2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/178700We have observed the rare decay K+ --> pi+ mu+ mu- and measured the branching ratio Gamma(K+ --> pi+ mu+ mu-)/Gamma(K+ --> all) = (5.0 +/- 0.4 (stat.) +/- 0.7 (sys.) +/- 0.6 (theor.)) x 10^{-8}. We compare this result with predictions from chiral perturbation theory and estimates based on the decay K+ --> pi+ e+ e-.9 pages, LaTeX, 2 Postscript figures. Version to be published in PRLHigh Energy Physics - Experimenttext