First Observation of K L →π ± e ± νe + e -

This Letter is the first report of the K L → π ± e ∓ ve + e - decay. Based on 19208 ± 144 events, we determine the branching fraction, B(K L → π ± e ∓ ve + e - ; M e + e - > 5MeV/c 2 , E * e + e - > 30 MeV) = (1.285 ± 0.041) X 10 -5 , and Γ(K e3ee ; M e + e - > 5 MeV/c 2 )/Γ(K e3 ) = [4.57 ± 0.04(stat) ± 0.14(syst)] X 10 -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.