New Results on Rare and Forbidden Semileptonic $K^+$ Decays

Abstract

Experiment E865 at the Brookhaven AGS was set up primarily to search for the lepton flavour violating decay $K^+\to π^+μ^+e^-$ ($K_{πμe}$) with high sensitivity. The flexibility of the apparatus allowed also to obtain more than an order of magnitude larger than previously available event samples on the following decay modes: $ π^+e^+e^- (K_{πee})$, $π^+μ^+μ^- (K_{πμμ})$, $π^+π^-e^+ν_e (K_{e4})$, $μ^+e^+e^-ν_μ$, and $e^+e^+e^-ν_e$. The report focusses on the $K_{πμe}$ results and those on other lepton flavour violating decays as well as the $K_{e4}$ data, from which a new, quite precise value for the s-wave $ππ$ scattering length can be deduced.