Experiment 865 at the Brookhaven AGS has observed the decay K + → e + νeµ + µ − . The branching ratio extracted is (1.72±0.37(stat)±0.17(syst)±0.19(model))×10 −8 where the third term in the error results from the use of a model to extrapolate into a kinematic region dominated by background.

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Erratum: High statistics measurement of K e4 decay properties [Phys. Rev. D 67, 072004 (2003)]

A new measurement of the properties of the rare decay K + → π + e + e − R. Appel 6,3 , G.S. A large low-background sample of events (10300) has been collected for the rare decay of kaons in flight K + → π + e + e − by experiment E865 at the Brookhaven AGS. The decay products were accepted by a broad band high-resolution charged particle spectrometer with particle identification. The branching ratio (2.94 ± 0.05 (stat.) ± 0.13 (syst.) ± 0.05 (model)) × 10 −7 was determined normalizing to events from the decay chain K + → π + π 0 ; π 0 → e + e − γ. From the analysis of the decay distributions the vector nature of this decay is firmly established now, and limits on scalar and tensor contributions are deduced. From the e + e − invariant mass distribution the decay form factor f (z) = f0(1 + δz) (z = M 2 ee /m 2 K ) is determined with δ = 2.14 ± 0.13 ± 0.15. Chiral QCD perturbation theory predictions for the form factor are also tested, and terms beyond leading order O(p 4 ) are found to be important. 13.20Eb

Chiral Perturbation Theory (ChPT) [1] has been a successful approach to describing the decays of pseudoscalar mesons. In the ChPT program radiative kaon decays can serve both as an important test and a source of input parameters for the theory. While the decay modes K + → e + νγ (K e2γ ) and µ + νγ (K µ2γ ) have allowed some study of the form factors involved [2,3,4], the decays) allow a more detailed investigation into the structure of these decays. We report here on such an investigation from Experiment 865 at the Brookhaven National Laboratory AGS with a 100-fold increase in the number of events in the former mode and 150-fold increase in the latter [5].The K e2ee and K µ2ee decays are assumed to proceed via exchange of a W + -boson (l + ν) and photon (e + e − ). The decay amplitude [6,7] includes inner bremsstrahlung (IB) corresponding to the tree diagrams in Fig. 1a,b, and structure dependent (SD) radiation (Fig. 1c) parameterized by vector F V , axial F A and R form factors. K e2γ and K µ2γ experiments were actually sensitive only to |F V + F A |. R, which contributes only to decays with an e + e − -pair, has not yet been measured. Inner bremsstrahlung is unambiguously predicted by the K → lν amplitude and is proportional to the kaon decay constant F K = 160 MeV. We included the kaon electromagnetic form factor in Fig. 1b (amplitude A 4 of