ISSN - 15507998, Physical Review D - Particles, Fields, Gravitation and Cosmology, 2013, vol. 88, p. -
Precision measurements of B[ψ(3686)→π+π -J/ψ] and B[J/ψ→l+l-]
Based on (106.41±0.86)×106 ψ(3686) events collected with the BESIII detector at the BEPCII collider, the branching fractions of ψ(3686)→π+π-J/ψ, J/ψ→e +e-, and J/ψ→μ+μ- are measured. We obtain B[ψ(3686)→π+π-J/ψ] =(34.98±0.02±0.45)%, B[J/ψ→e+e -]=(5.983±0.007±0.037)%, and B[J/ ψ→μ+μ-]=(5.973±0.007±0.038)%. The measurement of B[ψ(3686)→π+π-J/ψ] confirms the CLEO-c measurement, and is apparently larger than the others. The measured J/ψ leptonic decay branching fractions agree with previous experiments within one standard deviation. These results lead to B[J/ψ→l+l-]=(5.978±0.005±0.040)% by averaging over the e+e- and μ+μ - channels and a ratio of B[J/ψ→e+e -]/B[J/ψ→μ+μ-]=1.0017±0. 0017±0.0033, which tests e-μ universality at the four tenths of a percent level. All the measurements presented in this paper are the most precise in the world to date. © 2013 American Physical Society.