γ * → η γ and γ * → η ′ γ form factors to NNLO accuracy in perturbative QCD

We use conformal symmetry to calculate the next to next to leading order (NNLO) anomalous dimension matrix (three loops) for flavor-singlet axial-vector QCD operators for spin N ≤ 8 from a set of gauge invariant two-point correlation functions. Combining this result with the recent calculation of the two-loop coefficient functions, we carry out the calculation of the γγ* → η and γγ* → η' form factors at large momentum transfers to the NNLO accuracy in perturbative QCD.