A comparison of the rates for ψ(2S) → π + π − J/ψ, J/ψ → l + l − and J/ψ → anything is used to determine the J/ψ leptonic branching fractions. The results are B(J/ψ → e + e − ) = (5.90 ± 0.05 ± 0.10)% and B(J/ψ → µ + µ − ) = (5.84 ± 0.06 ± 0.10)%, where the first error is statistical and the second is systematic. Assuming lepton universality, the leptonic branching fraction of the J/ψ is B(J/ψ → ℓ + ℓ − ) = (5.87 ± 0.04 ± 0.09)% per species. This result is used to estimate the QCD scale factor Λ (4) M S and the strong coupling constant α s .

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