Observation of the Decay K⁺ → π⁺γγ

The first observation of the decay K⁺→π⁺γγ is reported. A total of 31 events was observed with an estimated background of 5.1±3.3 events in the π⁺ momentum range from 100 to 180 MeV/c. The corresponding partial branching ratio, B(K⁺→π⁺γγ,100 MeV/c<P_π⁺<180 MeV/c), is [6.0±1.5(stat)±0.7(syst)]×10^-7. No K⁺→π⁺γγ decay was observed in the π⁺ momentum region greater than 215 MeV/c. The observed π⁺ momentum spectrum is compared with the predictions of chiral perturbation theory.