Search for the Decay K+--> Pi + Micro+e-

We report here on a search for the lepton-family-number-violating decay ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\mu}}^{+}{e}^{\ensuremath{-}}$. We found no evidence for this decay mode and we establish an upper limit on its branching ratio of 1.1\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}9}$ at the 90% confidence level (C.L.) for a uniform phase-space decay distribution. We also use this result to establish an upper limit for the branching ratio of the decay ${\ensuremath{\pi}}^{0}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{e}^{\ensuremath{-}}$ of 7.8\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}8}$ (90% C.L.).