Generalized chiral perturbation theory and the reaction γγ→π0π

The cross section for γγ→π0π0 and the pion polarizabilities are computed, within generalized chiral perturbation theory, in the full one loop approximation, i.e. up to and including order O(p5). The result depends on the parameter αππ defining the tree level π-π scattering amplitude and on an additional low energy constant. The latter is shown to be related by an exact sum-rule to the e+e- data. The parameter αππ is related to the quark mass ratio r=ms/m via the expansion of pseudoscalar meson masses. The generalized one loop γγ→π0π0 amplitude agrees with the experimental data in the threshold region provided r=ms/m< or ∼10. Higher order corrections are estimated comparing our calculation with the dispersive approach. ((orig.))