We report a measurement of the branching ratio B(D + ! 0 ` + `)/ B(D + ! K 0 ` + `) from the Fermilab charm hadroproduction experiment E791. Based on signals of 49 17 events in the D + ! 0 e + e mode and 54 18 events in the D + ! 0 + mode, we measure Combining the results from both the electronic and muonic modes, we obtain 2 Semileptonic charm decays are useful in probing the dynamics of hadronic currents since the Cabibbo-Kobayashi-Maskawa matrix elements for the charm sector are well-known from unitarity constraints. Form factors for Cabibbo-suppressed (CS) c ! d semileptonic decays can be related via Heavy Quark EEective Theory (HQET) to those for b ! u semileptonic decays at the same four-velocity transfer 1]. Since knowledge of the form factors in b ! u transitions is vital for extracting V ub from b ! u semileptonic decays in a model-independent way, study of c ! d semileptonic decays can improve our knowledge of V ub. Although considerable progress has been made in studying CS semileptonic charm decays to pseudoscalar mesons 2], the only previous result on CS semileptonic charm decay to a vector meson is based on four D + ! 0 + events 3]. In this Letter, we report a new measurement from the Fermilab hadroproduction experiment E791 of B(D + ! 0 ` + `)=B(D + ! K 0 ` + `) based on more than 100 D + ! 0 ` + ` decays in the combined electronic and muonic modes. The E791 experiment 4] recorded 2 10 10 events from 500 GeV/c ? interactions in ve thin targets (one platinum, four diamond) separated by gaps of 1.34 to 1.39 cm. Precision tracking and vertexing information was provided by 23 silicon microstrip detectors (6 upstream and 17 downstream of the targets) and 35 drift chamber planes. Momentum was measured with two dipole magnets. Two segmented threshold Cerenkov counters provided =K separation in the 6 ? 60 GeV/c momentum range 5]. (charge-conjugate states are implied throughout this Letter) are selected by requiring a three-prong decay vertex of charge 1 with one of the decay particles being identiied as a lepton. A segmented lead and liquid-scintillator calorimeter 6] is used to identify the electrons, based on energy deposition and transverse shower shape. The probability that a (K) is misidentiied as …
