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)]

We report a study of 20 exclusive reactions measured at the AGS at 5.9 GeV/c incident momentum, 90" center of mass. This experiment confirms the strong quark flow dependence of two-body hadronhadron scattering at large angle. At 9.9 GeVIc an upper limit had been set for the ratio of cross sections for (pp + pp)/(pp i pp) at 90' c.m.. with the ratio less than 4%. The present experiment was performed a t lower energy to gain sensitivity, but was still within the fixed angle scaling region. A ratio R(pp/pp) % 1/40 was measured a t 5.9 GeV/c, 90" c.m. in comparison to a ratio near 1.7 for small angle scattering. In addition, many other reactions were measured, often for the first time at 90" c.m. in the scaling region, using beams of T * . K*, p, and p on a hydrogen target. There are similar large differences in cross sections for other reactions: R ( K -p + T + C -/ K -~ i n-C') % 1/12, for example. The relative magnitudes of the different cross sections are consistent with the dominance of quark interchange in these 90" reactions, and indicate that pure gluon exchange and quark-antiquark annihilation diagrams are much less important. The angular dependence of several elastic cross sections and the energy dependence a t a fixed angle of many of the reactions are also presented. PACS number(s): 13.75.-n, 13.85.Dz, 13.85.Fb GeV/c, the depentcurrent address: ~~~~~~d university, cambridge, MA dence of the cross sections flattened in the region of 90" 02139. c.m. These observations indicate that large angle scat- §Current address: University of Arizona, Tucson, AZ 85721. tering for Q2 > 5 ( G e v /~)~ has different dynamics from "current address: Niels Bohr Institute, Copenhagen, small angle scattering. Moreover, the observed power Denmark. of the energy dependence agrees well with dimensionalThe reactions are written as (beam + target) -+ (spectrometer particle + side particle). Reactions 1, 2, 3, 17, and 18 were measured with either final-state particle in the spectrometer. Meson-baryon reactions1 =+P -+ P+ 2 n-p + pn-3 K + p -+ pK+ 4 K -p -+ pK-5 n+p -+ PP+ 6

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