Observation of the DecayK+→π+γγ

The first observation of the decay $K^{+}\to {\pi }^{+}\gamma \gamma$ is reported. A total of 31 events was observed with an estimated background of $5.1\pm 3.3$ events in the ${\pi }^{+}$ momentum range from 100 to 180 MeV/c. The corresponding partial branching ratio, $B(K^{+}\to {\pi }^{+}\gamma \gamma ,100\mathrm{}\mathrm{MeV}/c<\mathrm{}{P}_{{\pi }^{+}}<180\mathrm{}\mathrm{MeV}/c)$, is $[6.0\pm 1.5(\mathrm{stat})\pm 0.7(\mathrm{syst}\left)\right]\times {10}^{-7\mathrm{}}$. No $K^{+}\to {\pi }^{+}\gamma \gamma$ decay was observed in the ${\pi }^{+}$ momentum region greater than 215 MeV/c. The observed ${\pi }^{+}$ momentum spectrum is compared with the predictions of chiral perturbation theory.