Measurements of the semileptonic branching ratios of charmed D+ and D0 mesons are presented, from data collected at the ψ(3770) in the Mark III detector at SPEAR. From a sample of events ...kinematically selected as charmed D+D- or D0 D-0 pairs, the branching ratios R(D+ → e+ + X) = (17.0 ± 1.9 ± 0.7)% and R(D0 → e+ + X) = ( 7.5 ± 1.1 ± 0.4)% are obtained. The ratio of these measurements can be interpreted as the ratio of D+ and D0 lifetimes $\frac{τ^+}{τ^0}$ = $2.3^{+0.5+0.1}_{-0.4-0.1}$ by neglect of the contribution of Cabibbo-suppressed decays to the total decay width.
The Mark III Collaboration has measured the {ital J}/{psi} leptonic branching fractions using the process {psi}(2{ital S}){r arrow}{pi}{sup +}{pi}{sup {minus}}{ital J}/{psi}, {ital J}/{psi}{r ...arrow}{ital l}{sup +}{ital l{minus}}. The results are {ital B}({ital J}/{psi}{r arrow}{ital e}{sup +}{ital e{minus}})=(5.92{plus minus}0.15{plus minus}0.20)% and {ital B}({ital J}/{psi}{r arrow}{mu}{sup +}{mu}{sup {minus}})=(5.90{plus minus}0.15{plus minus}0.19)%, where the first error is statistical and the second is systematic. Assuming lepton universality, the leptonic branching fraction of the {ital J}/{psi} is (5.91{plus minus}0.11{plus minus}0.20)%. This result is used to obtain the strong coupling constant {alpha}{sub {ital s}} and the QCD scale factor {Lambda}{sub MS} (M{bar S} denotes the modified minimal-subtraction scheme).
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