Measurement of the Branching Fractions for J/psi to ell^+ ell^-
  We present measurements of the branching fractions for J/psi --> e+e- and mu+mu- using 3M psi(2S) decays collected with the CLEO detector operating at the CESR e+e- collider. We obtain B(J/psi --> e+e-) = (5.945 +- 0.067 +- 0.042 )% and B(J/psi --> mu+mu-) = (5.960 +- 0.065 +- 0.050)%, leading to an average of B(J/psi --> l+l-) = (5.953 +- 0.056 +- 0.042)% and a ratio of B(J/psi --> e+e-) / B(J/psi --> mu+mu-) = (99.7 +- 1.2 +- 0.6)%, all consistent with, but more precise than, previous measurements. 
Z. Li, et al, CLEO Collaboration