a Study of the Rare Decays Long-Lived Kaon Going to Neutral Pion, Positron-Electron and Long-Lived Kaon Going to Positron-Electron Photon

An experimental study of the rare kaon decays K_{L} to pi^0 e^+ e^- and K_{L } to e^+ e^-gamma was conducted by the Brookhaven E845 collaboration. Long -lived neutral kaons were produced by a 24 GeV/c proton beam incident on a copper target. Measurements on the decay products were performed in a detector composed of a magnetic spectrometer to determine charged particle momenta, a hydrogen gas threshold Cerenkov counter to identify electrons, and a lead glass calorimeter to measure energies and positions of electrons and photons. The Standard Model predicts a branching ratio in the range 10^{-11} - 10^{-12} for the strangeness changing neutral current decay K _{L}topi^0e^+e ^-. Observation of the decay above this level could suggest a nonstandard model for the violation of CP invariance. E845 found no events consistent with this mode. The limit set by the experiment was B(K _{L}topi^0e^+e ^-) < 5.5 times 10^{-9} at the 90% confidence level, assuming a uniform Dalitz plot population for the decay. In addition, a sample of 919 K _{L}to e^+e^-gamma events was isolated and studied to determine both the branching ratio and form factor. The analysis of the charged pair mass spectrum indicated the presence of a non-trivial form factor which includes contributions from both pseudoscalar-pseudoscalar and vector-vector transitions. The measured branching ratio was B(K_{L }to e^+e^-gamma) = (9.1 +/- 0.4 _sp {-0.5}{+0.6}) times 10^{-6}.