First Observation of the DecayKL→π0e+e−γ

We report on the first observation of the decay $K_L\to {\pi }^0{e}^{+}{e}^{-}\gamma$ by the KTeV E799 experiment at Fermilab. Based upon a sample of 48 events with an estimated background of $3.6\pm 1.1$ events, we measure the $K_L\to {\pi }^0{e}^{+}{e}^{-}\gamma$ branching ratio to be $(2.34\pm 0.35\pm 0.13)\times {10}^{-8\mathrm{}}$. Our data agree with recent $O\left(p^6\right)$ calculations in chiral perturbation theory that include contributions from vector meson exchange through the parameter $a_V$. A fit was made to the $K_L\to {\pi }^0{e}^{+}{e}^{-}\gamma$ data for $a_V$ with the result $-0.67\mathrm{}\pm 0.21\pm 0.12$, which is consistent with previous results from KTeV.