Chiral Perturbation Theory tests at NA48 and NA62-RK experiments at CERN

Final results from an analysis of about 400 K ± → π ± γγ rare decay candidates collected by the NA48/2 and NA62-RK experiments at CERN during low intensity runs with minimum bias trigger configurations are presented. The results include a model-independent decay rate measurement and fits to Chiral Perturbation Theory (ChPT) description. The data support the ChPT prediction for a cusp in the di-photon invariant mass spectrum at the two pion threshold