resummation of alpha_s ln^2x effects.

NLO correction to form factor is under 20%.

Abstract

It has been pointed out that the recent BaBar data on the pi gamma^* -> gamma transition form factor F_{pi gamma}(Q^2) at low (high) momentum transfer squared Q^2 indicate an asymptotic (flat) pion distribution amplitude. These seemingly contradictory observations can be reconciled in the k_T factorization theorem: the increase of the measured Q^2F_{pi gamma}(Q^2) for Q^2 > 10 GeV^2 is explained by convoluting a k_T dependent hard kernel with a flat pion distribution amplitude, k_T being a parton transverse momentum. The low Q^2 data are accommodated by including the resummation of alpha_s ln^2x, x being a parton momentum fraction, which provides a stronger suppression at the endpoints of x. The next-to-leading-order correction to the pion transition form factor is found to be less than 20% in the considered range of Q^2.