First Observation of the Decay K_L → π⁰e⁺e^-γ

We report on the first observation of the decay KL→π0e⁺e^-γ by the KTeV E799 experiment at Fermilab. Based upon a sample of 48 events with an estimated background of 3.6±1.1 events, we measure the KL→π0e⁺e^-γ branching ratio to be (2.34±0.35±0.13)×10⁻⁸. Our data agree with recent O(p⁶) calculations in chiral perturbation theory that include contributions from vector meson exchange through the parameter aV. A fit was made to the KL→π0e⁺e^-γ data for aV with the result −0.67±0.21±0.12, which is consistent with previous results from KTeV.