Measurements of the Decay KL->e+e-mu+mu-
  The KTeV experiment at Fermilab has isolated a total of 132 events from the rare decay KL->e+e-mu+mu-, with an estimated background of 0.8 events. The branching ratio of this mode is determined to be 2.69 +/- 0.24 (stat) +/- 0.12 (syst) x 10^-9, with a radiative cutoff of M(eemm)^2/M(K)^2 > 0.95. The first measurement using this mode of the parameter alpha from the D'Ambrosio, Isidori, and Portoles model of the KLg*g* vertex yields a result of -1.59 +/- 0.37, consistent with values obtained from other decay modes. Because of the limited statistics, no sensitivity is found to the DIP parameter beta. The magnitude of the angular distribution asymmetry between the e+e- and mu+mu- planes, indicative of a CP-violating contribution to the decay, is found to be consistent with zero. We set a 90% C.L. upper limit of 4.12 x 10^(-11) on the branching ratio of the lepton flavor--violating mode KL->e+-e+-mu+-mu+-, a factor of three improvement over the current limit from the KTeV experiment. 
The KTeV Collaboration