Revisiting \gamma\gamma->\pi^+\pi^- at low energies
  We complete the recalculation of the available two-loop expressions for the reaction \gamma\gamma->\pi\pi in the framework of chiral perturbation theory. Here, we present the results for charged pions. The cross section and the values of the dipole polarizabilities agree very well with the earlier calculation, provided the same set of low-energy constants (LECs) is used. With updated values for the LECs at order p^4, we find for the dipole polarizabilities (\alpha_1-\beta_1)_{\pi^\pm} = (5.7\pm 1.0) 10^{-4} fm^3, which is in conflict with the experimental result recently reported by the MAMI Collaboration. 
Juerg Gasser, Mikhail A. Ivanov and Mikko E. Sainio