The authors observed a new neutral kaon decay mode, K{sub L} {yields} {pi}{sup {+-}}e{sup {-+}}{bar {nu}}e{sup +}e{sup -} for the first time. Based on the 20225 events including 1018 {+-} 25 background events, they determined the branching ratio, B(K{sub L} {yields} {pi}{sup {+-}}e{sup {-+}}{bar {nu}}e{sup +}e{sup -}; M{sub e{sup +}e{sup -}}> 5 MeV/c{sup 2}, E*{sub e{sup +}e{sup -}}> 30 MeV) = (1.281 {+-} 0.041) x 10{sup -5}. This branching ratio agrees with a theoretical prediction based on the chiral perturbation theory ({chi}PT) calculation at {Omicron}(p{sup 4}). Most of the kinematical distributions agree with the {chi}PT {Omicron}(p{sup 4}) calculation. They also measured one of the low energy coupling constants for the {chi}PT {Omicron}(p{sup 4}), L{sub 9}{sup r} = (8.0 {+-} 1.6) x 10{sup -3}. The M{sub e{sup +}e{sup -}} distribution below 100 MeV showed a 3 {sigma} deviation from the {chi}PT {Omicron}(p{sup 4}) calculation. This requires further studies in theory and experiments.

The authors observed a new neutral kaon decay mode, KL → ?±e∓$ar{v}$e+e- for the first time. Based on the 20225 events including 1018 ± 25 background events, they determined the branching ratio, B(KL → ?±e∓$ar{v}$e+e-; Me+e- > 5 MeV/c2, E*+e- > 30 MeV) = (1.281 ± 0.041) x 10 -5. This branching ratio agrees with a theoretical prediction based on the chiral perturbation theory (XPT) calculation at ?(p4). Most of the kinematical distributions agree with the XPT ?(p4) calculation. They also measured one of the low energy coupling constants for the XPT ?(p4), L$rtop{9}$= (8.0 ± 1.6) x 10-3. The Me+e- distribution below 100 MeV showed a 3 ? deviation from the XPT O(p4) calculation. This requires further studies in theory and experiments.

This thesis describes the analysis of the semileptonic decay D{sup 0} {yields} {bar K}{sup 0} {pi}{sup -} {mu}{sup +}{nu} using FOCUS data. FOCUS is a fixed target experiment at Fermilab that studies the physics of the charm quark. Particles containing charm are produced by photon-gluon fusion from the collision of a photon beam on a BeO target. The experiment is characterized by excellent vertex resolution and particle identification. The spectrometer consists of three systems for track reconstruction (two silicon systems and one multiwire proportional chamber system) and two magnets of opposite polarity. The polarity of the magnet is such that the events of e{sup +}e{sup -} pairs produced in the target (which constitutes the main background) travel through a central opening in the detectors without interactions. Particle momentum is measured from the deflection angle in the magnets. Three multicell Cerenkov counters are used for charged particle identification (for e, {pi}, K, and p). Two different tracking systems located after several interaction lengths of shielding material are used for muon identification. The energy of neutral pions and electrons is measured in two electromagnetic calorimeters, while an hadron calorimeter is used for measuring the neutron energy. During the last four years the FOCUS collaboration provided results on several charm topics: CP violation, D{sup 0}-{bar D}{sup 0} mixing, rare and forbidden decays, precision measurements of semileptonic decays, baryon and meson lifetimes, fully hadronic baryon and meson branching ratios, charm spectroscopy, Dalitz analyses of resonant structures, charm anti-charm production, QCD studies involving double charm particles, and pentaquarks. Semileptonic decays, besides having a clear signature for experiments, provide crucial information for theoretical studies. These decays carry information on the weak coupling of quarks since they can be used for measuring Cabibbo-Kobayashi-Maskawa matrix elements. Although the decay occurs through weak interaction, QCD effects due to quark confinement affect the decay amplitude. These effects can be included through the form factors, which are predicted by different theoretical approaches (quark models, lattice QCD, and sum rules). Experiments can measure form factor ratios, and from the comparison with theory they provide guidance for building a successful theory to describe hadrons.

We present an analysis of the exclusive semileptonic decay mode D° .-->. K−enu/sub e/. We have measured the ratio of decay rates GAMMA(D° .-->. K−enu/sub e/)/GAMMA(D° .-->. K−.pi./sup +/). After correcting for the reconstruction efficiencies and subtracting the contribution from other decay modes we have found the ratio to be equal to 0.77 +- 0.12(stat) +- 0.13(syst).

Results are reported from a Dalitz plot analysis of 17750 K/sup 0//sub e3/ candidates obtained in an experiment designed to study both the semileptonic and 3.pi. decay modes of the K/sup 0//sub L/ with high efficiency across the Dalitz plot.

This thesis describes the analysis of the semileptonic decay D[sup 0] [yields] [bar K][sup 0] [pi][sup -] [mu][sup +] [nu] using FOCUS data. FOCUS is a fixed target experiment at Fermilab that studies the physics of the charm quark. Particles containing charm are produced by photon--gluon fusion from the collision of a photon beam on a BeO target. The experiment is characterized by excellent vertex resolution and particle identification. The spectrometer consists of three systems for track reconstruction (two silicon systems and one multiwire proportional chamber system) and two magnets of opposite polarity. The polarity of the magnet is such that the events of e[sup +]e[sup -] pairs produced in the target (which constitutes the main background) travel through a central opening in the detectors without interactions. Particle momentum is measured from the deflection angle in the magnets. Three multi-cell Cerenkov counters are used for charged particle identification (for e, [pi], K, and p). Two different tracking systems located after several interaction lengths of shielding material are used for muon identification. The energy of neutral pions and electrons is measured in two electromagnetic calorimeters, while an hadron calorimeter is used for measuring the neutron energy.

AGS-787, BNL-Princeton-TRIUMF, began taking data in February 1988 on rare K decays into .pi. and non-interacting neutrals. Recent observations of B-/bar B/ mixing have sharpened the branching ratio prediction of the three generation standard model, (SM), to (approximately) 1-5 x 10−1° for .pi.+.nu./bar /nu//. The detector has 2.pi. acceptance for this decay and 4.pi. acceptance for background gammas. A very large window for viewing new physics exists between the SM value and the previous experimental limit of 1.4 x 10−7. Several other predicted rare K+ decays can be detected. The performance of the detector is described. 8 refs., 4 figs.