An Improved upper limit on the decay K^+ -> pi^+ mu^+ e^-
  Based on results of a search for the lepton-family-number-violating decay $K^+ \to \pi^+\mu^+ e^-$ with data collected by experiment E865 at the Alternating Gradient Synchrotron of Brookhaven National Laboratory, we place an upper limit on the branching ratio at $2.1 \times 10^{-11}$ (90% C.L.). Combining the results with earlier E865 data and those of a previous experiment, E777, an upper limit on the branching ratio of $1.3 \times 10^{-11}$ (90% C.L.) is obtained. 
Aleksey Sher, R. Appel, G.S. Atoyan, B. Bassalleck, D.R. Bergman, N.   Cheung, S. Dhawan, H. Do, J. Egger, S. Eilerts, W. Ha...