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)]

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

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

A search for lepton flavor violating decays, K+ --> mu+ mu+ pi-, K+ --> e+ e+ pi-, K+ --> pi+ e+ mu-, K+ --> mu+ e+ pi-, and pi0 --> e+ mu-, was performed using the data collected in Experiment E865 at the Brookhaven Alternating Gradient Synchrotron. No signal was found in any of the decay modes. At the 90% confidence level, the branching ratios are less than 3.0x10(-9), 6.4x10(-10), 5. 2x10(-10), 5.0x10(-10), and 3.4x10(-9), respectively.

We report experimental details and results of a new measurement of the decay K + → π + π − e + νe (Ke4). A sample of more than 400,000 Ke4 events with low background has been collected by Experiment 865 at the Brookhaven Alternate Gradient Synchrotron. From these data, the branching ratio (4.11 ± 0.01 ± 0.11) · 10 −5 and the ππ invariant mass dependence of the form factors F , G, and H of the weak hadronic current as well as the phase shift difference δ

E865 at the Brookhaven National Laboratory AGS collected about 70 000 K(+)(e3) events to measure the K(+)(e3) branching ratio relative to the observed K+-->pi(+)pi(0), K+-->pi(0)micro(+)nu, and K+-->pi(+)pi(0)pi(0) decays. The pi(0) in all the decays was detected using the e(+)e(-) pair from pi(0)-->e(+)e(-)gamma decay and no photons were required. Using the 2002 Particle Data Group branching ratios for the normalization decays, we obtain BR(K(+)(e3(gamma)))=(5.13+/-0.02(stat)+/-0.09(syst)+/-0.04(norm))%, where K(+)(e3(gamma)) includes the effect of virtual and real photons. This result is approximately 2.3sigma higher than the current Particle Data Group value. Implications for the V(us) element of the CKM matrix, and the matrix's unitarity are discussed.