Two Photon Distribution Amplitudes
  The factorization of the amplitude of the process gamma* gamma -> gamma gamma in the low energy and high photon virtuality region is demonstrated at the Born order and in the leading logarithmic approximation. The leading order two photon (generalized) distribution amplitudes exhibit a characteristic ln Q2 behaviour and obey new inhomogeneous evolution equations. 
M. El Beiyad, B. Pire, L. Szymanowski, S. Wallon