Observation of the Decay {ital K}{sup +} {r_arrow} {pi}{sup +}{mu}{sup +}{mu}{sup {minus}}

We have observed the rare decay K{sup +} {r_arrow} {pi}{sup +}{mu}{sup +}{mu}{sup {minus}} and measured the branching ratio {Gamma}(K{sup +} {r_arrow} {pi}{sup +}{mu}{sup +}{mu}{sup {minus}}) /{Gamma}(K{sup +} {r_arrow} all)=[5.0{plus_minus}0.4(stat){plus_minus} 0.7(syst){plus_minus}0.6(th)]{times}10{sup {minus}8}. We compare this result with predictions from chiral perturbation theory and estimates based on the decay K{sup +} {r_arrow} {pi}{sup +}e{sup +}e{sup {minus}}. {copyright} {ital 1997} {ital The American Physical Society}