An experiment on the decay K '-a'a-e'v was performed at the CERN proton synctirutron with sparkchamber and counter techniques. The IS, branching ratio has been measured relative to the T decay. The ax phase-shift difference 6: -61 and the form factors of the hadronic current have been determined as functions of the n a energy. The xa scattering length a : has been evaluated from the phase shifts with a phenomenological model. The results are compared with the theoretical predictions of current algebra and other models.

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...

In the reaction e*e~ -^"(3770), events containing either one to two electrons have been observed originating in semileptonic decays of D mesons. A comparison of these samples provides a determination of the branching,ratios: bif>^-*Xev) <4.0% (at 95% confidence level) and b(D + -*Xev) = (22.o!f ;f)%. These values imply that the ratio of D° and D + lifetimes is T(D + )/T(D°) >4.3 (at 95% confidence level).PACS numbers: 14.40.Pe, 13.65.+ iThe conventional model of the decay of charmed mesons 1 assumes that the light quarks are merely spectators and therefore predicts the D° and D + lifetimes to be equal. We report here a measurement of unequal lifetimes from a study of DD pairs produced me + e~ annihilations. 2 The analysis is based on a comparison of two data samples: In one, bothD and!) decay semileptonically, leading to events with a detected e + and e~ ("2e events"); in the other, only one semileptonic decay is observed ("le"). The data, recorded by the DELCO detector 3 at SPEAR, are selected from energies at the ty" and below. The ty" provides a pure sample of DD events with known charged and neutral composition, and the lower-energy data are used in the background measurements.The selection criteria for the le sample 4 require that events contain ^ 3 observed charged tracks, of which one is identified as an electron by having in-time Cherenkov-and shower-counter pulses. The backgrounds to the le sample come largely from r decays, accidental coincidences of a hadronic track and a Cherenkov pulse, misidentified photon conversions and Dalitz decays, and two-photon processes. Subtracting contributions from these backgrounds, we obtain 734 ± 44 le events due to charm decays at the */>".Events in the 2e sample must have two electrons and at least one nonelectron, defined as a track having momentum above 200 MeV/c, which passes through the Cherenkov counter but produces no Cherenkov pulse. To obtain unambiguous electron identification, we demand that only one track enter each triggered Cherenkov cell. The candidates are scanned to eliminate the majority of eey final states and misidentified photon conversions in hadronic events. A background due to ^'^7r + 7r" -~ e + e"ir +, n" is reduced by requiring an acollinearity of at least 20° between the e + and e~ in the azimuthal projection. Events involving 7T° or t] Dalitz decays are suppressed by requiring that the electron pair mass exceed m ^ . 21 2e events satisfy these criteria at the ip".The residual backgrounds in the 2e events result from (1) two real electrons, such as from twophoton processes or Dalitz decays; (2) one real electron plus one false electron, such as a D or r decay together with a hadron misidentified as an electron; and (3) two false electrons. We study le events at the ^(3100) to determine the mis identification probability for a track as an electron (1.9xl0" 3 ). Backgrounds (2) and (3) are then measured from the le sample at the ip" by randomly assigning a Cherenkov tag to eligible tracks at a rate equal to this misidentif ication probability....