Observation of the decay KL → πoγγ and of a form factor enhancement in the decay KL → e+e−γ

Abstract

The rare decay mode KL → oγγ has been observed in an experiment of the NA31 collaboration at CERN. From a signal of 21 events with a background of 1.5 ± 0.9 events, a branching ratio of (2.1 ± 0.6)10−6 is calculated for decays with an invariant γγ mass above 280 MeV. This result is compared with the values estimated from theoretical models and has implications for the CP-conserving contribution to KL → πoe+e−decay. In the same experiment, 1053 decays of the type KL → e+e−γ were observed. The branching ratio is measured to be (9.2 ± 0.5 ± 0.5)10−6 in good agreement with theoretical expectations. An enhancement is observed 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 vector mesons contributing to the photon propagator.