Measurement of the rate of the decay KL→e+e−γ and observation of a form factor in this decay

Abstract A large sample of Dalitz decays, K L → e + e − γ , has been observed. The branching ratio is Γ(K L →e + e − γ) Γ(K L → all )=(9.2±0.5±0.5)×10 −6 in good agreement with theoretical predictions. We observe an enhancement at high masses in the distribution of the invariant electron-positron pair mass, compared to the distribution expected from QED. This excess is interpreted as being due to virtual mesons contributing to the photon propagator.