New Limit on Muon and Electron Lepton Number Violation from $K^0_L \to   \mu^\pm e^\mp$ Decay
  The most sensitive experiment to date to search for the muon and electron lepton number violating decay $K^0_L \to \mu^\pm e^\mp$ has detected no events consistent with this process. Based on this result, the 90% confidence level upper limit on the branching fraction is $B(K^0_L \to \mu^\pm e^\mp) < 4.7\times10^{-12}$. 
D. Ambrose, et. al