New Measurement of the Rare DecayK+→π+μ+μ−

More than $400K^{+}\to {\pi }^{+}{\mu }^{+}{\mu }^{-}$ events were observed in a rare $K^{+}$ decay experiment at the Brookhaven Alternating Gradient Synchrotron. Normalized to the $K^{+}\to {\pi }^{+}{\pi }^{+}{\pi }^{-}$ decay, the branching ratio is determined to be $[9.22\pm 0.60(\mathrm{stat})\pm 0.49(\mathrm{syst}\left)\right]\times {10}^{-8\mathrm{}}$. This branching ratio and the $\mu \mu$ mass spectrum is in very good agreement with the measurement of the $K^{+}\to {\pi }^{+}{e}^{+}{e}^{-}$ decay, but deviates significantly from the previous measurement.