We have observed the rare decay $K^{+}\to {\pi }^{+}{\mu }^{+}{\mu }^{-}$ and measured the branching ratio $\gamma (K^{+}\to {\pi }^{+}{\mu }^{+}{\mu }^{-})/\gamma (K^{+}\to \mathrm{all})=[5.0\pm 0.4(\mathrm{stat})\pm 0.7(\mathrm{syst})\pm 0.6(\mathrm{th}\left)\right]\times {10}^{-8\mathrm{}}$. We compare this result with predictions from chiral perturbation theory and estimates based on the decay $K^{+}\to {\pi }^{+}{e}^{+}{e}^{-}$.

• Received 29 July 1997

DOI:https://doi.org/10.1103/PhysRevLett.79.4756