Search for the DecayK+→π+μ+e−

We report here on a search for the lepton-family-number-violating decay $K^{+}\to {\pi }^{+}{\mu }^{+}{e}^{-}$. We found no evidence for this decay mode and we establish an upper limit on its branching ratio of 1.1×${10}^{-9\mathrm{}}$ 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 ${\pi }^0\to {\mu }^{+}{e}^{-}$ of 7.8×${10}^{-8\mathrm{}}$ (90% C.L.).