Measurements of the Decay KL->e+e-mu+mu-

TL;DR

This paper reports the first measurement of the rare decay KL->e+e-mu+mu- with 132 events, determines its branching ratio, extracts the parameter alpha, and sets new limits on lepton flavor violation, providing insights into CP violation and decay dynamics.

Contribution

It provides the first measurement of the decay mode KL->e+e-mu+mu-, including branching ratio and decay parameters, improving constraints on lepton flavor violation.

Findings

Branching ratio measured as (2.69 +/- 0.24(stat) +/- 0.12(syst)) x 10^-9.

Parameter alpha determined to be -1.59 +/- 0.37, consistent with other modes.

Upper limit set on lepton flavor-violating decay at 4.12 x 10^-11 (90% C.L.).

Abstract

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.