First observation of the decay K+ -> e+ nu mu+ mu-

Experiment 865 at the Brookhaven AGS has observed the decay K^+ -> e^+ nu mu^+ mu^-. The branching ratio extracted is (1.72 +/- 0.37(stat) +/- 0.17(syst) +/- 0.19(model)) x 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.Comment: 4 pages, 6 figures, Revtex4. Correction to figure and minor text change

A new measurement of K+(e4) decay and the s-wave pi-pi-scattering length a00

A sample of 400000 events from the decay K+->pi+pi-e+nu(e)(K(e4)) has been collected in experiment E865 at the Brookhaven AGS. The analysis of these data yields new measurements of the K(e4) branching ratio (4.11+-0.01+-0.11)*10**(-5)), the s-wave pi-pi scattering length a00=0.228+-0.012+-0.003, and the form factors F, G, and H of the hadronic current and their dependence on the invariant pi-pi mass

New, high statistics measurement of the K+ -> pi0 e+ nu (Ke3) branching ratio

E865 at the Brookhaven National Laboratory AGS collected about 70,000 K+(e3) events with the purpose of measuring the relative K+(e3) branching ratio. The pi0 in all the decays was detected using the e+e- pair from pi0 -> e+e-gamma decay and no photons were required. Using the Particle Data Group branching ratios for the normalization decays we obtain BR(K+(e3(gamma))=(5.13+/-0.02(stat)+/-0.09(sys)+/-0.04(norm))%, where $K+(e3(gamma)) includes the effect of virtual and real photons. This result is 2.3 sigma higher than the current Particle Data Group value. The implications of this result for the$V_{us}$ element of the CKM matrix, and the matrix's unitarity are discussed.Comment: 4 pages, 5 figures; final version accepted by PR

An Improved upper limit on the decay K^+ -> pi^+ mu^+ e^-

Based on results of a search for the lepton-family-number-violating decay $K^+ \to \pi^+\mu^+ e^-$ with data collected by experiment E865 at the Alternating Gradient Synchrotron of Brookhaven National Laboratory, we place an upper limit on the branching ratio at $2.1 \times 10^{-11}$ (90% C.L.). Combining the results with earlier E865 data and those of a previous experiment, E777, an upper limit on the branching ratio of $1.3 \times 10^{-11}$ (90% C.L.) is obtained.Comment: v2: 13 pages, submitted to the Phys. Rev. D v3: 13 pages, resubmitted to Phys. Rev. D (corrections include: a more detailed overview of the combined analysis of the available experimntal data

Experimental Study of the Radiative Decays K+ -> mu+ nu e+e- and K+ -> e+ nu e+e-

Experiment 865 at the Brookhaven AGS obtained 410 K+ -> e+ nu e+e- and 2679 K+ -> mu+ nu e+e- events including 10% and 19% background. The branching ratios were measured to be (2.48+-0.14(stat.)+-0.14(syst.))x10^-8 (m_ee>150 MeV) and (7.06+-0.16+-0.26)x10^-8 (m_ee>145 MeV), respectively. Results for the decay form factors are presented.Comment: 4 pages, 3 figures, RevTeX

A new measurement of the properties of the rare decay K -> pi+ e+ e-

A large low-background sample of events (10300) has been collected for the rare decay of kaons in flight K+ -> pi+ 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+ -> pi+ pi0; pi0 -> e+ e- gamma. 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+ delta*z) (z=M(ee)**2/m(K)**2) is determined with delta=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.Comment: 4 pages, 5 figure