Bai, JZ (reprint author), Inst High Energy Phys, Beijing 100039, Peoples R China.
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(MS)((4)) and the strong coupling constant alpha(s). [S0556-2821(98)07021-0].