2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/178757More than 400 $K^{+}\toπ^{+}μ^+μ^-$ events were observed in a rare $K^+$ decay experiment at the AGS. Normalized to the $K^{+}\toπ^{+}π^+π^-$ decay, the branching ratio is determined to be $(9.22 \pm 0.60 (stat) \pm 0.49 (syst))\times 10^{-8}$. This branching ratio and the $μμ$ mass spectrum is in very good agreement with the measurement of the $K^{+}\toπ^{+}e^+e^-$ decay, but deviates significantly from the previous measurement.4 pages, 6 figures in 7 eps files. Paper to be submitted to Phys Rev LettHigh Energy Physics - Experimenttext