Determination of the J/psi Leptonic Branching Fraction via psi(2S) -> pi^+ pi^- J/psi

A comparison of the rates for psi(2S) -> pi^+ pi^- J/psi, J/psi -> l^+l^- and J/psi -> anything is used to determine the J/psi leptonic branching fractions. The results are B(J/psi -> e^+e^-) = (5.90 +/- 0.05 +/- 0.10)% and B(J/psi -> mu^+ mu^-) = (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/psi is B(J/psi -> l^+ l^-) = (5.87 +/- 0.04 +/- 0.09)% per species. This result is used to estimate the QCD scale factor Lambda_MSbar and the strong coupling constant alpha_s.