First observation of the decay K{sup +}{yields}e{sup +}{nu}{sub e}{mu}{sup +}{mu}{sup -}

Experiment 865 at the Brookhaven AGS has observed the decay K{sup +}{yields}e{sup +}{nu}{sub e}{mu}{sup +}{mu}{sup -}. The branching ratio extracted is (1.72{+-}0.37(stat){+-}0.17(syst){+-}0.19(model))x10{sup -8} where the third term in the error results from the use of a model to extrapolate into a kinematic region dominated by background.