First observation of KL -> pi+- e-+ nu e+ e-

This letter is the first report of the KL -> pi± e-+ nu e+ e- decay. Based on 19208±144 events, we determine the branching fraction, B(KL -> pi± e-+ nu e+ e-; Mee > 5 MeV/c^2, E*ee > 30 MeV) = (1.285±0.041)x10^-5, and Gamma(Ke3ee; Mee >5 MeV/c^2) / Gamma(Ke3) = [4.57±0.04(stat)±0.14(syst)]x10^-5. This ratio agrees with a theoretical prediction based on chiral perturbation theory (ChPT) calculated to O(p^4). The measured kinematical distributions agree with those predicted by just ChPT O(p^4), but show significant disagreement with ones predicted by leading order ChPT.