Pion transition form factor in kT factorization

It has been pointed out that the recent BaBar data on the πγ* → γ transition form factor Fπγ(Q2) at low (high) momentum transfer squared Q2 indicate an asymptotic (flat) pion distribution amplitude. These seemingly contradictory observations can be reconciled in the kT factorization theorem: the increase of the measured Q2FFπγ(Q2) for Q2 > 10 GeV2 is explained by convoluting a kT dependent hard kernel with a flat pion distribution amplitude, kT being a parton transverse momentum. The low Q2 data are accommodated by including the resummation of αs ln2x, 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 Q2. (orig.)