Measurements of the Neutral Charmed Meson Semileptonic Decays Neutral D Meson Going to - Positron Electron Neutrino and Neutral D Meson Going to - Positron Electron Neutrino.

Analyses of the semileptonic decays D ^0--> K^-e^+nu_ {e} and D^0--> pi^-e^+nu_{e} (charge conjugates are implied) are presented. The data for these analyses were collected in Experiment 687 at Fermi National Accelerator Laboratory during the 1990 and 1991 fixed target runs. Experiment 687 is a photoproduction experiment designed primarily to investigate the decays of particles containing the charm quark. Particle decays are reconstructed in a two magnet, multiparticle spectrometer with precision vertex measurement and particle identification. Semileptonic decays are defined by the presence of at least one hadron and exactly one lepton-neutrino pair in the final state. These decays are easier to understand than decays where only hadrons populate the final state because the lepton-neutrino system is well understood and cannot interact via the strong force. Semileptonic decays provide information about the hadronic sector through measurements of the Cabibbo-Kobayashi-Maskawa (CKM) maxtrix elements and hadronic form factors. We have reconstructed 237 +/- 18 events in the D^0--> K^ -e^+nu mode and 36 +/- 11 events in the D^0--> pi^-e^+nu mode. We measure the branching ratio of D^0--> pi^-e^+nu relative to D^0--> K^-e^+ nu to be:{B(D^0 -->pi^-e^+nu)over B(D^0--> K^-e^+nu) } = 0.173 +/- 0.053 ({rm statistical }) _sp{-0.008}{+0.013} ( {rm systematic})Using unitarity constraints on the CKM matrix, our measurement of the relative branching ratio, and theoretical predictions assuming nearest pole dominance, we calculate the ratio of form factors evaluated to be:mid{f_sp {+}{pi}(0)over f_sp {+}{K}(0)}mid = 1.31 +/- 0.20 ({rm statistical})_sp{ -0.07}{+0.05} ({rm systematic }). These results for the relative branching ratio and the ratio of form factors represent the world's best estimates and are in agreement with previous estimates from experiments and theory.