The electron energy spectrum in the process e + e" -* ij)" (3710) ~* e ± + (> 2 charged particles) has been used to determine the D(56% D° + 44% D + ) semileptonic branching ratio. Assuming the Glashow-Iliopoulos-Maiani model, it is found that the inclusive branching ratio is b(D~+Xev) = 0.080± 0.015 and the state X is dominated by K and Kit. The fraction of Knev is (37± 16)% if the Kir system is entirely K*(890) 9 or (55±21)% if the Kn system is nonresonant. Within the assumptions of the analysis, the # lifetime is calculated to be (2.5±1.6)xio" 13 sec.We have studied the final states in the semileptonic decays of D mesons by means of the electron energy spectrum observed \xie*e~ annihilations. 1 The analysis assumes the decays are predominantly D*K(nn)ev, n^O, following the Glashow-Illiopoulos-Maiani 2 (GIM) model. The value of the inclusive semileptonic branching ratio measures the strength of the nonleptonic enhancement analogous to that observed in K decays. Furthermore, since the rate T{D-*Kev) can be reliably calculated, 3 the lifetimes of theD 0 and D + can be determined from their Kev branching ratios. Additional interest in these decay modes arises from the quantum chromodynamics (QCD) calculation of the mass and lifetime of the charmed quark, which uses the measured lepton momentum spectrum and b(p -+Xev)S The data, recorded by the DELCO detector 5 at SPEAR, is taken from the ^"(3770) region, 3.76 <£ Cem-<3.78 GeV. This facilitates our analysis for several reasons: The charm cross section is resonant and therefore can be well measured; the charmed particles are produced simply in DD pairs (the D°D°:D + D~ ratio is 0.56:0.44 according to phase space) with a known, small velocity (IPJ-0.26 GeV/c). Therefore, the measurements suffer neither from uncertainties in the c -Z) fragmentation nor from substantial Lorentz smearing.For the purpose of this analysis, we select events with ^ 3 observed charged particles of which one and only one is identified as an electron by having in-time Cherenkov and shower counter pulses. The minimum pulse heights correspond to 0.07 photoelectron for the Cherenkov counter and 0.3 minimum-ionizing particle for the shower counter. The candidate electron track is required to have at least one hit in the two innermost cylindrical proportional chambers in order to decrease photon conversion backgrounds. To achieve unambiguous electron identification, we retain those events where only one track enters the triggered Cherenkov cell. A further requirement of at least one hit in the outer spark chambers (azimuthal view) ensures a momentum measurement accuracy of v p /P = [(0.052) 2 + (0.080P) 2 ] 1/2 , where P is the track momentum in GeV/cThe electron momentum spectrum of the 596 events which satisfy these criteria is shown in Fig. 1. These data are not yet corrected for backgrounds or Cherenkov detection efficiency (Fig. 2). The predominant background source of highenergy electrons is electronic r decays 6 (indicated by the solid line in Fig. 1). The remaining backgrounds come from t...
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